An Overview of Purple Bacteria: Systematics, Physiology, and Habitats

Abstract

Anoxygenic phototrophic purple bacteria are a major group of photosynthetic microorganisms widely distributed in nature, primarily
in aquatic habitats. Nearly 50 genera of these organisms are known and some have become prime model systems for the experimental
dissection of photosynthesis. Purple sulfur bacteria differ from purple nonsulfur bacteria on both metabolic and phylogenetic
grounds, but species of the two major groups often coexist in illuminated anoxic habitats in nature. Purple sulfur bacteria
are strong photoautotrophs and capable of limited photoheterotrophy, but they are poorly equipped for metabolism and growth
in the dark. By contrast, purple nonsulfur bacteria, nature’s preeminent photoheterotrophs, are capable of photoautotrophy,
and possess diverse capacities for dark metabolism and growth. Several purple bacteria inhabit extreme environments, including
extremes of temperature, pH, and salinity. Collectively, purple bacteria are important phototrophs because they (1) consume
a toxic substance, H2S, and contribute organic matter to anoxic environments by their autotrophic capacities; (2) consume organic compounds, primarily
non-fermentable organic compounds, in their roles as photoheterotrophs; and (3) offer scientists in the photosynthesis community
a smörgasbord of molecular diversity for the study of photosynthesis.

Full text

C. Neil Hunter, Fevzi Daldal, Marion C. Thurnauer and J. Thomas Beatty (eds): The Purple Phototrophic Bacteria, pp. 1–15.
Chapter 1
*Author for correspondence, email: madigan@micro.siu.edu
An Overview of Purple Bacteria: Systematics,
Physiology, and Habitats
Michael T. Madigan* and Deborah O. Jung
Department of Microbiology, Southern Illinois University, Carbondale, IL 62901, U.S.A.
Summary .................................................................................................................................................................. 2
I. Introduction ......................................................................................................................................................... 2
II. Systematics of Purple Bacteria .......................................................................................................................... 3
A. Purple Sulfur Bacteria .......................................................................................................................... 4
B. Purple Nonsulfur Bacteria .................................................................................................................... 4
III. Physiology of Purple Bacteria ............................................................................................................................ 4
A. Purple Sulfur Bacteria .......................................................................................................................... 4
B. Purple Nonsulfur Bacteria .................................................................................................................... 6
1. Photoheterotrophy ....................................................................................................................... 6
2. Dark Growth ................................................................................................................................ 7
3. Nitrogen Fixation ......................................................................................................................... 7
IV. Habitats of Purple Bacteria ................................................................................................................................. 7
A. Purple Sulfur Bacteria .......................................................................................................................... 7
1. Blooms in Strati ed Lakes ........................................................................................................... 7
2. Microbial Mats ............................................................................................................................. 8
B. Purple Nonsulfur Bacteria .................................................................................................................... 9
1. Sewage ....................................................................................................................................... 9
2. Purple Nonsulfur Bacteria in Waste Lagoons ............................................................................. 9
V. Purple Bacteria in Extreme Environments .......................................................................................................... 9
A. Thermophilic Purple Bacteria ............................................................................................................. 10
1. Thermochromatium tepidum .................................................................................................... 10
2. Other Thermophilic Purple Bacteria ..........................................................................................10
B. Halophilic and Alkaliphilic Purple Bacteria ......................................................................................... 10
C. Acidophilic Purple Bacteria ................................................................................................................ 11
D. Purple Bacteria from Permanently Cold Habitats ............................................................................... 11
E. Environmental Limits to Photosynthesis in Purple Bacteria ............................................................... 12
VI. Final Remarks .................................................................................................................................................. 12
Acknowledgments ................................................................................................................................................... 12
References .............................................................................................................................................................. 12
©2009 Springer Science + Business Media B.V.

2Michael T. Madigan and Deborah O. Jung
Summary
Anoxygenic phototrophic purple bacteria are a major group of photosynthetic microorganisms widely distributed
in nature, primarily in aquatic habitats. Nearly 50 genera of these organisms are known and some have become
prime model systems for the experimental dissection of photosynthesis. Purple sulfur bacteria differ from purple
nonsulfur bacteria on both metabolic and phylogenetic grounds, but species of the two major groups often
coexist in illuminated anoxic habitats in nature. Purple sulfur bacteria are strong photoautotrophs and capable
of limited photoheterotrophy, but they are poorly equipped for metabolism and growth in the dark. By contrast,
purple nonsulfur bacteria, nature’s preeminent photoheterotrophs, are capable of photoautotrophy, and possess
diverse capacities for dark metabolism and growth. Several purple bacteria inhabit extreme environments,
including extremes of temperature, pH, and salinity. Collectively, purple bacteria are important phototrophs
because they (1) consume a toxic substance, H2S, and contribute organic matter to anoxic environments by
their autotrophic capacities; (2) consume organic compounds, primarily non-fermentable organic compounds,
in their roles as photoheterotrophs; and (3) offer scientists in the photosynthesis community a smörgasbord of
molecular diversity for the study of photosynthesis.
Abbreviations: BChl – bacteriochlorophyll; LH – light-harvest-
ing; Rba. – Rhodobacter; Rcy. – Rhodocyclus; Rfx. – Rhodoferax;
Rps. – Rhodopseudomonas; Rsp. – Rhodospirillum; Tch. – Ther-
mochromatium
I. Introduction
Anoxygenic phototrophic pur ple bacteria are a major
group of phototrophic microorganisms that inhabit
aquatic and terrestrial environments. Purple bacte-
ria that inhabit oxic habitats and which carry out
photosynthesis only aerobically are called ‘aerobic
anoxygenic phototrophs’ and are covered in Chapter
3 (Yurkov and Csotonyi). The current chapter cov-
ers only the classical purple bacteria: purple sulfur
bacteria and purple nonsulfur bacteria.
Purple bacteria are photosynthetic gram-negative
prokaryotes that convert light energy into chemical
energy by the process of anoxygenic photosynthesis.
Purple bacteria contain photosynthetic pigments–bac-
teriochlorophylls and carotenoids — and can grow
autotrophically with CO2 as sole carbon source.
Many genera of purple bacteria are known and the
organisms share many basic properties with their non-
phototrophic relatives. Some general characteristics
of purple bacteria are listed in Table 1.
Purple bacteria share with oxygenic phototrophic
prokaryotes — the cyanobacteria — the ability to
conserve energy by photophosphorylation. How-
ever, unlike cyanobacteria and aerobic anoxygenic
phototrophs, photosynthesis in purple bacteria only
occurs under anoxic (O2-free) conditions. This is also
true of the other classical anoxygenic phototrophs:
green sulfur bacteria, green nonsulfur bacteria, and
the heliobacteria (Blankenship et al., 1995). Purple
bacteria require anoxic conditions for phototrophic
growth because pigment synthesis in these organisms
is repressed by molecular oxygen (Cohen-Bazire et
al., 1957). Thus, the competitive success of purple
bacteria in nature requires both light and anoxic
conditions. This combination is most commonly
found in lakes, ponds, estuaries, and other aquatic
environments where H2S is present (Pfennig, 1967,
1978a, 1989). Once these general conditions are
met, the exact physiochemical nature of the habitat
(sulfi de concentration, pH, light quality and intensity,
temperature) controls the abundance and diversity of
purple bacteria that develop there (Pfennig, 1978a,
1989; Madigan, 1988).
Purple bacteria participate in the anoxic cycling
of carbon both as primary producers (CO2 fi xation,
photoautotrophy) and as light-stimulated consumers
of reduced organic compounds (photoheterotrophy).
In certain habitats particularly favorable for their
development, purple bacteria have been shown to
be signifi cant primary producers (Czeczuga, 1968;
Takahashi and Ichimura, 1968; Overmann et al.,
1994, 1996, 1999). However, in most illuminated
sulfi dic habitats the role of purple bacteria as H2S
consumers is probably more important than any
contribution they make to primary production; H2S is
a highly poisonous substance for plants and animals
and also for many bacteria. The oxidation of sulfi de
by purple bacteria yields nontoxic forms of sulfur,
such as elemental sulfur (S0) and sulfate (SO4
2–).
Sulfi de oxidation thus allows the upper waters of a
lake to remain oxic and suitable for plants, animals,
and aerobic bacteria.

3
Chapter 1 Overview of Purple Bacteria
II. Systematics of Purple Bacteria
Purple sulfur bacteria and purple nonsulfur bacte-
ria were originally distinguished on physiological
grounds based on their tolerance and utilization
of sulfi de. Purple sulfur bacteria were species that
tolerated millimolar levels of sulfi de and oxidized
sulfi de to sulfur globules stored intracellularly (Fig.
1a), while purple nonsulfur bacteria were species that
did neither (van Niel, 1932, 1944). However, classic
chemostat experiments by Hansen and van Gemerden
(1972) showed that these criteria for classifying
purple bacteria were not absolute. At low levels of
sulfi de, typically less than 0.5 mM, most species of
purple nonsulfur bacteria will grow and in so doing,
oxidize sulfi de to S0, S4O6
2–, or SO4
2–. Nevertheless,
an important distinction in the sulfi de metabolism
of purple sulfur and purple nonsulfur bacteria re-
mains: any S0 formed by purple nonsulfur bacteria
is not stored intracellularly, but instead is deposited
outside the cell (Hansen and van Gemerden, 1972;
Brune, 1995) (species of Ectothiorhodospiraceae are
an exception here). Thus, when grown on sulfi de, it
is easy to differentiate a purple sulfur from a purple
nonsulfur bacterium because of the microscopically
obvious globules of S0 formed (Fig. 1a).
Subsequent isolations of purple nonsulfur bac-
teria from highly sulfi dic habitats have shown that
many species of this group are actually quite sulfi de
tolerant. For example, both the marine species
Rhodobacter (originally Rhodopseudomonas, Rps.)
sulfi dophilus and the cold-active species Rhodoferax
antarcticus (isolated from the sulfi dic bottom waters
Table 1. General properties of anoxygenic purple phototrophic bacteriaa
Property Examples
Groups/phylogeny Purple sulfur bacteria (gammaproteobacteria); purple nonsulfur bacteria (alpha-
or betaproteobacteria)
Major species studied Allochromatium vinosum and Thiocapsa roseopersicina (purple sulfur bacteria);
Rhodobacter capsulatus,Rhodobacter sphaeroides,Rhodospirillum rubrum, and
Rhodopseudomonas palustris (purple nonsulfur bacteria)
Pigments/color of dense cell suspensions BChl a or b; major carotenoids include spirilloxanthin, spheroidene, lycopene,
and rhodopsin, and their derivatives; cell suspensions purple, purple-red, purple-
violet, red, orange, brown, or yellow brown (BChl a-containing species); green or
yellow (BChl b-containing species)
Location of pigments in cells Within intracytoplasmic membranes arranged as membrane vesicles, tubes,
bundled tubes, or in stacks resembling lamellae
Absorption maxima of living cells BChl a-containing species: near 800 nm, and anywhere from 815–960 nm; BChl
b-containing species: 835–850 nm and 1010–1040 nm
Electron donors/sulfur globulesbH2S, S0, S2O3
2–, H2, Fe2+; if S0 is produced from the oxidation of sulfi de, the S0 is
stored intracellularly only in certain purple sulfur bacteria (see Fig. 1a)
Photoheterotrophy/dark respiratory growth Purple sulfur bacteria limited on both accounts; purple nonsulfur bacteria
typically diverse on both accounts
aAll purple bacteria are gram-negative prokaryotes. All species contain peptidoglycan and an outer membrane containing lipopolysac-
charide. bVirtually all purple bacteria are capable of autotrophic growth. When growing autotrophically, the Calvin cycle (reductive
pentose phosphate cycle) is used as the mechanism for CO2 fi xation
Fig. 1. Phase photomicrographs of phototrophic purple bacteria.
(a) Cells of a strain of the purple sulfur bacterium Thermochro-
matium tepidum isolated from a New Mexico (USA) hot spring.
Note the bright refractile intracellular sulfur globules (arrows).
(b) Cells of Rhodobaca bogoriensis, an alkaliphilic purple non-
sulfur bacterium isolated from Lake Bogoria (Kenya). If purple
nonsulfur bacteria oxidize sulfi de, any S0 they produce remains
outside the cell. Cells of Tch. tepidum are about 1.5 µm wide and
cells of Rbc. bogoriensis are about 0.8 µm wide.

4Michael T. Madigan and Deborah O. Jung
of the permanently frozen freshwater Lake Fryxell,
McMurdo Dry Valleys) can tolerate over 4 mM
sulfi de (Hansen and Veldkamp, 1973; Jung et al.,
2004). This concentration of sulfi de is toxic to many
purple sulfur bacteria (Pfennig, 1967, 1978a, 1989)!
The original classifi cation of purple bacteria on the
basis of sulfi de metabolism has been supported by
molecular criteria. Phylogenetic analyses of purple
bacteria based on comparative 16S rRNA sequencing
have shown that purple sulfur bacteria are species of
gammaproteobacteria while purple nonsulfur bacteria
are either alpha- or betaproteobacteria (Imhoff et al.,
2005) (Tables 1 and 2).
A. Purple Sulfur Bacteria
Over 25 genera of purple sulfur bacteria are now
recognized, consisting of a variety of morphological
types (Table 2). Purple sulfur bacteria include both
species that store S0 inside the cell (family Chromatia-
ceae, see Fig. 1a), and those that produce extracellular
S0 (Ectothiorhodospiraceae) (Table 1). [It should
be noted that research on the mechanism of sulfi de
oxidation by Allochromatium vinosum has shown that
the ‘intracellular sulfur’ produced by this organism
actually accumulates in the periplasm rather than in
the cytoplasm (Pattaragulwanit et al., 1998); this is
likely true for all species of Chromatiaceae as well].
Most laboratory studies of purple sulfur bacteria have
focused on Allochromatium and Thiocapsa species
(Table 1) since these are the most easily grown. Many
species of purple sulfur bacteria are ‘extremophilic’
species, including in particular, species that grow
best at high salt and/or pH (Table 2).
B. Purple Nonsulfur Bacteria
Twenty genera of purple nonsulfur bacteria are now
recognized (Table 2). Species of Rhodobacter and
Rhodopseudomonas have been the workhorses for
laboratory studies of anoxygenic photosynthesis
(Table 1). But many other interesting species, some
of which have one or more unusual metabolic fea-
tures, are also known. For example, extremophilic
species inhabiting hot, cold, salty, alkaline (Fig. 1b),
and acidic environments have been isolated and will
be discussed in a later section of this chapter (see
section V).
As shown in Table 2, all purple nonsulfur bacteria
are proteobacteria, and phylogenetic trees show the
various species to be closely related to nonphototro-
phic species (Imhoff et al., 2005). If one considers
the fact that the pigments and photocomplexes in
the different species of purple nonsulfur bacteria
are very similar, this suggests that the acquisition of
phototrophic capacity in purple nonsulfur bacteria
has occurred by lateral gene transfer. Sequence
analyses of photocomplex proteins have confi rmed
this (Nagashima et al., 1997). Table 2 also lists the
three letter genus name abbreviations for both purple
sulfur and nonsulfur bacteria; these abbreviations will
be used throughout this book.
III. Physiology of Purple Bacteria
Purple bacteria are relatively easy to grow in labo-
ratory culture; in most cases all that is needed is an
anoxic mineral medium supplemented with either
sulfi de plus bicarbonate (photoautotrophic growth) or
an organic compound (photoheterotrophic growth).
Because of this, and because anoxygenic photosyn-
thesis is a simpler form of photosynthesis than the
oxygenic process, purple bacteria have emerged as
ideal model systems for dissecting the physiology,
biochemistry and molecular biology of photosynthe-
sis. Moreover, anoxygenic photosynthesis preceded
oxygenic photosynthesis on Earth by billions of
years. Thus, studies of purple and other anoxygenic
phototrophs have contributed in major ways to our
understanding of the evolution of photosynthesis
(Raymond et al., 2003; Chapter 2, Swingley et al.).
A. Purple Sulfur Bacteria
The physiology of purple sulfur bacteria is inti-
mately linked to sulfi de, and large populations of
purple sulfur bacteria are observed in nature only in
illuminated environments where sulfi de is present
(Pfennig, 1967, 1978a, 1989). This implies that the
growth of purple sulfur bacteria in nature is primarily
phototrophic. If growth is photoautotrophic, sulfi de,
thiosulfate or H2 are used as photosynthetic electron
donors (Trüper and Fischer, 1982; Madigan, 1988;
Brune, 1995). A few species can also use ferrous
(Fe2+) iron as an electron donor, oxidizing it to fer-
ric (Fe3+) iron (Ehrenreich and Widdel, 1994), and at
least one species, a strain of Thiocapsa, can use nitrite
(NO2
–) as photosynthetic electron donor, oxidizing it
to nitrate (NO3
–.
In addition to autotrophic growth, a few organic
carbon sources are photoassimilated by purple sulfur

5
Chapter 1 Overview of Purple Bacteria
Table 2. Genera of anoxygenic phototrophic purple bacteria
Taxonomy/Phylogeny Genus Genus abbreviationaMorphology
Purple Nonsulfur Bacteria
Alphaproteobacteria RhodobacacRca. Cocci to short rods
Rhodobacter Rba. Rods
Rhodovulum Rdv. Rods-Cocci
RhodopseudomonascRps. Budding rods
RhodoblastuscRbl. Budding rods
Blastochloris Blc. Budding rods
Rhodomicrobium Rmi. Budding rods
Rhodobium Rbi. Rods
Rhodoplanes Rpl. Rods
RhodocistacRcs. Spirilla
Rhodospirillum Rsp. Spirilla
Phaeospirillum Phs. Spirilla
RhodopilacRpi. Cocci
Rhodospira Rsa. Spirilla
RhodovibriocRhv. Vibrio
RhodothallasiumcRts. Spirilla
Roseospira Ros. Spirilla
Roseospirillum Rss. Spirilla
Betaproteobacteria Rhodocyclus Rcy. Curled vibrios
RhodoferaxcRfx. Rods, vibrios
Rubrivivax Rvi. Rods, curved rods
Purple Sulfur Bacteria
Gammaproteobacteria
Family ChromatiaceaebAllochromatium Alc. Rods
Amoebobacter Amb. Cocci in plates or clumps
Chromatium Chr. Rods
HalochromatiumcHch. Rods
Isochromatium Isc. Rods
Lamprobacter Lpb. Rods
Lamprocystis Lpc. Cocci in clusters
Marichromatium Mch. Rods
Rhabdochromatium Rbc. Rods
ThermochromatiumcTch. Rods
ThioalkalicoccuscTac. Cocci
Thiobaca Tba. Rods
Thiocapsa Tca. Cocci
Thiococcus Tco. Cocci
Thiocystis Tcs. Cocci to short rods
Thiodictyon Tdc. Rods forming aggregates
Thiofl avicoccus Tfc. Cocci
ThiohalocapsacThc. Cocci
Thiolamprovum Tlp. Cocci
Thiopedia Tpd. Cocci, often in plates
Thiorhodococcus Trc. Cocci
Thiorhodovibrio Trv. Vibrios to spirilla
Thiospirillum Tsp. Spirilla
Family EctothiorhodospiraceaebEctothiorhodospiracEct. Vibrios to spirilla
HalorhodospiracHlr. Vibrios to spirilla
ThiorhodospiracTrs. Vibrios to spirilla
Ectothiorhodosinus Ets. Rods
aAbbreviations in accordance with Imhoff and Madigan (2004). bSpecies of Chromatiaceae store sulfur from the oxidation of sulfi de
intracellularly (see Fig. 1A); species of Ectothiorhodospiraceae do not. cContain one or more extremophilic species growing at an
extreme of temperature, pH, or salinity greater than marine salinity

6Michael T. Madigan and Deborah O. Jung
bacteria. Organic acids and fatty acids are the pre-
ferred substrates, but short-chain alcohols and even
carbohydrates are used by certain species (Sojka,
1978). Photoheterotrophic growth of Allochroma-
tium vinosum and other Allochromatium species
that are capable of assimilatory sulfate reduction
does not require sulfi de. However, some purple
sulfur bacteria will not grow without sulfi de and are
also nutritionally quite restricted. These include the
large-celled Chromatium species such as Chromatium
okenii and Chromatium weissei, and Thiospirillum
(Trüper, 1978), as well as the thermophilic species
Thermochromatium (Tch.) tepidum (Madigan, 1986)
(Fig. 1a). Sulfi de is required for growth of these
species and the only organic compounds that are
photoassimilated are acetate and pyruvate (Trüper
1981; Madigan, 1986).
Dark growth of some purple sulfur bacteria is pos-
sible. For example, certain Chromatiaceae, including
species of Allochromatium, Thiocystis, Amoebo-
bacter, and Thiocapsa, can grow in darkness as
either chemoorganotrophs or chemolithotrophs when
the oxygen concentration is signifi cantly reduced
[microaerobic growth; Kämpf and Pfennig, 1980].
Thiocapsa roseopersicina and Thiocystis violacea
are the most oxygen tolerant purple sulfur bacte-
ria (Kondratieva et al., 1976; Kämpf and Pfennig,
1980); however, respiratory growth of these species
is very slow compared with phototrophic growth.
If one considers that dark growth of purple sulfur
bacteria in nature puts them in direct competition
with nonphototrophic bacteria as well as with purple
nonsulfur bacteria, the ecological signifi cance of dark
metabolism by purple sulfur bacteria is probably mi-
nor. It is more likely that dark energy metabolism by
purple sulfur bacteria helps these organisms survive
intermittently oxygenated environments or is used as
a means to generate ATP at night, rather than being a
major means of supporting extended growth in nature
(van Gemerden, 1968).
B. Purple Nonsulfur Bacteria
Purple nonsulfur bacteria are a physiologically versa-
tile group of purple bacteria that can grow well both
phototrophically and in darkness. Growth of some
purple nonsulfur bacteria, for example, Rhodobacter
(Rba.) capsulatus, is possible under phototrophic
conditions with either CO2 or organic carbon, or in
darkness by respiration, fermentation, or chemoli-
thotrophy. This makes Rba. capsulatus probably the
most metabolically versatile of all known bacteria
(Madigan and Gest, 1979). Carbon metabolism in
purple nonsulfur bacteria has been summarized in
the excellent reviews by Tabita (1995) and by Gibson
and Harwood (1995); see also Chapter 28, Romagnoli
andTabita; and Chapter 29, Harwood.
1. Photoheterotrophy
Under phototrophic (anoxic/light) conditions,
typical purple nonsulfur bacteria can grow photo-
autotrophically with H2 or low levels of sulfi de as
electron donors; a few species can use S2O3
2– or Fe2+
as photosynthetic electron donors (Ehrenreich and
Widdel, 1994; Brune, 1995). However, most purple
nonsulfur bacteria grow best as photoheterotrophs in
media containing a readily useable organic compound,
such as malate or pyruvate, and ammonia as nitrogen
source (Sojka, 1978).
Yeast extract is a common addition to media
formulated for purple nonsulfur bacteria (Biebl and
Pfennig, 1981). Yeast extract is a source of B-vitamins,
one or more of which are required by the majority
of recognized species of purple nonsulfur bacteria.
Requirements for thiamine, nicotinic acid, biotin,
and p-aminobenzoic acid are the most common. Re-
quirements for B-complex vitamins have never been
observed in purple sulfur bacteria, although many spe-
cies require vitamin B12, a growth factor required by
only a handful of purple nonsulfur bacteria (Pfennig,
1978b; Siefert and Koppenhagen, 1982). However,
beyond its role as a source of vitamins, yeast extract
also stimulates the growth of purple nonsulfur bac-
teria because of its assortment of organic compounds
that can fuel photoheterotrophic growth.
Several individual organic compounds support
photoheterotrophic growth of purple nonsulfur
bacteria. Organic acids, amino acids, fatty acids,
alcohols, carbohydrates, and even C-1 compounds
are metabolized by different species (Sojka, 1978;
Trüper and Pfennig, 1981). With minor exceptions,
the citric acid cycle intermediates malate, succinate,
and fumarate are universally used, as are pyruvate
and acetate; many species also use ethanol, lactate,
and propionate (Sojka, 1978; Trüper and Pfennig,
1981). A few purple nonsulfur bacteria photoassimi-
late aromatic compounds such as benzoate, hydroxy
derivatives of benzoate, and cyclohexane carboxylate
(Gibson and Harwood, 1995). Enrichment cultures
employing benzoate as carbon source typically yield
strains of Phaeospirillum (formerly Rhodospirillum)

7
Chapter 1 Overview of Purple Bacteria
fulvum or Rps. palustris (Gibson and Harwood, 1995).
Benzene is not utilized by these or other purple non-
sulfur bacteria, but at least one aromatic hydrocarbon,
toluene, supports photoheterotrophic growth of cer-
tain strains of Blastochloris sulfoviridis (Zengler et
al., 1999). Growth of purple nonsulfur bacteria on
aliphatic hydrocarbons has not been described.
2. Dark Growth
Many of the same organic compounds that are photo-
assimilated by purple nonsulfur bacteria can also be
used as electron donors and carbon sources for dark
respiratory growth. Oxygen tolerances for respira-
tory growth vary among species, but some, such as
Rhodobacter species, can be grown with vigorous
aeration (Madigan, 1988). Certain purple nonsulfur
bacteria can grow under anoxic dark conditions by
either fermentation or anaerobic respiration. For
example, pyruvate (Uffen and Wolfe, 1970; Gurgen
et al., 1976) and certain sugars (Madigan and Gest,
1978; Schultz and Weaver, 1982) support fermenta-
tive growth of some purple nonsulfur bacteria, most
notably Rhodospirillum (Rsp.) rubrum and Rba.
capsulatus. Extensive fermentative growth of Rba.
capsulatus requires addition of an accessory oxidant
such as dimethyl sulfoxide or trimethylamine-N-oxide
(Madigan and Gest, 1978; Schultz and Weaver, 1982).
Rba. sphaeroides is capable of true denitrifi cation,
reducing NO3
– to N2 using nonfermentable carbon
sources as electron donors (Satoh et al., 1976).
Dark chemolithotrophic growth of certain species
of purple nonsulfur bacteria is possible using H2 or
S2O3
2– as electron donors. In Rba. capsulatus, chemo-
lithotrophic growth on H2 occurs and the organism
can be grown in a synthetic medium supplied with
the gases H2, O2, and CO2 as electron donor, electron
acceptor, and carbon source, respectively (Madigan
and Gest, 1979). Whether chemolithotrophy is a
signifi cant growth strategy for purple bacteria in
nature is unknown, but it is likely that the ability
to conserve energy from the oxidation of inorganic
electron donors gives purple bacteria an added
physiological dimension in competition with non-
phototrophic bacteria.
3. Nitrogen Fixation
With only a couple of known exceptions, purple non-
sulfur bacteria can fi x nitrogen (N2 + 8H → 2NH3 +
H2) (Madigan, 1995). The Rhodobacter species Rba.
capsulatus and Rba. sphaeroides grow most rapidly
with N2 as sole nitrogen source and show the highest
rates of in vivo nitrogenase activity (Madigan et al.,
1984). Consequently, Rhodobacter species tend to
dominate enrichment cultures for purple nonsulfur
bacteria that employ nitrogen fi xation as a selective
condition. Because in general purple nonsulfur bac-
teria are excellent nitrogen-fi xing bacteria (Madigan
et al., 1984), it is likely that the capacity for di azo-
tro phy confers a signifi cant competitive advantage
on them in anoxic environments that are limited in
fi xed nitrogen.
IV. Habitats of Purple Bacteria
A. Purple Sulfur Bacteria
A detailed description of the major habitats of purple
bacteria including a wealth of specifi c examples can
be found in the reviews of Madigan (1988), Pfennig
(1967, 1978a, 1989), and van Gemerden and Mas
(1995).
Large masses (blooms) of purple sulfur bacteria
often develop in sulfi dic aquatic ecosystems exposed
to light. Although blooms of phototrophic sulfur
bacteria may occur in shallow lagoons polluted by
sewage (which triggers the activities of sulfate-re-
ducing bacteria), densely stratifi ed ‘plates’ of purple
bacteria form only in the deep waters of lakes pro-
tected from excessive wind mixing and which contain
suffi cient sulfate to support sulfate reduction in the
sediments.
1. Blooms in Stratified Lakes
Intense microbial activity occurs in the sediments of
productive stratifi ed lakes. Organic material reaching
the bottom waters is catabolized by fermentation,
which releases a variety of reduced organic products,
including lactate, ethanol, and fatty acids. In fresh-
water lakes containing low levels of sulfate, these
fermentation products can be photoassimilated by
purple nonsulfur bacteria or converted to methane
by the cooperative interactions of syntrophic bacteria
and methanogenic Archaea. Alternatively, if electron
acceptors such as Fe3+ or NO3
– are available, the
fermentation products will fuel anaerobic respira-
tions supported by these electron acceptors before
extensive occurs.
If sulfate is present, sulfate-reducing bacteria

8Michael T. Madigan and Deborah O. Jung
will be active in the sediments forming sulfi de; the
sulfi de diffuses upwards from the sediments into the
water column forming a gradient. Sulfi de triggers the
growth of purple sulfur bacteria, which develop in
specifi c zones of the water column where light and
sulfi de are optimal (Pfennig, 1967, 1975, 1978a).
If cell numbers are suffi ciently high, the lake water
itself will become pigmented red, purple, or reddish-
brown (Pfennig 1978a, 1989; Overmann et al., 1994,
1996, 1999). When this occurs, it is often possible to
identify the major genera of purple bacteria present
by simple microscopic examination. In many strati-
fi ed lakes the bloom of purple bacteria consists of a
mixture of species (Caldwell and Tiedje, 1975a,b),
while in others, the bloom may contain only a single
species (Overmann et al., 1994, 1996, 1999).
A nice example of the layering of phototrophic
purple bacteria in stratifi ed lakes can be found in
the work of Caldwell and Tiedje (1975a,b). These
workers examined water samples collected at 1 m
intervals from two eutrophic lakes — Wintergreen
and Burke — located in southwest Michigan (USA).
In these studies, done before the days of molecular
microbial ecology, the purple sulfur bacteria could
be adequately identifi ed by their characteristic mor-
phologies. Both lakes contained species of Thiopedia,
Thiospirillum, Thiocystis, and Chromatium. However,
in Burke Lake the Thiospirillum population domi-
nated while in Wintergreen Lake, the Thiopedia popu-
lation dominated. Although physiochemical profi les
of the two lakes were not performed, the dominant
population in each case was likely selected by the
major characteristics — light, sulfi de, pH, dissolved
organic carbon, and the like — that defi ned each lake
(Caldwell and Tiedje, 1975a,b).
Both Burke and Wintergreen lakes also contained
green sulfur bacteria in the waters beneath the
purple sulfur bacteria. The green bacterial popula-
tion contained phototrophic consortia (Overmann
and Schubert, 2002) such as ‘Chlorochromatium’
(Caldwell and Tiedje, 1975a,b). Green bacteria can
outcompete purple bacteria at low light intensities
because they possess large antenna pigment struc-
tures called chlorosomes (Kimble and Madigan,
2002; Frigaard and Bryant, 2004). Green bacteria
also use different regions of the spectrum than do
purple bacteria and are typically much more sulfi de
tolerant. Thus, green bacteria can exist beneath a
layer of purple bacteria in a water column, and this
pattern is common in stratifi ed lakes (Pfennig, 1967,
1975, 1978a, 1989).
In hypersulfi dic shallow lakes, such as those in the
karstic Banyoles area of northern Spain, millimolar
levels of sulfi de are present and phototrophic sulfur
bacteria bloom throughout the water column. In Lake
Cisó, the best studied of these lakes, the entire water
column is anoxic, and the lake turns bright red during
an active bloom (Guerrero et al., 1985). Both green
and purple sulfur bacteria are present in Lake Cisó
but the lake is dominated by a Chromatium (probably
Allochromatium) species during periods of the most
extensive bloom.
2. Microbial Mats
Purple sulfur bacteria are also common in microbial
mats, including mats that form in marine or hyper-
saline environments (van Gemerden and Mas, 1995)
and in the effl uents of thermal springs (Castenholz
and Pierson, 1995). Microbial mats are laminated
organo-sedimentary structures composed primar-
ily of fi lamentous cyanobacteria and anoxygenic
phototrophs, such as Chlorofl exus, but often contain
purple bacteria as well. As new growth occurs from
the top of the mat, the lower mat layers decompose
and sulfate-reduction typically occurs. This supplies
the sulfi de necessary to trigger growth of purple
bacteria, usually purple sulfur bacteria. Mat thick-
ness can vary considerably. In siliceous alkaline hot
spring microbial mats, mats can be 4–5 cm thick.
Mats containing only purple bacteria, such as those of
Tch. tepidum that form in the Mammoth hot springs
of Yellowstone are much thinner, up to 0.5 cm in
thickness (Ward et al., 1989).
Thiocapsa and Allochromatium species are com-
mon inhabitants of marine microbial mats. These
purple sulfur bacteria often form a dense pigmented
layer between the cyanobacteria and the lower layers
of the mat. In this niche purple bacteria oxidize sulfi de
that diffuses upwards from below before it reaches
the cyanobacterial layers (van Gemerden and Mas,
1995). Thiocapsa roseopersicina, in particular, is
very common in marine mats, probably because of
its metabolic versatility. Besides its photoautotrophic
and photoheterotrophic capacities, this purple sulfur
bacterium can grow in darkness by heterotrophic and
chemolithotrophic means (Kondratieva et al., 1976).
This versatility allows Thiocapsa roseopersicina to
take full advantage of the variable growth conditions
that characterize different layers of microbial mats
(van Gemerden and Mas, 1995).

9
Chapter 1 Overview of Purple Bacteria
B. Purple Nonsulfur Bacteria
Purple nonsulfur bacteria occasionally form dense
blooms in habitats where levels of sulfi de are either
low or undetectable. Purple nonsulfur bacteria are
usually present in only low numbers in blooms of
purple sulfur bacteria, probably because of their
sulfi de sensitivity. Instead of photoautotrophy, purple
nonsulfur bacteria specialize in photoheterotrophy.
Although this puts them in competition with hetero-
trophs for organic compounds, photoheterotrophic ca-
pacity likely confers a signifi cant selective advantage
on purple nonsulfur bacteria. This is because unlike
heterotrophs, phototrophs do not need to conserve
energy from the carbon sources they photoassimilate;
carbon goes almost quantitatively into cell material.
However, no known purple nonsulfur bacteria can
hydrolyze major polymeric substances such as cel-
lulose or starch, and so ultimately, the phototrophs
depend on the heterotrophs to generate the low-
molecular-weight compounds they photoassimilate
(Pfennig, 1978a).
1. Sewage
Purple nonsulfur bacteria are present in sewage (Holm
and Vennes 1970; Siefert et al., 1978). In a detailed
study by Siefert et al. (1978), it was shown that the
mean number of purple nonsulfur bacteria in a sew-
age plant in Göttingen, Germany was highest in the
activated sludge stage of the treatment process; counts
fl uctuated between 105 and 106 cells ml–1 but were
never greater than 106 cells ml–1 (as measured using
plate counting techniques). Purple sulfur bacteria,
on the other hand, were quantitatively insignifi cant
in sewage and were detectable by culture only from
activated sludge (~103 cells ml–1).
A variety of purple nonsulfur bacteria were identi-
fi ed in the sewage plant, including Rba. sphaeroides
and Rba. capsulatus,Rps. palustris and Rps. (now
Blastochloris)viridis,Rhodocyclus (now Rubrivivax)
gelatinosus and Rhodocyclus (Rcy.) tenuis and Rsp.
photometricum.Rba. sphaeroides,Rubrivivax gela-
tinosus,Rps. palustris, and Rba. capsulatus made up
the bulk of the purple bacteria present. Despite these
relatively high numbers, it was concluded that pur ple
nonsulfur bacteria probably played only a minor role
in organic matter transformations in sewage compared
with heterotrophic bacteria, which were present at
108 to 109 cells ml–1. Purple nonsulfur bacteria were
also detectable in the strictly anaerobic (and dark)
sewage sludge digestor, but these likely represented
only transient cells traveling through the system
(Siefert et al., 1978).
2. Purple Nonsulfur Bacteria in Waste
Lagoons
Waste lagoons offer excellent conditions for growth
of purple nonsulfur bacteria (Jones, 1956; Cooper
et al., 1975; Kobayashi, 1975). For example, a
pigmented bloom of purple nonsulfur bacteria was
reported from the waste lagoon of a vegetable canning
plant in Minnesota (USA); prolifi c growth leading
to intensely red-colored lagoons occurred and the
bloom was associated with a signifi cant reduction in
odor (Cooper et al., 1975). Rba. sphaeroides,Rba.
capsulatus, and Rps. palustris were the key species
in this bloom, and it is likely that their consumption
of volatile fatty acids produced by fermentation led
to the odor reduction observed.
The morphologically and phylogenetically unique
purple nonsulfur bacterium Rcy. purpureus, iso-
lated by Norbert Pfennig over 30 years ago, was the
dominant phototroph in a swine waste lagoon in Iowa
(USA) (Pfennig, 1978b). This organism, of which
only a single strain has ever been isolated, probably
thrived on the combination of organic constituents
present in the waste materials. However, it is of interest
that Rcy. purpureus, one of the only purple nonsulfur
bacteria to lack a nitrogenase system (Madigan et al.,
1984; Madigan, 1995), was the dominant purple bac-
terium in this particular habitat and that it has never
been reported from elsewhere. One would expect a
swine waste lagoon to be high in amines and thus
that nitrogen fi xation would be unnecessary. It is thus
possible that Rcy. purpureus is somehow selected for
in otherwise suitable habitats for purple nonsulfur
bacteria that are very high in ammonia and volatile
amines. Since Rcy. purpureus is easily cultured, an
enrichment study of its distribution in nature using
media containing elevated levels of ammonia and
amines could yield insight on its ecology.
V. Purple Bacteria in Extreme
Environments
Purple bacteria have been isolated from extreme
environments, including hot, cold, acidic, alkaline,
and hypersaline (Madigan, 2003). Unfortunately, with
a few exceptions, these inherently interesting ‘ ex-

10 Michael T. Madigan and Deborah O. Jung
tremophilic’ purple bacteria have been little utilized
in the study of photosynthesis thus far. However, the
success of these unusual purple bacteria in their harsh
habitats implies that they have evolved important
solutions to photosynthesis under stress conditions.
We can therefore lear n much from studying them. For
example, molecular adaptations linked to photosyn-
thesis under extreme conditions should be relatively
easy to identify in extremophilic purple bacteria by
applying a bioinformatics/structural biology approach
to their genomes.
A. Thermophilic Purple Bacteria
The fi rst extremophilic purple bacteria were discov-
ered in the 1960s and were either halophiles or acido-
philes, including extremely halophilic species of the
genus Ectothiorhodospira. Into the late 1970s, several
new halophilic and haloalkaliphilic purple bacteria
were discovered. In the 1980s, the fi rst thermophilic
purple bacterium was isolated, Tch. tepidum. Since
then, a large diversity of alkaliphilic and halophilic
purple bacteria has been isolated. Psychrophilic
phototrophs have been described only very recently,
with two representatives currently in culture.
1. Thermochromatium tepidum
Purple bacteria were fi rst identifi ed in Yellowstone
microbial mats over 30 years ago (Castenholz, 1977).
But it was not until the 1980s that the purple sulfur
bacterium Thermochromatium (originally Chroma-
tium) tepidum was isolated in pure culture (Madigan,
1984, 1986). Tch. tepidum (Fig. 1a) is thermophilic
(optimumtemperature ~50 ºC, maximum temperature
57 ºC) and produces a novel light-harvesting (LH) 1
photopigment complex that absorbs maximally near
920 nm (Garcia et al., 1986; Nozawa et al., 1986).
The Tch. tepidum LH1 (core) antenna complex has
been studied in connection with the mechanism of
energy transfer to the reaction center. A biophysical
conundrum exists with this photocomplex in that its
absorption maximum is 50 nm to the red of that of
the reaction center. Nevertheless, because there is a
small overlap between spectra of the two components,
effi cient energy transfer occurs from the Tch. tepidum
LH1 complex to the reaction center (Kramer and
Amesz, 1996). Examples of long-wavelength-absorb-
ing core antenna complexes even more spectacular
than that of Tch. tepidum have been discovered,
indicating that in purple bacteria, LH1 complexes
that absorb very far to the red (963 nm) can still
transfer energy to the reaction center (Permentier
et al., 2001).
The photosynthetic reaction center of Tch. tepidum
is similar in most respects to that of other purple
bacteria, except for its increased thermal stability
(Nozawa and Madigan, 1991). To probe the mecha-
nism behind this, the Tch. tepidum reaction center
was crystallized. From this work, key substitutions
were identifi ed in the L and M subunits of the Tch.
tepidum reaction center that are likely responsible for
the thermostability of this photocomplex (Nogi et al.,
2000). In addition to thermal stable photocomplexes,
a thermophilic ribulose bisphosphate carboxylase (a
key enzyme of the Calvin cycle) of the green plant
type was characterized from Tch. tepidum and shown
to be stable to at least 60 ºC (Heda and Madigan,
1988. 1989).
2. Other Thermophilic Purple Bacteria
Other mildly thermophilic purple bacteria (optimum
growth temperature ~40 ºC) have been cultured
from hot spring microbial mats. These include the
BChl b-containing species Rhodopseudomonas sp.
strain GI, isolated from a New Mexico hot spring
(Resnick and Madigan, 1989), and Rps. cryptolactis
(Statwald-Demchick et al., 1990) and Rsp. centenum
(Rhodocista centenaria) (Favinger et al., 1989), both
isolated from a Thermopolis (Wyoming, USA) hot
spring. Rhodocista centenaria in particular has been
useful as a model organism for biochemical/genetic
research on phototaxis and related issues of motil-
ity (see, for example McClain et al., 2002), and its
genome has recently been sequenced (C. E. Bauer,
personal communication).
B. Halophilic and Alkaliphilic Purple Bacteria
Several extremophilic purple bacteria are halophilic
or haloalkaliphilic (Imhoff et al., 1978, 1979). These
include purple sulfur bacteria such as Ectothio-
rhodospira,Halorhodospira,Halochromatium,
Marichromatium, and Thiohalocapsa, and purple
nonsulfur bacteria such as Rhodovibrio, Rhodothalas-
sium,Rhodobium,Rhodovulum, and Roseospira.
Collectively, these purple bacteria have salt optima
that range from seawater salinities to over 20% NaCl
(Imhoff, 2001). Interestingly, the Dead Sea purple
nonsulfur bacterium Rhodovibrio sodomensis shows a
distinct intermediate level salt requirement (optimum

11
Chapter 1 Overview of Purple Bacteria
at 8–11% NaCl) (Mack et al., 1993), which is very
near that of its habitat. Some purple sulfur bacteria
can grow in saturated salt solutions, making them the
most halophilic of all known phototrophic bacteria
(Imhoff, 2001).
In the 1990s several new purple bacteria were
isolated from low salinity soda lakes. Most of these
differed dramatically from known halophilic or halo-
alkaliphilic species in that they required little if any
NaCl for growth. These isolates are, however, strongly
alkaliphilic (pH optima near 9) and phylogenetically
distinct. These include purple nonsulfur bacteria
such as Rhodobaca (Milford et al., 2000), and purple
sulfur bacteria such as Thioalkalicoccus (Bryantseva
et al., 2000), and Thiorhodospira (Bryantseva et al.,
1999). Rhodobaca (Fig. 1b) is of particular interest
because it lacks a peripheral (LH2) antenna complex,
a rarity among purple nonsulfur bacteria (Glaeser and
Overmann, 1999), and it produces several unusual
carotenoids that render phototrophic cultures yellow
in color (Takaichi et al., 2001). Rhodobaca shows
various metabolic peculiarities as well, including an
inability to fi x N2 and to grow photoautotrophically
(Milford et al., 2000); both of these properties are
hallmarks of purple nonsulfur bacteria (Madigan,
1988).
C. Acidophilic Purple Bacteria
The list of acid-loving anoxygenic phototrophs is
short, as only two genera (three species) are known.
Rhodoblastus acidophilus (formerly Rhodopseudo-
monas acidophila) is common in mildly acidic en-
vironments, such as bogs, marshes, and acidic lakes.
In the original description of this organism, several
strains were described, some containing orange/brown
carotenoids and others purple/red carotenoids. The
strains are otherwise similar, and all show a lower
limit for growth near pH 4 (Pfennig, 1969). A very
similar organism is the species Rhodoblastus sphag-
nolica (Kulichevskaya et al., 2006).
Rhodopila globiformis was isolated from acidic
warm sulfi de springs (pH 3.5–4) that fl ow out along
the Gibbon River in Yellowstone National Park (Pfen-
nig, 1974). Rhodpila globiformis-like organisms have
also been obtained from springs that feed into Nymph
Lake, a warm, acidic and sulfi dic lake adjacent to the
Gibbon River; the pH of the Nymph Lake springs is 3
(Madigan, 2003). Rhodopila globiformis has a low pH
optimum for growth similar to that of Rhodoblastus
species, but, if carefully tested, would likely be more
acid tolerant than Rhodoblastus species because of the
more strongly acidic nature of its habitat. Phylogeneti-
cally, Rhodopila and Rhodoblastus are quite distinct.
No acidophilic purple sulfur bacteria are known and
this is likely because at acid pH, sulfi de would exist
exclusively as H2S, its most toxic form.
D. Purple Bacteria from Permanently Cold
Habitats
Permanently cold environments are habitats for
cold-active purple bacteria (Burke and Burton,
1988; Madigan, 1998). The Madigan laboratory
has been studying phototrophs that inhabit lakes in
the McMurdo Dry Valleys of Antarctica. These are
closed basin lakes with a biology that is exclusively
microbial, and they remain permanently frozen with
ice covers of 4–7 m. Purple nonsulfur bacteria have
been isolated from samples of microbial mats that
develop along the edge of the lakes as well as from
water under the ice. Molecular studies using pufM
as a measure of the biodiversity of purple bacteria
suggest that species related to known purple nonsulfur
bacteria reside in these lakes (Karr et al., 2003).
The purple bacterium Rhodoferax antarcticus
inhabits both microbial mats and the water column
of Lake Fryxell (77º S latitude), a lake supporting
major sulfur cycling activities (Sattley and Madigan,
2006). Lake Fryxell is unmixed and weakly stratifi ed,
with saline bottom waters overlain by freshwater. A
gradient of sulfi de is present in Lake Fryxell from
micromolar levels at a depth of 9 m to nearly 1.5
millimolar near the sediments (~18 m). Despite these
nearly perfect conditions for the development of
phototrophic sulfur bacteria, no evidence for purple
sulfur bacteria has emerged from enrichment culture
or nucleic acid probing studies using pufM (Achen-
bach et al., 2001; Karr et al., 2003). Instead, Rfx.
antarcticus, a very sulfi de tolerant purple nonsulfur
bacterium (Jung et al., 2004), seems to dominate
anoxygenic photosynthesis in Lake Fryxell.
Although Rfx. antarcticus is not strongly psychro-
philic (optimal growth occurs at 18 ºC and no growth
occurs above 24 ºC), it is the fi rst anoxygenic photo-
troph to show a distinct cold adaptation and growth at
0 ºC (Madigan et al., 2000). Also in the water column
of Lake Fryxell, gas vesiculate purple nonsulfur
bacteria are present, including a morphologically
unique strain of Rfx. antarcticus (betaproteobacteria)
and a rod-shaped organism related to Rhodobacter
species (alphaproteobacteria). These gas vesiculate

12 Michael T. Madigan and Deborah O. Jung
purple bacteria position themselves in zones of the
water column where photosynthesis can occur opti-
mally (Karr et al. 2003; Jung et al., 2004). Biomass
measurements have shown that they localize in the
water column at a depth of 10 m and that cell numbers
drop off sharply below this depth and are undetectable
above this depth (Madigan, unpublished).
E. Environmental Limits to Photosynthesis in
Purple Bacteria
Work with extremophilic purple bacteria has given a
good indication of the limits to which photosynthesis
can be pushed. Photosynthesis in purple bacteria can
occur at temperatures up to at least 57 ºC and down
to 0 ºC, pH values as low as 3 or as high as 11, and
at salinities up to and including saturated solutions
of NaCl (~32%). Although these are probably not the
absolute physiochemical limits to photosynthesis in
anoxygenic phototrophs, they are likely to be very
close to the limits. It is notable that the green non-
sulfur bacterium Chlorofl exus aurantiacus, which
contains a purple bacterial-type photosynthetic re-
action center (Achenbach et al., 2001), can grow up
to 70 °C (Castenholz and Pierson, 1995). This holds
out hope that purple bacteria capable of growth at
temperatures above 57 °C (the upper limit for Tch.
tepidum, Madigan, 1986) may exist in nature. Fur-
ther exploration for new purple bacteria in sulfi dic
hot springs with temperatures above 60 °C should
answer this question.
VI. Final Remarks
Our understanding of purple bacteria goes far beyond
what has been discussed here. Through the years
purple bacteria have become increasingly important
as research tools for the study of basic problems in
photosynthesis and have contributed in major ways
to our understanding of the biochemistry, genetics
and evolution of photosynthesis. And impor tantly, the
beautiful colors and metabolic versatility of purple
bacteria continue to attract talented young people to
the fi eld. The reader will see the fruits of their labors
as well as those of the more seasoned investigators in
the following chapters where the biology of purple
bacteria will unfold in a spectacular way.
Acknowledgments
Research in the Madigan laboratory is supported by
the United States National Science Foundation, most
recently by grant MCB0237576. MTM thanks all of
his former students and postdoctoral colleagues who
did research on purple bacteria in his laboratory.
These include Rich Masters, Rick Stegeman, Glenn
Wright, Carletta Ooten, Erin Mack, Joseph Mayers,
Ghanshyam Heda,Vasiliki Karayiannis, Sol Resnick,
Chad Rubin, Gerrit Hoogewerf, Ike Pantazopoulous,
Amy Milford, Linda Kimble, Amy Stevenson, Terry
Locke, Tiffany Full, Jen Carey, Elizabeth Karr, Jill
Crespi, Mahmoud Tayah, Tom Wahlund, Matt Sattley,
and Marie Asao.
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