[Show abstract][Hide abstract] ABSTRACT: Here, we present sedimentological, trace metal, and molecular evidence
for tracking bottom water redox-state conditions during the past 12,500
years in nowadays sulfidic and meromictic Lake Cadagno (Switzerland). A
10.5 m long sediment core from the lake covering the Holocene period was
investigated for concentration variations of the trace metals Mn and Mo
(XRF core scanning and ICP-MS measurements), and for the presence of
anoxygenic phototrophic sulfur bacteria (carotenoid pigment analysis and
16S rDNA real time PCR). Our trace metal analysis documents an
oxic-intermediate-sulfidic redox-transition period beginning shortly
after the lake formation ˜12.5 kyr ago. The oxic period is
characterized by low sedimentary Mn and Mo concentrations, as well as by
the absence of any remnants of anoxygenic phototrophic sulfur bacteria.
Enhanced accumulation/preservation of Mn (up to 5.6 wt%) in the
sediments indicates an intermediate, Mn-enriched oxygenation state with
fluctuating redox conditions during a ˜2300-year long transition
interval between ˜12.1 and 9.8 kyr BP. We propose that the high Mn
concentrations are the result of enhanced Mn2+ leaching from
the sediments during reducing conditions and subsequent rapid
precipitation of Mn-(oxyhydr)oxide minerals during episodic and
short-term water-column mixing events mainly due to flood-induced
underflows. At 9800 ± 130 cal yr BP, a rapid transition to fully
sulfidic conditions is indicated by the marked enrichment of Mo in the
sediments (up to 490 ppm), accompanied by an abrupt drop in Mn
concentrations and the increase of molecular biomarkers that indicate
the presence of anoxygenic photosynthetic bacteria in the water column.
Persistently high Mo concentrations >80 ppm provide evidence that
sulfidic conditions prevailed thereafter until modern times, without any
lasting hypolimnetic ventilation and reoxygenation. Hence, Lake Cadagno
with its persistently stable chemocline offers a framework to study in
great temporal detail over ˜12 kyr the development of phototrophic
sulfur bacteria communities and redox processes in a sulfidic
environment, possibly depicting analogous conditions in an ancient
ocean. Our study underscores the value of combining sedimentological,
geochemical, and microbiological approaches to characterize
paleo-environmental and -redox conditions in lacustrine and marine
Full-text · Article · Jun 2013 · Geochimica et Cosmochimica Acta
[Show abstract][Hide abstract] ABSTRACT: Lake Cadagno and the permanently stratified lakes, in general, represent optimal models for studying aquatic microbial populations. The lake Cadagno is particularly interesting for the presence of phototrophic sulfur bacteria and sulfate reducing bacteria populations. In recent years we have seen that the phototrophic sulfur bacteria candidatus Thiodictyon syntropicum Cad16 and the sulfate reducing bacteria Desulfocapsa thiozymogenes Cad626 are able to associate into specie-specific structures called aggregates. Until now a detailed study on the aggregate has been hampered by the lack of techniques that allowed to reproduce specific aggregation under controlled environmental and laboratory conditions. With this work, for the first time, we developed methods that would allow generating an experimental model for aggregation as starting point to determine the ecological and metabolic reasons and mechanisms at the basis of aggregation.
[Show abstract][Hide abstract] ABSTRACT: Lake Cadagno is a meromictic high-alpine lake situated in the Piora
valley in the southern Alps of Switzerland (1921 m asl, 0.26 km2, max.
water depth 21 m). The inflow of sulfate-rich waters from subaquatic
springs situated in dolomitic bedrock results in a permanent chemocline
at 10 to 13 m water depth with sulfidic conditions in the hypolimnion.
As yet, it is unknown how far back in time euxinia has prevailed in the
Lake Cadagno water column, maintaining a diverse anaerobic community of
bacteria at and below the redox-transition zone. Long sediment cores
(10.5 m) covering the past ~12,000 years were, for the first time,
retrieved from the centre of Lake Cadagno using an UWITEC piston coring
system. Here, we present results documenting the lake's redox-state
evolution through the Holocene based on sedimentological, geochemical
and molecular analyses. Sediment stratigraphy reveals an autochthonous
lacustrine background sedimentation frequently intercalated by flood
(accounting for 30% of the sediment succession) and mass-movement
deposits (49%) that are the result of lake-slope failures. Trace metal
analysis by XRF core scanning and ICP-MS measurements document the
transition from oxic conditions after the lake formation (~12,000 cal yr
BP) to the onset of sulfidic conditions at ~9800 cal yr BP. Enhanced
accumulation/preservation of Mn (up to 5.9 wt% of Mn) in the sediments
indicates an intermediate (i.e. manganous) oxygenation state with
fluctuating redox conditions during a millennial transition period. We
propose that the high Mn concentrations are the result of Mn2+ leaching
from the sediments during reducing conditions and subsequent rapid
precipitation of Mn-oxide minerals during episodic and short-term mixing
events. Sulfidic conditions, which are indicated in the sediment record
by high Mo burial rates (Mo concentrations of up to 470 ppm), prevailed
thereafter until modern times without any lasting hypolimnetic
oxygenation. We speculate that the onset of euxinia in Lake Cadagno
corresponds to the actuation of subaquatic springs originating from
dolomitic bedrock after the last glacial termination. This result of a
stable and persistent chemocline offers a framework to study over ~10
kyr the evolution of anaerobic bacterial communities in an extreme
lacustrine environment that possibly resemble conditions more common in
[Show abstract][Hide abstract] ABSTRACT: Lake Cadagno is a crenogenic meromictic lake situated in the southern range of the Swiss Alps characterized by a compact chemocline that has been the object of many ecological studies. The population dynamics of phototrophic sulfur bacteria in the chemocline has been monitored since 1994 with molecular methods such as 16S rRNA gene clone library analysis. To reconstruct paleo-microbial community dynamics, we developed a quantitative real-time PCR methodology for specific detection of 16S rRNA gene sequences of purple and green sulfur bacteria populations from sediment samples. We detected fossil 16S rDNA of nine populations of phototrophic sulfur bacteria down to 9-m sediment depth, corresponding to about 9500 years of the lake's biogeological history. These results provide the first evidence for the presence of 16S rDNA of anoxygenic phototrophic bacteria in Holocene sediments of an alpine meromictic lake and indicate that the water column stratification and the bacterial plume were already present in Lake Cadagno thousands of years ago. The finding of Chlorobium clathratiforme remains in all the samples analyzed shows that this population, identified in the water column only in 2001, was already a part of the lake's biota in the past.
[Show abstract][Hide abstract] ABSTRACT: Strain Cad16(T) is a small-celled purple sulfur bacterium (PSB) isolated from the chemocline of crenogenic meromictic Lake Cadagno, Switzerland. Long term in situ observations showed that Cad16(T) regularly grows in very compact clumps of cells in association with bacteria belonging to the genus Desulfocapsa in a cell-to-cell three dimensional structure. Previously assigned to the genus Lamprocystis, Cad16(T), was here reclassified and assigned to the genus Thiodictyon. Based on comparative 16S rRNA gene sequences analysis, isolate Cad16(T) was closely related to Thiodictyon bacillosum DSM234(T) and Thiodictyon elegans DSM232(T) with sequence similarities of 99.2% and 98.9%, respectively. Moreover, matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) analysis separated Cad16(T) from other PSB genera, Lamprocystis and Thiocystis. Major differences in cell morphology (oval-sphere compared to rod-shaped) and arrangement (no netlike cell aggregates), carotenoid group (presence of okenone instead of rhodopinal), chemolithotrophic growth as well as the ability to form syntrophic associations with a sulfate-reducing bacteria of the genus Desulfocapsa suggested a different species within the genus Thiodictyon. This isolate is therefore proposed and described as Candidatus "Thiodictyon syntrophicum" sp. nov., a provisionally novel species within the genus Thiodictyon.
No preview · Article · Mar 2012 · Systematic and Applied Microbiology
[Show abstract][Hide abstract] ABSTRACT: Two isolates, designated CadH11(T) and Cad448(T), representing uncultured purple sulfur bacterial populations H and 448, respectively, in the chemocline of Lake Cadagno, a crenogenic meromictic lake in Switzerland, were obtained using enrichment and isolation conditions that resembled those used for cultured members of the genus Thiocystis. Phenotypic, genotypic and phylogenetic analyses of these isolates confirmed their assignment to the genus Thiocystis. However, 16S rRNA gene sequence similarities of 98.2 % between CadH11(T) and Cad448(T), and similarities of 97.7 and 98.5 %, respectively, with their closest cultured relative Thiocystis gelatinosa DSM 215(T), as well as differences in DNA G+C content and carbon source utilization suggested that the isolates belonged to two distinct species. DNA-DNA hybridization of CadH11(T) and Cad448(T) with T. gelatinosa DSM 215(T) showed relatedness values of 46.4 and 60.8 %, respectively; the relatedness value between CadH11(T) and Cad448(T) was 59.2 %. Based on this evidence, strains CadH11(T) and Cad448(T) represent two novel species within the genus Thiocystis, for which the names Thiocystis chemoclinalis sp. nov. and Thiocystis cadagnonensis sp. nov. are proposed, respectively. The type strains of T. chemoclinalis sp. nov. and T. cadagnonensis sp. nov. are CadH11(T) ( = JCM 15112(T) = KCTC 5954(T)) and Cad448(T) ( = JCM 15111(T) = KCTC 15001(T)), respectively.
Full-text · Article · Jul 2011 · International Journal of Systematic and Evolutionary Microbiology
[Show abstract][Hide abstract] ABSTRACT: Primary production in the meromictic Lake Cadagno, Switzerland, is dominated by anoxygenic photosynthesis. The green sulfur bacterium Chlorobium clathratiforme is the dominant phototrophic organism in the lake, comprising more than half of the bacterial population, and its biomass increases 3.8-fold over the summer. Cells from four positions in the water column were used for comparative analysis of the Chl. clathratiforme proteome in order to investigate changes in protein composition in response to the chemical and physical gradient in their environment, with special focus on how the bacteria survive in the dark. Although metagenomic data are not available for Lake Cadagno, proteome analysis was possible based on the completely sequenced genome of an isolated strain of Chl. clathratiforme. Using LC-MS/MS we identified 1321 Chl. clathratiforme proteins in Lake Cadagno and quantitatively compared 621 of these in the four samples. Our results showed that compared with cells obtained from the photic zone, cells collected from the dark part of the water column had the same expression level of key enzymes involved in carbon metabolism and photosynthetic light harvesting. However, most proteins participating in nitrogen and sulfur metabolism were twofold less abundant in the dark. From the proteome analysis we were able to show that Chl. clathratiforme in the photic zone contains enzymes for fixation of N(2) and the complete oxidation of sulfide to sulfate while these processes are probably not active in the dark. Instead we propose that Chl. clathratiforme cells in the dark part of the water column obtain energy for maintenance from the fermentation of polyglucose. Based on the observed protein compositions we have constructed possible pathways for C, N and S metabolism in Chl. clathratiforme.
No preview · Article · Jan 2011 · Environmental Microbiology
[Show abstract][Hide abstract] ABSTRACT: For many years, the chemocline of the meromictic Lake Cadagno, Switzerland, was dominated by purple sulfur bacteria. However, following a major community shift in recent years, green sulfur bacteria (GSB) have come to dominate. We investigated this community by performing microbial diversity surveys using FISH cell counting and population multilocus sequence typing [clone library sequence analysis of the small subunit (SSU) rRNA locus and two loci involved in photosynthesis in GSB: fmoA and csmCA]. All bacterial populations clearly stratified according to water column chemistry. The GSB population peaked in the chemocline (c. 8 x 10(6) GSB cells mL(-1)) and constituted about 50% of all cells in the anoxic zones of the water column. At least 99.5% of these GSB cells had SSU rRNA, fmoA, and csmCA sequences essentially identical to that of the previously isolated and genome-sequenced GSB Chlorobium clathratiforme strain BU-1 (DSM 5477). This ribotype was not detected in Lake Cadagno before the bloom of GSB. These observations suggest that the C. clathratiforme population that has stabilized in Lake Cadagno is clonal. We speculate that such a clonal bloom could be caused by environmental disturbance, mutational adaptation, or invasion.
Full-text · Article · Aug 2009 · FEMS Microbiology Ecology
[Show abstract][Hide abstract] ABSTRACT: The nitrogen cycling of Lake Cadagno was investigated by using a combination of biogeochemical and molecular ecological techniques. In the upper oxic freshwater zone inorganic nitrogen concentrations were low (up to approximately 3.4 microM nitrate at the base of the oxic zone), while in the lower anoxic zone there were high concentrations of ammonium (up to 40 microM). Between these zones, a narrow zone was characterized by no measurable inorganic nitrogen, but high microbial biomass (up to 4 x 10(7) cells ml(-1)). Incubation experiments with (15)N-nitrite revealed nitrogen loss occurring in the chemocline through denitrification (approximately 3 nM N h(-1)). At the same depth, incubations experiments with (15)N(2)- and (13)C(DIC)-labelled bicarbonate, indicated substantial N(2) fixation (31.7-42.1 pM h(-1)) and inorganic carbon assimilation (40-85 nM h(-1)). Catalysed reporter deposition fluorescence in situ hybridization (CARD-FISH) and sequencing of 16S rRNA genes showed that the microbial community at the chemocline was dominated by the phototrophic green sulfur bacterium Chlorobium clathratiforme. Phylogenetic analyses of the nifH genes expressed as mRNA revealed a high diversity of N(2) fixers, with the highest expression levels right at the chemocline. The majority of N(2) fixers were related to Chlorobium tepidum/C. phaeobacteroides. By using Halogen In Situ Hybridization-Secondary Ion Mass Spectroscopy (HISH-SIMS), we could for the first time directly link Chlorobium to N(2) fixation in the environment. Moreover, our results show that N(2) fixation could partly compensate for the N loss and that both processes occur at the same locale at the same time as suggested for the ancient Ocean.
Full-text · Article · May 2009 · Environmental Microbiology
[Show abstract][Hide abstract] ABSTRACT: Phototrophic sulfur bacteria in the chemocline of Lake Cadagno, Switzerland, were dominated by purple sulfur bacteria before 1998, but their composition shifted after a presumed disturbance to consist of mainly green sulfur bacteria. This study focused on comparative analyses of the distribution of the green sulfur compared to the purple sulfur bacteria by analyzing specific populations along fine scale depth profiles of the chemocline of Lake Cadagno. Water samples were collected from the chemocline on October 14 th , 1998, and on September 28 th , 2004. A detailed analysis of chemocline depth profiles revealed that total biomass of phototrophic sulfur bacteria was three times higher in 2004 than in 1998. The three-fold increase of biomass was entirely due to increments in abundance of one population of green sulfur bacteria, identified as Chlorobium clathratiforme. Abundance of purple sulfur bacteria remained unchanged with respect to overall numbers in the chemocline, but also with respect to distribution of different populations in depth in the chemocline. Aggregates of small-celled purple sulfur bacteria decreased in size about four-fold, but remained associated with sulfate-reducing bacteria of the genus Desulfocapsa. Compared to 1998, these had increased in numbers about three times in 2004, resulting in about ten times higher numbers per aggregate. These results demonstrate long-term effects of a presumed disturbance in autumn 1999 and 2000, on environmental conditions and on green sulfur and sulfate-reducing bacteria in the chemocline, however, without changes in the abundance and in the distribution of purple sulfur bacteria.
[Show abstract][Hide abstract] ABSTRACT: Quantitative information on the ecophysiology of individual microorganisms is generally limited because it is difficult to assign specific metabolic activities to identified single cells. Here, we develop and apply a method, Halogen In Situ Hybridization-Secondary Ion Mass Spectroscopy (HISH-SIMS), and show that it allows simultaneous phylogenetic identification and quantitation of metabolic activities of single microbial cells in the environment. Using HISH-SIMS, individual cells of the anaerobic, phototropic bacteria Chromatium okenii, Lamprocystis purpurea, and Chlorobium clathratiforme inhabiting the oligotrophic, meromictic Lake Cadagno were analyzed with respect to H(13)CO(3)(-) and (15)NH(4)(+) assimilation. Metabolic rates were found to vary greatly between individual cells of the same species, showing that microbial populations in the environment are heterogeneous, being comprised of physiologically distinct individuals. Furthermore, C. okenii, the least abundant species representing approximately 0.3% of the total cell number, contributed more than 40% of the total uptake of ammonium and 70% of the total uptake of carbon in the system, thereby emphasizing that numerically inconspicuous microbes can play a significant role in the nitrogen and carbon cycles in the environment. By introducing this quantification method for the ecophysiological roles of individual cells, our study opens a variety of possibilities of research in environmental microbiology, especially by increasing the ability to examine the ecophysiological roles of individual cells, including those of less abundant and less active microbes, and by the capacity to track not only nitrogen and carbon but also phosphorus, sulfur, and other biological element flows within microbial communities.
Full-text · Article · Dec 2008 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: The idea and concepts of «extra-muros» University lead to the establishment in the Cantone Ticino of the Laboratoire d'écologie microbienne (LEM), the Microbial ecology group of the University of Geneva. The LEM is located and based in the facilities of Cantonal Institute of Microbiology. Thanks to the impulsion of our University in a region without an academic tradition we were able to develop a scientific research line graduate and undergraduate education and training in clinical and environmental microbiology. Our scientific activities focused on bacterial genera naturally developing in freshwater habitats which can contaminate human beings under given circumstances and thus are of importance in clinical microbiology, and on microbial genera which are major key players in biogeochemical cycles in natural and human impacted freshwaters habitats. The present contribution collect the cornerstones of our scientific work by describing the researched bacterial genera together with the evolution of more and more powerful analytical tools over more than 30 years of activity. New technological advances in microbiology and molecular biology permit the detection and tracking of emerging opportunistic pathogens such as, Aeromonas, Yersinia and Legionella in the environment and help to resolve their path from the environment to human contamination and ultimately disease. A permanently stratified freshwater ecosystem, the meromictic lake Cadagno located nearby the Alpine Biology Centre, was taken as a model for the study of biogeochemical cycles in freshwater habitats. Major interests focused on the biological filter developing in the chemocline, retaining toxic compounds such as sulfide which is mainly driven by the anaerobic key genera such as Chromatium and Lamprocystis. Moreover, the symbiotic collaboration between Geneva and Ticino promoted and allowed the accomplishment of several PhD research thesis, the foundation of the Alpine Biology Centre in Piora as well as the maintenance of an academic site in the new building of the Cantonal Institute of Microbiology in Bellinzona.
[Show abstract][Hide abstract] ABSTRACT: Lake Cadagno, a crenogenic meromictic lake located in the catchment area of a dolomite vein rich in gypsum in the Piora Valley in the southern Alps of Switzerland, is characterized by a compact chemocline with high concentrations of sulfate, steep gradients of oxygen, sulfide and light and a turbidity maximum that correlates to large numbers of bacteria (up to 10 7 cells ml -1). The most abun-dant taxa in the chemocline are large-and small-celled purple sulfur bacteria, which account for up to 35% of all bacteria, and sul-fate-reducing bacteria that represent up to 23% of all bacteria. Depending on the season, as much as 45% of all bacteria in the chemocline are associated in aggregates consisting of different populations of small-celled purple sulfur bacteria of the genus Lam-procystis (up to 35% of all bacteria) and sulfate-reducing bacteria of the family Desulfobulbaceae (up to 12% of all bacteria) that are almost completely represented by bacteria closely related to Desulfocapsa thiozymogenes. Their association in aggregates is re-stricted to small-celled purple sulfur bacteria of the genus Lamprocystis, but not obligate since non-associated cells of bacteria re-lated to D. thiozymogenes are frequently found, especially under limited light conditions in winter and early summer. Aggregate formation and concomitant growth enhancement of isolates of both partners of this association suggests synergistic interactions that might resemble a sulfide-based source-sink relationship between the sulfate-reducing bacterium that is able to sustain growth by a disproportionation of inorganic sulfur compounds (sulfur, thiosulfate, sulfite), with the purple sulfur bacteria acting as a biotic scav-enger. The availability of these isolates opens up the door for future studies considering other facets of potential interactions in ag-gregates since both types of organisms are metabolically highly versatile and interactions may not be limited to sulfur compounds only.
[Show abstract][Hide abstract] ABSTRACT: Abstract In situ hybridization with specific oligonucleotide probes was used to monitor enrichment cultures of yet uncultured populations of sulfate-reducing and small-celled purple sulfur bacteria found to associate into aggregates in the chemocline of meromictic Lake Cadagno, Switzerland, and to select potential isolates. Enrichment and isolation conditions resembled those of their nearest cultured relatives, the sulfate-reducing bacterium Desulfocapsa thiozymogenes and small-celled purple sulfur bacteria belonging to the genus Lamprocystis, respectively. Based on comparative 16S rRNA analysis and physiological characterization, isolate Cad626 was found to resemble D. thiozymogenes although it differed from the type strain by its ability to grow on lactate and pyruvate. Like D. thiozymogenes, isolate Cad626 was able to disproportionate inorganic sulfur compounds (sulfur, thiosulfate, sulfite) and to grow, although growth on sulfur required a sulfide scavenger (FeOOH). Isolate Cad16 represented small-celled purple sulfur bacteria that belonged to a previously detected, but uncultured population designated F and was related to Lamprocystis purpurea as evidenced by comparative 16S rRNA analysis and the presence of bacteriochlorophyll a and the carotenoid okenone. Mixed cultures of isolates Cad626 and Cad16 resulted in their association in aggregates similar to those observed in the chemocline of Lake Cadagno. Concomitant growth enhancement of both isolates in mixed culture suggested synergistic interactions that presumably resemble a source-sink relationship for sulfide between the sulfate-reducing bacterium growing by sulfur disproportionation and the purple sulfur bacteria acting as biotic scavenger.
Full-text · Article · Aug 2003 · FEMS Microbiology Ecology
[Show abstract][Hide abstract] ABSTRACT: Abstract In situ hybridization was used to study the spatio-temporal distribution of phototrophic sulfur bacteria in the permanent chemocline of meromictic Lake Cadagno, Switzerland. At all four sampling times during the year the numerically most important phototrophic sulfur bacteria in the chemocline were small-celled purple sulfur bacteria of two yet uncultured populations designated D and F. Other small-celled purple sulfur bacteria (Amoebobacter purpureus and Lamprocystis roseopersicina) were found in numbers about one order of magnitude lower. These numbers were similar to those of large-celled purple sulfur bacteria (Chromatium okenii) and green sulfur bacteria that almost entirely consisted of Chlorobium phaeobacteroides. In March and June when low light intensities reached the chemocline, cell densities of all populations, with the exception of L. roseopersicina, were about one order of magnitude lower than in August and October when light intensities were much higher. Most populations were evenly distributed throughout the whole chemocline during March and June, while in August and October a microstratification of populations was detected suggesting specific eco-physiological adaptations of different populations of phototrophic sulfur bacteria to the steep physico-chemical gradients in the chemocline of Lake Cadagno.
Full-text · Article · Mar 2003 · FEMS Microbiology Ecology
[Show abstract][Hide abstract] ABSTRACT: Sulfate-reducing bacteria were analyzed in the chemocline of meromictic Lake Cadagno, Switzerland, in March, June, August and October using in situ hybridization. Numbers of sulfate-reducing bacteria determined as the sum of cells hybridizing to Probes SRB385Db targeting Desulfobacteriaceae and SRB385 targeting Desulfovibrionaceae were similar at all samplings and accounted for 13 to 18% of the total microbial community, respectively. Abundance of cells detected with either probe, however, changed during the year, with cell numbers detected with SRB385Db being larger in early summer (June) and those detected with SRB385 being larger in late summer (October). Increasing cell numbers detected with SRB385Db were mainly caused by a yet uncultured and phylogenetically unidentified bacterium with a peculiar morphology ('Morphotype R') that followed the sulfide profile in June as well as in August with increasing numbers at increasing concentrations with depth. From the fraction of cells detected with SRB385, only a minor part represented a yet uncultured population without close cultured relatives. At all samplings, the majority of cells detected with SRB385 (93 to 99%) represented populations phylogenetically related to Desulfocapsa thiozymogenes DSM7269. Their cells were generally found in association with aggregates of small-celled phototrophic sulfur bacteria. This association was not specific for 1 of the 4 populations representing all small-celled phototrophic sulfur bacteria in Lake Cadagno. The association was also not obligate since non-associated cells were frequently found, especially in winter and early summer when limited light conditions caused by snow and ice cover reduced the abundance of small-celled phototrophic sulfur bacteria to about 50% of the values found in late summer. Nonetheless, the association between populations related to D. thiozymogenes and small-celled phototrophic sulfur bacteria suggests an ecological advantage to both organisms under appropriate environmental conditions.
No preview · Article · Jan 2003 · Aquatic Microbial Ecology