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RETICULATED FILAMENTS IN CAVE POOL
SPELEOTHEMS: MICROBE OR MINERAL?
LESLIE A. MELIM
1
,DIANA E. NORTHUP
2
,MICHAEL N. SPILDE
3
,BRIAN JONES
4
,PENELOPE J. BOSTON
5
,AND
REBECCA J. BIXBY
2
Abstract: We report on a reticulated filament found in modern and fossil cave samples
that cannot be correlated to any known microorganism or organism part. These
filaments were found in moist environments in five limestone caves (four in New Mexico,
U.S.A., one in Tabasco, Mexico), and a basalt lava tube in the Cape Verde Islands. Most
of the filaments are fossils revealed by etching into calcitic speleothems but two are on
the surface of samples. One hundred eighty individual reticulated filaments were imaged
from 16 different samples using scanning electron microscopy. The filaments are up to
75 mm (average 12 mm) long, but all filaments appear broken. These reticulated filaments
are elongate, commonly hollow, tubes with an open mesh reminiscent of a fish net or
honeycomb. Two different cross-hatched patterns occur; 77% of filaments have
hexagonal chambers aligned parallel to the filament and 23% of filaments have
diamond-shaped chambers that spiral along the filament. The filaments range from
300 nm to 1000 nm in diameter, but there are two somewhat overlapping populations;
one 200–400 nm in size and the other 500–700 nm. Individual chambers range from 40 to
100 nm with 30–40 nm thick walls. Similar morphologies to the cave reticulated
filaments do exist in the microbial world, but all can be ruled out due to the absence of
silica (diatoms), different size (diatoms, S-layers), or the presence of iron (Leptothrix sp.).
Given the wide range of locations that contain reticulated filaments, we speculate that
they are a significant cave microorganism albeit with unknown living habits.
INTRODUCTION
Microbes are well known from cave systems (Danielli
and Edington, 1983; Northup and Lavoie, 2001; Barton et
al., 2001; Barton and Northup, 2007). Scanning electron
microscopy of the surface of speleothems commonly
reveals a variety of spheroid or filamentous features,
interpreted as either microorganisms or biofilms (mucous/
EPS) (Jones and Motyka, 1987; Jones and Kahle, 1986;
Jones, 2001; Vlasceanu et al., 2000; Baskar et al., 2006).
Etching calcitic samples with weak acid sometimes reveals
fossil microorganisms and/or biofilms that were entombed
in the calcite (Melim et al., 2001; Boston et al., 2001). We
report herein on an unusual reticulated filament found in
modern and fossil cave samples that cannot be correlated
to any known microorganism or organism part. Since all of
our samples are from caves, we cannot comment on their
possible wider distribution. We document here the
morphology, distribution and context of these reticulated
filaments.
FIELD LOCATIONS
The samples for this study (Table 1) came from caves in
the Guadalupe Mountains of southeastern New Mexico,
from a lava tube in the Cape Verde Islands, and a cave in
Tabasco, Mexico (Fig. 1). All locations are from the
aphotic zone of the cave; all but one are from speleothems
that formed underwater (pool fingers) or in wet areas (cave
pearls).
The Guadalupe Mountains include over 300 known
caves (DuChene and Martinez, 2000) administered vari-
ously by Carlsbad Caverns National Park, the National
Forest Service, and the Bureau of Land Management.
Hidden Cave and Cottonwood Cave are located in the
Guadalupe Ranger District of the Lincoln National Forest
in southeastern New Mexico (Fig. 2). The samples from
Hidden Cave are giant pool fingers (Fig. 3a), pendant
features that form underwater (Davis et al., 1990; Melim et
al., 2001). In Cottonwood Cave, the samples are thin pool
fingers with abundant u-loops; curved connections between
fingers (Fig. 3b; Davis et al., 1990). Both areas are
currently dry but the features formed when the pools were
full (unpublished data; Hill, 1987 Melim et al., 2001).
Endless Cave is located on Bureau of Land Management
property in the McKittrick Hill area of the Guadalupe
Mountains (Fig. 2). The sample is a warclub (Hill and
Forti, 1997) from the Warclub Room, a currently dry
room. A warclub forms when the end of a stalactite is
1
Dept. of Geology, Western Illinois University, Macomb, IL 61455, USA
2
Biology Dept., University of New Mexico, Albuquerque, NM 87131, USA
3
Institute of Meteoritics, University of New Mexico, Albuquerque, NM 87131, USA
4
Dept. of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB,
T6G 2E3, Canada
5
Dept. of Earth and Environmental Science, New Mexico Institute of Mining and
Technology, Socorro, NM 87801, USA
L.A. Melim, D.E. Northup, M.N. Spilde, B. Jones, P.J. Boston, and R.J. Bixby – Reticulated filaments in cave pool speleothems:
microbe or mineral? Journal of Cave and Karst Studies, v. 70, no. 3, p. 135–141.
Journal of Cave and Karst Studies, December 2008 N135
submerged by a rising pool level and is then coated in pool
spar. Our sample is a surface piece of this pool spar. Two
more samples came from Carlsbad Cavern in Carlsbad
Caverns National Park (Fig. 2). One sample is a thin pool
finger from an active pool, whereas the other is webulite
from a dry pool in Lower Cave, a portion of the cave
generally closed to the public. Webulite is a thin draping
calcite web that connects adjacent pool fingers (Davis et
al., 1990, Queen and Melim, 2006).
The Cape Verde sample is from a small, unnamed lava
tube on Fogo Island. The sample came from a purple and
white crust on the wall of the basalt lava tube (Peter Roe,
personal communication). The Mexican sample is a cave
pearl collected from Cueva de Las Canicas, a cave in
Tabasco famous for containing millions of cave pearls
(Pisarowicz and Snow, 2003).
METHODS
All samples were collected under permit from the
appropriate agency (U.S. National Park Service, U.S.
Forest Service, and Bureau of Land Management) or
landowner permission (Sr. Arturo Cano, Cueva de Las
Can˜icas) and were selected from those broken by past
visitors to avoid disturbance of intact speleothems. Thin
sections were cut from all samples from Hidden Cave,
Cottonwood Cave, and Carlsbad Cavern and examined for
possible microbial fabrics in a standard petrographic
microscope. The cave pearl was cut in half, ground flat,
and then etched. For SEM analysis, samples were either
etched with 5% hydrochloric acid for 10–15 seconds before
rinsing in distilled water, drying and mounting on SEM
stubs (Hidden Cave, Cottonwood Cave, Carlsbad Cavern
Table 1. List of locations containing reticulated filaments.
Cave Location Speleothem Collector
Number of
Imaged Filaments
Hidden Cave Guadalupe Mountains, New
Mexico, USA
Pool fingers Authors 92
Cottonwood Cave Guadalupe Mountains, New
Mexico, USA
Pool fingers and U-loops Authors 65
Endless Cave Guadalupe Mountains, New
Mexico, USA
Warclub Authors 9
Carlsbad Cavern Guadalupe Mountains, New
Mexico, USA
Webulite Authors 5
Cueva de Las Canicas Tabasco, Mexico Cave pearls Arturo C. Conde 2
Fogo Island, Basalt
Lava Tube
Cape Verde Islands Coating on basalt lava
flow
Peter Roe 11
Figure 1. World map of locations where reticulated filaments have been found by this study: 1. Guadalupe Mountains, New
Mexico, U.S., 2. Tabasco, Mexico, 3. Cape Verde Islands. Also shown are locations for two examples found by other workers:
4. Poland, Gradzinski (2003); 5. Frasassi Caves, Italy, Macalady (pers. comm.).
RETICULATED FILAMENTS IN CAVE POOL SPELEOTHEMS:MICROBE OR MINERAL
136 NJournal of Cave and Karst Studies, December 2008
and Cueva de Las Canicas) or mounted without etching
(Cape Verde lava tube and Endless Cave).
All samples were examined with either a JEOL 5800LV
scanning electron microscope at the University of New
Mexico (UNM) or a JSM 6301FXV field emission
scanning electron microscope at the University of Alberta.
The UNM samples were coated with approximately 200
angstroms of gold-palladium alloy in an evaporative
coater. The thin film of gold-palladium provides a
conductive layer that is relatively free of artifacts and
allows light element x-rays of carbon to pass with only
moderate attenuation. The University of Alberta samples
were coated in gold alone, which resulted in minor artifacts
at higher magnification. The UNM SEM is equipped with
an Oxford Isis 300 Energy Dispersive X-ray (EDX)
analyzer. This modern EDX system utilizes a thin
polymer-film window, which allows the analysis of low-
energy x-rays of light elements such as boron and carbon
(Z .5). The SEM was operated at 15 kV accelerating
voltage, the beam current was 10 picoamps as measured in
a Faraday cup. This gives a beam diameter of less than
50 nm. However, the beam samples a greater volume. With
these operating conditions, the EDX provides a qualitative
estimate of elements present in the upper 2–3 mmofa
calcitic sample.
Three samples were analyzed for total organic carbon
using a Carlo Erba elemental analyzer from samples
dissolved in 6 N hydrochloric acid to remove carbonate.
OCCURRENCE OF RETICULATED FILAMENTS
Smooth, commonly hollow, filaments are locally
abundant in our cave samples and have been interpreted
as fossil microbes (Melim et al., 2001; Boston et al., 2001).
We have also found a less common reticulated filament
that is similar in size to the smooth filaments but has a
reticulated form (Fig. 4). Over 180 individual examples of
these unusual reticulated filaments were found in 16
different cave samples (out of 22 samples examined in this
study). Most (87%) of these filaments are in pool fingers
from Hidden and Cottonwood Caves (Figs. 2 and 3), but
this at least partly reflects the extended time we have spent
with these samples. In addition, Gradzinski (2000, 2003)
imaged an example from Polish cave pearls and J.
Macalady has found them in the Frasassi caves in Italy
(Macalady, personal communication).
Filaments are preferentially found in dense micritic
calcite, but can also occur in clear spar (Melim et al., 2001).
All but two of our samples come from cave pool biothems,
Figure 2. Locations of major caves in the Guadalupe
Mountains. We have found reticulated filaments in samples
from Hidden Cave, Carlsbad Cavern, Cottonwood Cave, and
Endless Cave. (Map, after Palmer and Palmer, 2000.)
Figure 3. (a) Giant pool fingers in Hidden Cave, NM. (b) Small pool fingers coating an earlier stalactite in Cottonwood
Cave, NM (Spanish moss of Hill, 1987).
L.A. MELIM, D.E. NORTHUP, M.N. SPILDE,B.JONES, P.J. BOSTON,AND R.J. BIXBY
Journal of Cave and Karst Studies, December 2008 N137
features with external morphology suggestive of microbial
involvement (Cunningham et al., 1995; Queen and Melim,
2006). This, however, could be an artifact of our study, as
we have not closely examined many other cave pool
precipitates, other speleothems or similar features in non-
cave environments. Other speleothems or other environ-
ments may contain similar filaments, but etching of
samples and meticulous, very high magnification SEM
work is required to find them.
Reticulated filaments have an open cross-hatched
pattern reminiscent of a fish net or a honeycomb (Fig. 4).
Preservation of filaments varies substantially between
individuals. The best preserved are hollow tubes that are
partly collapsed (Fig. 4a). Others are torn open (Fig. 4b) or
completely filled with calcite (Fig. 4c). Recently living
filaments are hollow tubes (Fig. 4d). The filaments are up
to 75 mm (average 12 mm) long. The measured length,
however, is largely an artifact of preservation as virtually
all of the specimens are torn, broken or have their ends
buried in the matrix. Filament diameter is 300–1000 nm
(average 590 nm) but this overall range encompasses two
overlapping populations; one with an average diameter of
200–400 nm and the other with an average diameter of
500–700 nm. Both populations are found in the same
samples (Fig. 5).
The filaments are characterized by two different styles
of cross-hatched patterns, both with individual chambers
40 to 100 nm long and walls between chambers that are 30–
40 nm wide. The larger diameter filaments typically have
larger chambers, but not always. The more common form
(77% of imaged filaments) has approximately hexagonal
chambers that align in rows parallel to the length of the
filament (Figs. 4b, 5). The less common form (23%) has
more diamond-shaped chambers that spiral along the
filament (Fig. 4c). The cross-hatch pattern does not
correlate to filament size.
COMPOSITION OF FILAMENTS
In etched samples, the reticulated filaments occur within
the sample and are partially revealed by the removal of the
surrounding calcite (Fig. 4). Preliminary data show 1–2%
organic carbon in the sample containing the most filaments
(from Cottonwood Cave). EDX analysis of individual
Figure 4. Scanning electron micrographs of reticulated filaments. (a) Hollow reticulated filament with diamond-shaped
chambers that form a spiral. Hidden Cave pool finger, etched sample. (b) Reticulated filament that is hollow and torn open,
with chambers that align along length of filament. Hidden Cave pool finger, etched sample. (c) Reticulated filament that is solid
with diamond-shaped chambers that spiral. Hidden Cave pool finger, etched sample. (d) Reticulated filament from recently
living sample (not etched). Note similarity to etched samples, particularly (a). Cape Verde lava tube.
RETICULATED FILAMENTS IN CAVE POOL SPELEOTHEMS:MICROBE OR MINERAL
138 NJournal of Cave and Karst Studies, December 2008
filaments commonly shows calcium, oxygen, and more
carbon than the surrounding calcite (Fig. 6). Since the
depth of analysis (2–3 mm) is greater than the thickness of
the filament, the results are a mixture of the surrounding
calcite and the filament. We speculate that the extra carbon
found is either a coating on the filament, protecting it from
the acid etch (Melim et al., 2001), or the filaments are
simply composed of a carbon-rich (hence organic) material
and the etch has removed the surrounding calcite. Since the
filaments from surface samples (not etched, Fig. 4d) and
the embedded samples (etched out, Fig. 4a–c) have
identical textures, it seems more likely that the filaments
are preserved organic material and not carbon-coated
calcitized filaments.
POSSIBLE ORIGINS
The morphology and high carbon content of these
filaments suggests that they are biogenic. An extensive
survey of known microorganisms and associated struc-
tures, and consultation with colleagues who work with a
number of different microbial forms, however, shows
nothing similar to the reticulated filaments in the same
size range. Although the general filament shape is a
common morphology, reticulated chambers are not. We
are uncertain as to the origin of the reticulated chambers,
as known microbes lack any structure of comparable size.
Larger chambers are common; for example, some diatoms
(Bacillariophyceae) have similar complex structures in their
cell walls (areolae), but they are far larger in scale and very
different in aspect ratio. Filamentous diatoms such as those
in the genus Aulacoseira have similar cell morphology with
regular patterns of pores termed areolae, but these
filaments are significantly larger in diameter (3–30 mm
Figure 5. Scanning electron micrographs showing the two different sizes of reticulated filaments with parallel hexagonal
chambers. Note the scale is identical in these images. (a) Larger filament. (b) Smaller filament. Hidden Cave pool finger.
Figure 6. Scanning electron micrograph of reticulated
filament and two EDX analyses; one centered on the filament
and one centered on the calcite next to the filament. The
analyses extend 2–3 mm into the sample so both plots show
calcite (Ca +C+O), trace amounts of clays (Mg +Si +O)
and the gold-palladium coating medium (Au +Pd). Note that
the filament contains more carbon than the calcite sample
indicating that the filament itself is carbon-rich.
L.A. MELIM, D.E. NORTHUP, M.N. SPILDE,B.JONES, P.J. BOSTON,AND R.J. BIXBY
Journal of Cave and Karst Studies, December 2008 N139
diameter, 2.5–24 mm mantle height; Krammer and Lange-
Bertalot, 1991). In general, diatoms in valve view range
from eight or ten microns to a millimeter in length and are
commonly 10–20 microns in diameter. At the extreme size
ranges, diatoms can be as small as one micron in diameter
(rarely) and as wide as 30–40 microns in diameter (Round
et al., 2000). Some diatoms, including Aulacoseira, produce
resting cells of similar sizes to the normal cells (Round et
al., 2000). Thus, there is only the most marginal overlap in
size with our observed structures. More importantly, all
diatoms contain biogenic silica as a major component of
their cell walls (Round et al., 2000). The reticulated
filaments found in the cave samples completely lack silica.
Fungi, especially the filamentous, microscopic forms,
were eliminated from consideration due to their larger size,
generally two microns in diameter or larger. Thus, while
they are often tens of microns in length, their diameter is
insufficiently small to qualify as a bona fide candidate. The
fungal morphologies we examined, or with which we are
experienced, lack this form of reticulated patterning.
Actinomycetes, a filamentous group of bacteria, com-
monly occur in caves, are tens of microns in length at times
and have similar diameters, but all known strains that we
have investigated in the literature lack this kind of
reticulated patterning. The authors have examined many
cultures and environmental samples of actinomycetes using
scanning electron microscopy and have never observed this
morphology nor seen it in the literature
There is the possibility that the reticulate structure is a
form of S-layer, a symmetrical arrangement of hexagonal
protein units in the outer cell surface layer that has been
observed in some Gram-negative bacteria (e.g. Schultze-
Lam et al., 1992) and in Archaea (Messner et al., 1986).
Interestingly, S-layers have a cross-hatched pattern when
imaged using TEM (Phoenix et al., 2005) that is akin to the
chamber patterns found in the reticulated filaments. The
hexagonal units of the S-layer, however, are generally
,50 nm wide (Messner et al., 1986; Schultze-Lam et al.,
1992; Phoenix et al., 2005) and are therefore smaller than
70–100 nm long hexagonal units found in the reticulated
filament. In the reticulate filaments, the center-to-center
distance between adjacent chambers is generally two to
three times larger than found in S-layer units. In addition,
images of S-layers give the impression of a solid lattice
layer rather than the open chambers of the reticulated
filaments that we have observed in our cave samples.
In reviewing bacteria described in Bergey’s Manual of
Systematic Bacteriology (Holt, 1984–1989; Garrity, 2001–),
only three bacteria were found with a similar morphology.
The first of these is Nitrosomonas, which is pictured with
‘‘an additional cell wall layer’’ with units that are ,20 nm
(see Holt 1984–1989, p. 1824). The units in this extra cell
wall layer in Nitrosomonas are three to four times smaller
than the hexagonal units found in the cave reticulated
filaments and also give the impression of a solid layer.
Nitrosomonas’ additional layer may represent an S-layer, as
it is similar in morphology. The image of Prosthecomicro-
bium polyspheroidum (Garrity, 2001–) depicts cells with
numerous short prosthecae that give the appearance of a
corn cob, superficially resembling the reticulate structures,
but the comparison breaks down on closer inspection. The
third possibility is an image of Helicobacter bilis (Garrity,
2001–) with ‘‘tightly wound periplasmic fibers and multi-
ple sheathed flagella.’’ Although this organism is similar
in diameter, it is only three microns in length.
In a study of putative Leptothrix sp. from a pool
enriched in iron oxides in Carlsbad Cavern, Caldwell and
Caldwell (1980) described filaments with ‘‘a hexagonal
matrix over the surface of the cells.’’ These hexagonal
subunits, 0.1 microns in diameter, are more irregular in
shape and are composed of iron, thus ruling out Leptothrix
as a candidate for our filaments.
Thus, similar morphologies to the cave reticulated
filaments do exist in the microbial world, but all can be
ruled out due to the presence of silica (diatoms), size
(diatoms, S-layers), or the presence of iron (Leptothrix sp.),
leading us to conclude that this is a heretofore unreported
morphology probably bacterial in nature.
CONCLUSIONS
Reticulated filaments are common in speleothems from
moist or wet environments. These reticulated filaments are
tubes of cross-hatched mesh with either hexagonal or
diamond-shaped chambers. They are up to 75 mm in length
(avg. 12 mm) but this size range is partly an artifact of
preservation. Two overlapping populations occur, one
200–400 nm in diameter and the other 500–700 nm. EDX
indicates that the filaments are composed of predominantly
carbon, hence they are not mineral. We speculate that
reticulated filaments are from an unknown, but possibly
common, subsurface type of microorganism or group of
microorganisms that prefer moist cave environments. We
continue to look for more examples, particularly living
examples, in the hope of culturing and/or obtaining DNA
in order to more precisely identify their phylogenetic
position and to understand their role in the cave ecosystem.
ACKNOWLEDGEMENTS
This material is based upon work supported by the
National Science Foundation under Grants No. 0719710,
0719507 and 0719669. We thank the U.S. Forest Service
for permission to sample in Hidden and Cottonwood
Caves, the U.S. National Park Service for permission to
sample in Carlsbad Caverns National Park, and the U.S.
Bureau of Land Management for permission to sample in
Endless Cave. In addition, special thanks are owed to
USFS employees Jerry Trout, Ransom Turner, Mike Baca,
Deanna Younger, and Kevin Glover; USNPS employees
Dale Pate, Paul Burger, and Jason Richards; and BLM
RETICULATED FILAMENTS IN CAVE POOL SPELEOTHEMS:MICROBE OR MINERAL
140 NJournal of Cave and Karst Studies, December 2008
employee James Goodbar. J. Michael Queen, Ginny Rust,
Neil Shannon and Andy Brehm all assisted in the field
work. Special thanks to Susan Herpin, who managed to
train L. Melim in vertical work for entering Hidden Cave
despite the odds and who assisted with field work and
other logistics. Western Illinois students Kristen Shinglman
Woody, Neil Shannon and Randi Liescheidt assisted in the
UNM SEM work. Peter Roe collected the sample from the
Cape Verde Islands. We extend our gratitude to Sen˜or
Arturo Cano Conde, owner of Cueva de Las Can˜ icas, in
Teapa, Mexico, for cave pearl samples. Many thanks to
George Braybrook who took the SEM images at the
University of Alberta. Special thanks to Kenneth Ingham
who took many great photographs of our sampling sites
and pool precipitates in the field. Figure 3 images courtesy
of K. Ingham.
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