Cyanobacterially deposited speleothems: Subaerial stromatolites

Stalagmites, which appear to be deposited by cyanobacterial action, have been identified in two caves in New South Wales (Australia). These have a characteristic morphology, which has given rise to local names likening them to crustaceans. We have studied these speleothems, and the cyanobacteria covering them, by microscopic and chemical techniques. Both calcite precipitation and aeolian sediment trapping are involved in their deposition, which is strongly controlled by environmental conditions. They can be regarded as stromatolites within currently accepted definitions of the term.     

Unusual preservation of late Quaternary millipedes from Jamaica

The first terrestrial arthropods recorded from the Jamaican fossil record are millipedes (Diplopoda), representing at least three species, from a late Pleistocene or Holocene cave deposit. Taxa identified include Rhinocricus sp. or spp., aff Chondrotropis sp. and Caraibodesmus verrucosus (Pocock). The associated fauna includes diverse vertebrates and gastropods, with rare isopods, nonmarine crab claws and ostracodes. Preservation of millipedes has been enabled by calcite coating, lining and impregnating the chitinous exoskeleton, which had a high original calcium content and acted as a nucleus for precipitation. Delicate structures such as limbs, antennae, gonopods and eyes may all be preserved. This preservation has been facilitated by the bottle-shaped cave with an opening at its apex, the surrounding limestone, the clastic infill, and the seasonality of precipitation with very high rainfalls during tropical storms and hurricanes. Millipedes most probably drowned in the cave during storms and formed a nucleus for the precipitation of calcite from solution. This suite of conditions is most likely to occur in the tropics. □ Quaternary, caves, Jamaica, millipedes, Diplopoda, preservation, diagenesis.     

Microbial mats and physicochemistry in a saltern in the Bretagne (France) and in a laboratory scale saltern model

Abstract A saltern near La Baule (Bretagne, France) was remodeled in a programmable temperature and humidity controlled walk-in environmental chamber resembling the characteristics of the original saltern. The saltern showed different types of microbial mats predominantly composed of algae, oxy- and anoxyphotobacteria, and associated chemoorganotrophic bacteria, fungi and animals. Well-developed microbial mats were found up to a salinity of 10% during the three or four months in summer when salinity gradients and NaCl precipitation were established. The main phototrophic organisms were diatoms, the cyanobacteria Aphanothece, Microcoleus, Spirulina , and Oscillatoria , and Chromatiaceae. At higher salinity, Halobacterium sp., diatoms, and Dunaliella were dominant. Typical microbial mats and saltern-typical invertebrate, algal and bacterial species also developed in the saltern model, building up a stable community. The ionic composition of the brines and physicochemical parameters were similar to those determined for the original saltern. Different photosynthetic organisms, e.g. a filamentous purple bacterium and a hypersaline Chloroflexus -like organism, could be enriched within the microbial mats by changing the light regime.     

Biomicrospheres Generate Ooids in the Laboratory

Precipitation of calcium carbonate is widespread in microbial communities (biofilms, microbial mats, etc.) and plays an important role in the nucleation and formation of stromatolites, onkolites and oolites. Here we report carbonate precipitation under laboratory conditions in defined spherical microbial communities. These microbial communities represent a symbiosis of cyanobacteria, diatoms and heterotrophic bacteria. Calcification structures always reflect the shape of microbial aggregates resulting from the formation of coalescent grains in chains. Position of the single carbonate crystals follows the spherical arrangement of the filamentous cyanobacteria and results in hollow spheres. The structure of the spherical biofilm determines the form of the carbonate layer. Tiny calcite scleres, deposited by spherical microbial communities in concentric arrangement are observed continuously, documented and described in the laboratory cultures. They may represent the precursors of oolites and are the first reproducible laboratory system generating ooids without a hard nucleus within an otherwise laminated microbial mat community.     

Quaternary dune carbonates from the Mediterranean Coast of Egypt: Petrography and diagenesis

Summary The Quaternary carbonates of the Mediterranean coast of Egypt between Alexandria and Salum appear as parallel limestone ridges rising up to 100 m above sea level. These ridges are dominated by dunal carbonates which differ not only in their primary composition but also by distinct grades of meteoric water diagenesis. Oolitic facies dominates the younger aeolianites of the first and second ridges. Bioclastic facies with abundant coralline algae, benthonic foraminifers, molluscs, echinoderms and intraclasts represents the major rock type in the older aeolianites. Features of meteoric water diagenesis include precipitation of increasing amounts of avoid-filling low Mg-calcite spar, dissolution of aragonite and stabilization of aragonite and high Mg-calcite to low Mg-calcite. Aeolianites below paleosol horizons contain abundant calcrete cements, micritized fossils and detrital grains which are commonly corroded and replaced by calcite. Three stages of progressive meteoric diagenesis are recognised in the Egyptian Quaternary aeolianites. In stage 1 minor precipitation of low Mg-calcite occurs at the grain boundaries. Stage 2 is marked by partial dissolution of aragonite, partial loss of high Mg-calcite and precipitation of low Mg-calcite in some pore spaces. In stage 3, most of the remaining pores are occluded by cementation. At the end of stage 3, Mg is removed from high Mg-calcite but some aragonite still persists. The early vadose cements are represented by miniscus, bridge and pendant cements. The phreatic cements were precipitated as bladed spar in the isopachous rims and equant spar in the intergranular and mouldic porosity. The late vadose cements are represented by micritic cements that were related to calcrete formation. Elemental behaviour during meteoric water diagenesis indicates that it leads to a gradual decrease in bulk Sr along with Na in progressively altered aeolianites. Mn distribution is controlled by the carbonate mineralogy (aragonite versus calcite) as well as the proximity of the aeolianites to the overlying paleosol horizons.     

Microbial mats associated with bryozoans (Coorong Lagoon, South Australia)

Summary Bryostromatolites are laminated carbonate rocks composed of bryozoan zoarial laminae. The laminated texture is frequently caused by patterns of bryozoan self overgrowth as a regular defensive tactic against microbial fouling. In the Coorong Lagoon (South Australia), another type of bryostromatolite is present where the laminated growth of the weakly calcifying bryozoan speciesConopeum aciculata is postmortally stabilized by cyanobacterial mats at the surface, and fungal mats settling in the zooecial cavities. A tough extracellular slime network produced by benthic cyanobacteria is a trap for sediment particles, provides a method of adhesion to the bryozoan substrate, and produces a biological lamination by the vertical stratification of dead bryozoan skeletons. These slimes are also important for the preservation of cell structures and for their fossilization. Seasonal fluctuations in salinity and water level are the most important regional control factors, causing a phase displacement in the growth optima of microbial mats and bryozoans, thereby resulting in a rigid bryostromatolitic fabric.     

Autotrophy of green non-sulphur bacteria in hot spring microbial mats: biological explanations for isotopically heavy organic carbon in the geological record

Inferences about the evidence of life recorded in organic compounds within the Earth's ancient rocks have depended on ¹³C contents low enough to be characteristic of biological debris produced by the well-known CO₂ fixation pathway, the Calvin cycle. 'Atypically' high values have been attributed to isotopic alteration of sedimentary organic carbon by thermal metamorphism. We examined the possibility that organic carbon characterized by a relatively high ¹³C content could have arisen biologically from recently discovered autotrophic pathways. We focused on the green non-sulphur bacterium Chloroflexus aurantiacus that uses the 3-hydroxypropionate pathway for inorganic carbon fixation and is geologically significant as it forms modern mat communities analogous to stromatolites. Organic matter in mats constructed by Chloroflexus spp. alone had relatively high ¹³C contents (−14.9‰) and lipids diagnostic of Chloroflexus that were also isotopically heavy (−8.9‰ to −18.5‰). Organic matter in mats constructed by Chloroflexus in conjunction with cyanobacteria had a more typical Calvin cycle signature (−23.5‰). However, lipids diagnostic of Chloroflexus were isotopically enriched (−15.1‰ to −24.1‰) relative to lipids typical of cyanobacteria (−33.9‰ to −36.3‰). This suggests that, in mats formed by both cyanobacteria and Chloroflexus , autotrophy must have a greater effect on Chloroflexus carbon metabolism than the photoheterotrophic consumption of cyanobacterial photosynthate. Chloroflexus cell components were also selectively preserved. Hence, Chloroflexus autotrophy and selective preservation of its products constitute one purely biological mechanism by which isotopically heavy organic carbon could have been introduced into important Precambrian geological features.     

A microbial role in the construction of Mono Lake carbonate chimneys?

Lacustrine carbonate chimneys are striking, metre‐scale constructions. If these were bioinfluenced constructions, they could be priority targets in the search for early and extraterrestrial microbial life. However, there are questions over whether such chimneys are built on a geobiological framework or are solely abiotic geomorphological features produced by mixing of lake and spring waters. Here, we use correlative microscopy to show that microbes were living around Pleistocene Mono Lake carbonate chimneys during their growth. A plausible interpretation, in line with some recent works by others on other lacustrine carbonates, is that benthic cyanobacteria and their associated extracellular organic material (EOM) formed tubular biofilms around rising sublacustrine spring vent waters, binding calcium ions and trapping and binding detrital silicate sediment. Decay of these biofilms would locally have increased calcium and carbonate ion activity, inducing calcite precipitation on and around the biofilms. Early manganese carbonate mineralisation was directly associated with cell walls, potentially related to microbial activity though the precise mechanism remains to be elucidated. Much of the calcite crystal growth was likely abiotic, and no strong evidence for either authigenic silicate growth or a clay mineral precursor framework was observed. Nevertheless, it seems likely that the biofilms provided initial sites for calcite nucleation and encouraged the primary organised crystal growth. We suggest that the nano‐, micro‐ and macroscale fabrics of these Pleistocene Mono Lake chimneys were affected by the presence of centimetre‐thick tubular and vertically stacked calcifying microbial mats. Such carbonate chimneys represent a promising macroscale target in the exploration for ancient or extraterrestrial life.     

A likely role for anoxygenic photosynthetic microbes in the formation of ancient stromatolites

Although cyanobacteria are the dominant primary producers in modern stromatolites and other microbialites, the oldest stromatolites pre-date geochemical evidence for oxygenic photosynthesis and cyanobacteria in the rock record. As a step towards the development of laboratory models of stromatolite growth, we tested the potential of a metabolically ancient anoxygenic photosynthetic bacterium to build stromatolites. This organism, Rhodopseudomonas palustris, stimulates the precipitation of calcite in solutions already highly saturated with respect to calcium carbonate, and greatly facilitates the incorporation of carbonate grains into proto-lamina (i.e. crusts). The appreciable stimulation of the growth of proto-lamina by a nonfilamentous anoxygenic microbe suggests that similar microbes may have played a greater role in the formation of Archean stromatolites than previously assumed.     

Mineralisation of filamentous cyanobacteria in Lake Thetis stromatolites,Western Australia

Stromatolites are cited as some of the earliest evidence for life on Earth, but problems remain in reconciling the paucity of microfossils in ancient carbonate examples with the abundance of microbes that help construct modern analogues. Here, we trace the mineralisation pathway of filamentous cyanobacteria within stromatolites from Lake Thetis, Western Australia, providing new insights into microfossil preservation in carbonate stromatolites. Lake Thetis cyanobacteria exhibit a spectrum of mineralisation processes that include early precipitation of Mg‐silicates, largely controlled by the morphochemical features of the cyanobacteria, followed by aragonite formation that is inferred to be driven by heterotrophic activity. Fossilised cyanobacteria with high‐quality morphological preservation are characterised by a significant volume of authigenic Mg‐silicates, which have preferentially nucleated in/on extracellular organic material and on cell walls, and now replicate the region once occupied by the cyanobacterial sheath. In such specimens, aragonite is restricted to the outer sheath margin and parts of the cell interior. Cyanobacteria that display more significant degradation appear to possess a higher ratio of aragonite to Mg‐silicate. In these specimens, aragonite forms micronodules in the sheath zone and is spatially associated with the inferred remains of heterotrophic bacteria. Aragonite also occurs as an advancing front from the outer margin of the sheath where it is commonly intergrown with Mg‐silicates. Where there is no evidence of Mg‐silicates within filaments, the fidelity of microfossil preservation is poor. In these cases, individual filaments may no longer be visible under light microscopy, and little organic material remains, but filament traces remain detectable using electron microscopy due to variations in aragonite texture. These data provide further evidence that authigenic silicate minerals play a crucial role in the fossilisation of micro‐organisms; in their absence, carbonate crystal growth potentially mediated by heterotrophic microbial decay may largely obliterate morphological evidence for life within stromatolites, although mineralogical traces may still be detectable using electron microscopy.     

Diversity of nifH Genotypes in Floating Periphyton Mats Along a Nutrient Gradient in the Florida Everglades

Periphyton mats are an important component of many wetland ecosystems, performing a range of vital ecosystem functions, including nitrogen fixation. The composition and integrity of these mats are affected by nutrient additions, which might result in changes in their function. The overall objective of this study was to investigate the distribution of nifH sequences in floating periphyton mats collected along a nutrient gradient in the Florida Everglades. Distribution of nifH clone libraries indicated nutrient enrichment selected primarily for sequences branching deeply within the heterocystous cyanobacteria and within a novel group of cyanobacteria; sequences from low-nutrient sites were broadly distributed, with no clear dominance of sequences associated with heterocystous and nonheterocystous cyanobacteria and alpha-, gamma-, and delta-proteobacteria. The dominance of heterocystous cyanobacteria in nutrient-enriched sites and the lack of clear dominance by heterocystous cyanobacteria is consistent with previously reported diurnal cycles of nitrogen fixation rates in these systems. Sequences clustering with those harbored by methanotrophs were also identified; sequences from nutrient-impacted and transition regions clustered with those characteristic of type II methanotrophs, and sequences from oligotrophic regions clustered with type I methanotrophs.     

Interaction between Calcite and Phosphorus in Biomineralization Processes in Tufa Carbonates

Using electron microscopy techniques (SEM, LTSEM) coupled with analytical methods (XRD and EDS) the role of phosphorus has been assessed in the formation of freshwater calcite deposits (tufa) in a small pond of the Ruidera Lakes (Spain). Differences between the cell walls and sheaths of bacteria and eukaryotic algae as well as the existence of additional layers of extracellular polymeric substances (EPS) were features that lead to differences in the process of induced calcite biomineralization. Phosphorus has influence in the biomineralization of the EPS, sheaths and cell walls of cyanobacteria allowing for fossil preservation whereas does not participate in the calcite precipitation around algae and mosses. This variability may explain the different positive or negative roles played by natural or artificial inputs of phosphorus in hard water lakes and the different morphological features of calcite precipitates associated with eukaryotic and cyanobacteria picoplankton found in natural environments. The biomineralization observed is in agreement with the isotopic composition of the tufa layers that reflect the variations in environmental conditions around biological communities. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

One-step electrospinning of cross-linked chitosan fibers

Chitin is a nitrogen-rich polysaccharide that is abundant in crustaceans, mollusks, insects, and fungi and is the second most abundant organic material found in nature next to cellulose. Chitosan, the N-deacetylated derivative of chitin, is environmentally friendly, nontoxic, biodegradable, and antibacterial. Fibrous mats are typically used in industries for filter media, catalysis, and sensors. Decreasing fiber diameters within these mats causes many beneficial effects such as increased specific surface area to volume ratios. When the intrinsically beneficial effects of chitosan are combined with the enhanced properties of nanofibrous mats, applications arise in a wide range of fields, including medical, packaging, agricultural, and automotive. This is particularly important as innovative technologies that focus around bio-based materials are currently of high urgency, as they can decrease dependencies on fossil fuels. We have demonstrated that Schiff base cross-linked chitosan fibrous mats can be produced utilizing a one-step electrospinning process that is 25 times faster and, therefore, more economical than a previously reported two-step vapor-cross-linking method. These fibrous mats are insoluble in acidic, basic, and aqueous solutions for 72 h. Additionally, this improved production method results in a decreased average fiber diameter, which measures 128 +/- 40 nm. Chemical and structural analyses were conducted utilizing Fourier transform infrared spectroscopy, solubility studies, and scanning electron microscopy.     

Structure and function of natural sulphide-oxidizing microbial mats under dynamic input of light and chemical energy

We studied the interaction between phototrophic and chemolithoautotrophic sulphide-oxidizing microorganisms in natural microbial mats forming in sulphidic streams. The structure of these mats varied between two end-members: one characterized by a layer dominated by large sulphur-oxidizing bacteria (SOB; mostly Beggiatoa-like) on top of a cyanobacterial layer (B/C mats) and the other with an inverted structure (C/B mats). C/B mats formed where the availability of oxygen from the water column was limited (<5 μm). Aerobic chemolithotrophic activity of the SOB depended entirely on oxygen produced locally by cyanobacteria during high light conditions. In contrast, B/C mats formed at locations where oxygen in the water column was comparatively abundant (>45 μM) and continuously present. Here SOB were independent of the photosynthetic activity of cyanobacteria and outcompeted the cyanobacteria in the uppermost layer of the mat where energy sources for both functional groups were concentrated. Outcompetition of photosynthetic microbes in the presence of light was facilitated by the decoupling of aerobic chemolithotrophy and oxygenic phototrophy. Remarkably, the B/C mats conserved much less energy than the C/B mats, although similar amounts of light and chemical energy were available. Thus ecosystems do not necessarily develop towards optimal energy usage. Our data suggest that, when two independent sources of energy are available, the structure and activity of microbial communities is primarily determined by the continuous rather than the intermittent energy source, even if the time-integrated energy flux of the intermittent energy source is greater.     

Early colonization of thermal niches in a silica-depositing hot spring in central Tibet

Thermophilic microbial mats dominated by the anoxygenic phototroph Roseiflexus castenholzii commonly develop around sinter-depositing geysers in the Daggyai Tso geothermal field of central Tibet. In this study we used morphological and molecular genetic techniques to reveal a diverse pioneer biofilm community including both archaea and bacteria involved in early colonization of such thermal niches at temperatures ranging from 46 to 77 °C. Sinter precipitation and biomineralization were evident at all locations, but the latter was selective between taxa and most evident on filamentous cells. Evidence for possible indirect biosignatures from biofilms overwhelmed by sinter deposition was found. Succession to a mature community appeared to relate to the growth rate for key taxa outpacing that of silicification within an optimum temperature range of 54–61 °C. The thin surface layer of silicification-resistant cyanobacteria that developed on the surface of mature mats may play a role in preventing biomineralization of the susceptible R. castenholzii beneath within these communities.     

Bio-deposition of a calcium carbonate layer on degraded limestone by Bacillus species

To obtain a restoring and protective calcite layer on degraded limestone, five different strains of the Bacillus sphaericus group and one strain of Bacillus lentus were tested for their ureolytic driven calcium carbonate precipitation. Although all the Bacillus strains were capable of depositing calcium carbonate, differences occurred in the amount of precipitated calcium carbonate on agar plate colonies. Seven parameters involved in the process were examined: calcite deposition on limestone cubes, pH increase, urea degrading capacity, extracellular polymeric substances (EPS)-production, biofilm formation, ζ-potential and deposition of dense crystal layers. The strain selection for optimal deposition of a dense CaCO₃ layer on limestone, was based on decrease in water absorption rate by treated limestone. Not all of the bacterial strains were effective in the restoration of deteriorated Euville limestone. The best calcite precipitating strains were characterised by high ureolytic efficiency, homogeneous calcite deposition on limestone cubes and a very negative ζ-potential.     

Cyanobacterial diversity and activity in modern conical microbialites

Modern conical microbialites are similar to some ancient conical stromatolites, but growth, behavior and diversity of cyanobacteria in modern conical microbialites remain poorly characterized. Here, we analyze the diversity of cyanobacterial 16S rRNA gene sequences in conical microbialites from 14 ponds fed by four thermal sources in Yellowstone National Park and compare cyanobacterial activity in the tips of cones and in the surrounding topographic lows (mats), respectively, by high‐resolution mapping of labeled carbon. Cones and adjacent mats contain similar 16S rRNA gene sequences from genetically distinct clusters of filamentous, non‐heterocystous cyanobacteria from Subsection III and unicellular cyanobacteria from Subsection I. These sequences vary among different ponds and between two sampling years, suggesting that coniform mats through time and space contain a number of cyanobacteria capable of vertical aggregation, filamentous cyanobacteria incapable of initiating cone formation and unicellular cyanobacteria. Unicellular cyanobacteria are more diverse in topographic lows, where some of these organisms respond to nutrient pulses more rapidly than thin filamentous cyanobacteria. The densest active cyanobacteria are found below the upper 50 μm of the cone tip, whereas cyanobacterial cells in mats are less dense, and are more commonly degraded or encrusted by silica. These spatial differences in cellular activity and density within macroscopic coniform mats imply a strong role for diffusion limitation in the development and the persistence of the conical shape. Similar mechanisms may have controlled the growth, morphology and persistence of small coniform stromatolites in shallow, quiet environments throughout geologic history.     

Unravelling the microbial role in ooid formation – results of an in situ experiment in modern freshwater Lake Geneva in Switzerland

The microbial role in the formation of the cortex of low‐Mg calcite freshwater ooids in western part of Lake Geneva in Switzerland has been suggested previously, but not demonstrated conclusively. Early work mostly concentrated in hypersaline milieus, and hence little is known about their genesis in freshwater environments. We designed an in situ experiment to mimic the natural process of low‐Mg calcite precipitation. A special device was placed in the ooid‐rich bank of the lake. It contained frosted glass (SiO₂) slides, while quartz (SiO₂) is the most abundant mineral composition of ooid nuclei that acted as artificial substrates to favour microbial colonization. Microscopic inspection of the slides revealed a clear seasonal pattern of carbonate precipitates, which were always closely associated with biofilms that developed on the surface of the frosted slides containing extracellular polymeric substance, coccoid and filamentous cyanobacteria, diatoms and heterotrophic bacteria. Carbonate precipitation peaks during early spring and late summer, and low‐Mg calcite crystals mostly occur in close association with filamentous and coccoid cyanobacteria (e.g. Tolypothrix, Oscillatoria and Synechococcus, Anacystis, respectively). Further scanning electron microscope inspection of the samples revealed low‐Mg calcite with crystal forms varying from anhedral to euhedral rhombohedra, depending on the seasons. Liquid cultures corroborate the in situ observations and demonstrate that under the same physicochemical conditions the absence of biofilms prevents the precipitation of low‐Mg calcite crystals. These results illustrate that biofilms play a substantial role in low‐Mg calcite ooid cortex formation. It further demonstrates the involvement of microbes in the early stages of ooid development. Combined with ongoing microbial cultures under laboratory‐controlled conditions, the outcome of our investigation favoured the hypothesis of external microbial precipitation of low‐Mg calcite as the main mechanism involved in the early stage of ooid formation in freshwater Lake Geneva.     

Identification and characterization of Rhodopseudomonas spp., a purple, non-sulfur bacterium from microbial mats

A species of facultative photo-organotrophic, purple, non-sulfur bacterium was isolated from mixed-species microbial mats, characterized and examined for metal tolerance and bioremediation potential. Contributing mats were natural consortia of microbes, dominated by cyanobacteria and containing several species of bacteria arranged in a laminar structure, stabilized within a gel matrix. Constructed microbial mats were used for bioremediation of heavy metals and organic chemical pollutants. Purple, non-sulfur bacteria are characteristically found in lower strata of intact mats, but their contributing function in mats survival and function by mediating the chemical environment has not been explored. The gram-negative rod-shaped bacterium, reported here, produced a dark red culture under phototrophic conditions, reproduced by budding and formed a lamellar intracytoplasmic membrane (ICM) system parallel to cytoplasmic membrane, which contained bacteriochlorophyll a and carotenoids. This strain was found to have multiple metal resistances and to be effective in the reductive removal of Cr(VI) and the degradation of 2,4,6-trichlorophenol. Based on the results obtained from morphology, nutrient requirements, major bacteriochlorophyll content, GC content, random amplified polymorphic DNA-polymerase chain reaction (RAPD-PCR) profile and 16S-rDNA phylogenetic analysis, this member of the microbial mats may be identified as a new strain of the genus Rhodopseudomonas.     

Diversity of endolithic fungal communities in dolomite and limestone rocks from Nanjiang Canyon in Guizhou karst area, China

The endolithic environment, the tiny pores and cracks in rocks, buffer microbial communities from a number of physical stresses, such as desiccation, rapid temperature variations, and UV radiation. Considerable knowledge has been acquired about the diversity of microorganisms in these ecosystems, but few culture-independent studies have been carried out on the diversity of fungi to date. Scanning electron microscopy of carbonate rock fragments has revealed that the rock samples contain certain kinds of filamentous fungi. We evaluated endolithic fungal communities from bare dolomite and limestone rocks collected from Nanjiang Canyon (a typical karst canyon in China) using culture-independent methods. Results showed that Ascomycota was absolutely dominant both in the dolomite and limestone fungal clone libraries. Basidiomycota and other eukaryotic groups (Bryophyta and Chlorophyta) were only detected occasionally or at low frequencies. The most common genus in the investigated carbonate rocks was Verrucaria. Some other lichen-forming fungi (e.g., Caloplaca, Exophiala, and Botryolepraria), Aspergillus, and Penicillium were also identified from the rock samples. The results provide a cross-section of the endolithic fungal communities in carbonate rocks and help us understand more about the role of microbes (fungi and other rock-inhabiting microorganisms) in rock weathering and pedogenesis.