Microbial Mats in Australian Coastal Environments

Mineralogy and Microbial Diversity of the Microbialites in the Hypersaline Storr's Lake, the Bahamas

Microbialites found in the low-light-intensity, hypersaline waters of Storr's Lake (SL), San Salvador Island, the Bahamas, were investigated with respect to their morphology, mineralogy, and microbial diversity. Previously described microbialite morphologies, as well as a newly identified "multi-cuspate" morphology, were observed at various depths. Electron microscopy analysis revealed the presence of angular, blocky, and needle-shaped crystals with mineralized cyanobacterial filaments and remains of exopolymeric substances. X-ray diffraction studies confirmed the presence of both Mg-calcite and aragonite in the plateau-mushroom and pinnacle mound microbialites, whereas only Mg-calcite was identified in the other microbialite morphotypes. A comprehensive molecular analysis using barcoded pyrosequencing of five different microbial mat communities identified at least 12 dominant bacterial phyla. Cyanobacteria were generally low in abundance and ranged from ∼0.01% in the deeper pinnacle mounds to ∼3.2% in the shallow calcareous knobs. Other photosynthetic members included green nonsulfur bacteria of the phylum Chloroflexi and purple sulfur bacteria of the class Gammaproteobacteria. All mat types contained significant amounts of sulfate-reducing and dehalogenating bacteria. The low light intensity reaching the deeper microbialites, the lack of dominant cyanobacteria, and the abundance of sulfate reducers and Chloroflexi collectively suggest that sulfate reduction and anoxygenic photosynthetic processes influence the carbonate biomineralization process in these systems.

Inside the alkalinity engine: the role of electron donors in the organomineralization potential of sulfate-reducing bacteria

Mineral precipitation in microbial mats may have been the key to their preservation as fossil stromatolites, potentially documenting evidence of the earliest life on Earth. Two factors that contribute to carbonate mineral precipitation are the saturation index (SI) and the presence of nucleation sites. Both of these can be influenced by micro-organisms, which can either alter SI through their metabolisms, or produce and consume organic substances such as extracellular polymeric substances (EPS) that can affect nucleation. It is the balance of individual metabolisms within the mat community that determines the pH and the dissolved inorganic carbon concentration, thereby potentially increasing the alkalinity and consequently the SI. Sulfate-reducing bacteria (SRB) are an important component of this 'alkalinity engine.' The activity of SRB often peaks in layers where CaCO(3) precipitates, and mineral precipitation has been demonstrated in SRB cultures; however, the effect of their metabolism on the alkalinity engine and actual contribution to mineral precipitation is the subject of controversy. Here, we show through culture experiments, theoretical calculations, and geochemical modeling studies that the pH, alkalinity, and organomineralization potential will vary depending on the type of electron donor. Specifically, hydrogen and formate can increase the pH, but electron donors like lactate and ethanol, and to a lesser extent glycolate, decrease the pH. The implication of this for the lithification of mats is that the combination of processes supplying electron donors and the utilization of these compounds by SRB may be critical to promoting mineral precipitation.

Bacterial, archaeal and eukaryotic diversity of smooth and pustular microbial mat communities in the hypersaline lagoon of Shark Bay

The bacterial, archaeal and eukaryotic populations of nonlithifying mats with pustular and smooth morphology from Hamelin Pool, Shark Bay were characterised using small subunit rRNA gene analysis and microbial isolation. A highly diverse bacterial population was detected for each mat, with 16S rDNA clones related to Actinobacteria, Bacteroidetes, Chloroflexi, Cyanobacteria, Gemmatimonas, Planctomycetes, Alphaproteobacteria, Gammaproteobacteria, Deltaproteobacteria, Verrucomicrobia and candidate division TM6 present in each mat. Spirochaetes were detected in the smooth mat only, whereas candidate division OP11 was only detected in the pustular mat. Targeting populations with specific primers revealed additional cyanobacterial diversity. The archaeal population of the pustular mat was comprised purely of Halobacteriales, whereas the smooth mat contained 16S rDNA clones from the Halobacteriales, two groups of Euryarchaea with no close characterised matches, and the Thaumarchaea. Nematodes and fungi were present in each mat type, with diatom 18S rDNA clones only obtained from the smooth mat, and tardigrade and microalgae clones only retrieved from the pustular mat. Cultured isolates belonged to the Firmicutes, Gammaproteobacteria, Alphaproteobacteria, Bacteroidetes, Actinobacteria, Cyanobacteria, and Halobacteriales. The mat populations were significantly more diverse than those previously reported for Hamelin Pool stromatolites, suggesting specific microbial populations may be associated with the nonlithifying and lithifying microbial communities of Hamelin Pool.

Regulation of photosynthesis and oxygen consumption in a hypersaline cyanobacterial mat (Camargue, France) by irradiance, temperature and salinity

Short-term effects of irradiance (0-1560 micromol photons m(-2) s(-1)), temperature (10-25 degrees C), and salinity (40-160) on oxygenic photosynthesis and oxygen consumption in a hypersaline mat (Salin-de-Giraud, France) were investigated with microsensors under controlled laboratory conditions. Dark O(2) consumption rates were mainly regulated by the mass transfer limitations imposed by the diffusive boundary layer. Areal rates of net photosynthesis increased with irradiance and saturated at irradiances >400 micromol photons m(-2) s(-1). At low irradiances, oxygen consumption increased more strongly with temperature than photosynthesis, whereas the opposite was observed at saturating irradiances. Net photosynthesis vs. irradiance curves were almost unaffected by decreasing salinity (100 to 40), whereas increasing salinities (100 to 160) led to a decrease of net photosynthesis at each irradiance. Dark O(2) consumption rates, maximal gross and net photosynthesis at light saturation were relatively constant over a broad salinity range (60-100) and decreased at salinities above the in situ salinity of 100. Within the range of natural variation, temperature was more important than salinity in regulating photosynthesis and oxygen consumption. At higher salinities the inhibitory impact of salinity on these processes and therefore the importance of salinity as a regulating environmental parameter increased, indicating that in more hypersaline systems, salinity has a stronger limiting effect on microbial activity.

Microbial lithification in marine stromatolites and hypersaline mats

Lithification in microbial ecosystems occurs when precipitation of minerals outweighs dissolution. Although the formation of various minerals can result from microbial metabolism, carbonate precipitation is possibly the most important process that impacts global carbon cycling. Recent investigations have produced models for stromatolite formation in open marine environments and lithification in shallow hypersaline lakes, which could be highly relevant for interpreting the rock record and searching for extraterrestrial life. Two factors that are controlled by microbial processes and physicochemical characteristics determine precipitation: exopolymeric substances and the saturation index, the latter being determined by the pH, {Ca(2+)} and {CO(3)(2-)}. Here, we evaluate community metabolism in microbial mats and hypothesize why these organosedimentary biofilms sometimes lithify and sometimes do not.

Biogeochemistry of an iron-rich hypersaline microbial mat (Camargue, France)

In situ microsensor measurements were combined with biogeochemical methods to determine oxygen, sulfur, and carbon cycling in microbial mats growing in a solar saltern (Salin-de-Giraud, France). Sulfate reduction rates closely followed the daily temperature changes and were highest during the day at 25 degrees C and lowest during the night at 11 degrees C, most probably fueled by direct substrate interactions between cyanobacteria and sulfate-reducing bacteria. Sulfate reduction was the major mineralization process during the night and the contribution of aerobic respiration to nighttime DIC production decreased. This decrease of aerobic respiration led to an increasing contribution of sulfide (and iron) oxidation to nighttime O2 consumption. A peak of elemental sulfur in a layer of high sulfate reduction at low sulfide concentration underneath the oxic zone indicated anoxygenic photosynthesis and/or sulfide oxidation by iron, which strongly contributed to sulfide consumption. We found a significant internal carbon cycling in the mat, and sulfate reduction directly supplied DIC for photosynthesis. The mats were characterized by a high iron content of 56 micromol Fe cm(-3), and iron cycling strongly controlled the sulfur cycle in the mat. This included sulfide precipitation resulting in high FeS contents with depth, and reactions of iron oxides with sulfide, especially after sunset, leading to a pronounced gap between oxygen and sulfide gradients and an unusual persistence of a pH peak in the uppermost mat layer until midnight.