/pdf/processes-of-carbonate-precipitation-in-modern-microbial-176vfhaypi.pdf

Processes of carbonate precipitation in modern microbial mats

Microbial cells (ie bacteria, archaea, microeukaryotes) in oceans secrete a diverse array of large molecules, collectively called extracellular polymeric substances (EPS) or simply exopolymers These secretions facilitate attachment to surfaces that lead to the formation of structured 'biofilm' communities In open-water environments, they also lead to formation of organic colloids, and larger aggregations of cells, called ‘marine snow’ Secretion of EPS is now recognized as a fundamental microbial adaptation, occurring under many environmental conditions, and one that influences many ocean processes This relatively recent realization has revolutionized our understanding of microbial impacts on ocean systems EPS occur in a range of molecular sizes, conformations and physical/chemical properties, and polysaccharides, proteins, lipids and even nucleic acids are actively-secreted components Interestingly, however, the physical ultrastructure of how individual EPS interact with each other is poorly-understood Together, the EPS matrix molecules form a three-dimensional architecture from which cells may localize extracellular activities and conduct cooperative/ antagonistic interactions that cannot be accomplished efficiently by free-living cells EPS alter optical signatures of sediments and seawater, and are involved in biogeomineral precipitation and the construction of microbial macrostructures, and horizontal-transfers of genetic information In the water-column, they contribute to the formation of marine snow, transparent exopolymer particles (TEP), sea-surface microlayer biofilm, and marine oil snow (MOS) Excessive production of EPS occurs during later-stages of phytoplankton blooms as an excess metabolic byproduct and releases a carbon pool that transitions among dissolved-, colloidal-, and gel-states Some EPS are highly-labile carbon forms, while other forms appear quite refractory to degradation Emerging studies suggest that EPS contribute to efficient trophic-transfer of environmental contaminants, and may provide a protective refugia for pathogenic cells within marine systems; one that enhances their survival/persistence Finally, these secretions are prominent in ‘extreme’ environments ranging from sea-ice communities to hypersaline systems to the high-temperatures/pressures of hydrothermal-vent systems This overview summarizes some of the roles of exopolymer in oceans

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Microbial Extracellular Polymeric Substances (EPSs) in Ocean Systems.

https://appliedecology.cals.ncsu.edu/absci/wp-content/uploads/33_Baumgartner2006.pdf

Sulfate reducing bacteria in microbial mats: Changing paradigms, new discoveries

Photosynthetic uptake of inorganic carbon can raise the pH adjacent to cyanobacterial cells, promoting CaCO 3 precipitation. This effect is enhanced by CO 2 concentrating mechanisms that actively transport into cells for carbon fixation. CO 2 concentrating mechanisms presumably developed in response to atmospheric decrease in CO 2 and increase in O 2 over geological timescales. In present-day cyanobacteria, CO 2 concentrating mechanisms are induced when the atmospheric partial pressure of CO 2 (p CO2 ) falls below ∼ 0.4%. Reduction in p CO2 during the Proterozoic may have had two successive effects on cyanobacterial calcification. First, fall in p CO2 below ∼ 1% (33 times present atmospheric level, PAL) resulted in lower dissolved inorganic carbon (DIC) concentrations that reduced pH buffering sufficiently for isolated CaCO 3 crystals to begin to nucleate adjacent to cyanobacterial cells. As a result, blooms of planktic cyanobacteria induced precipitated ‘whitings’ of carbonate mud in the water column whose sedimentary accumulation began to dominate carbonate platforms ∼ 1400‐ 1300 Ma. Second, fall in p CO2 below ∼ 0.4% (10 PAL) induced CO 2 -concentrating mechanisms that further increased pH rise adjacent to cells and promoted in vivo cyanobacterial sheath calcification. Crossing of this second threshold is indicated in the fossil record by the appearance of Girvanella 750‐700 Ma. Coeval acquisition of CO 2 concentrating mechanisms by planktic cyanobacteria further stimulated whiting production. These inferences, that p CO2 fell below ∼ 1% ∼ 1400‐1300 Ma and below ~0.4% 750‐700 Ma, are consistent with empirical and modelled palaeo-atmosphere estimates. Development of CO 2 concentrating mechanisms was probably temporarily slowed by global cooling ∼ 700‐570 Ma that favoured diffusive entry of CO 2 into cells. Lower levels of temperature and DIC at this time would have reduced seawater carbonate saturation state, also hindering cyanobacterial calcification. It is suggested that as Earth emerged from ‘Snowball’ glaciations in the late Neoproterozoic, global warming and O 2 rise reactivated the development of CO 2 concentrating mechanisms. At the same time, rising levels of temperature, calcium ions and DIC increased seawater carbonate saturation state, stimulating widespread cyanobacterial in vivo sheath calcification in the Early Cambrian. This biocalcification event promoted rapid widespread development of calcified cyanobacterial reefs and transformed benthic microbial carbonate fabrics.

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Cyanobacterial calcification, carbon dioxide concentrating mechanisms, and Proterozoic–Cambrian changes in atmospheric composition

Dolomite was successfully precipitated in culture experiments that simulated microbiogeochemical conditions prevailing during late stages of evaporation in ephemeral, hypersaline dolomitic lakes of the Coorong region, South Australia. Analyses of lake- and pore-water samples document rapid geochemical changes with time and depth in both dolomitic and non-dolomitic lakes. Extremely high sulphate and magnesium ion concentrations in lake waters decline rapidly with depth in pore waters throughout the sulphate-reduction zone, whereas carbonate concentrations in pore waters reach levels up to 100 times those of normal sea water. Ultimately, sulphate is totally consumed and no solid sulphate is recorded in the dolomitic lake sediments. ‘Most probable number’ calculations of lake sediment samples record the presence of large populations of sulphate-reducing bacteria, whereas sulphur-isotope analyses of lake-water samples indicate microbial fractionation in all the lakes studied. Viable populations of microbes from the lake sediments were cultured in anoxic conditions in the laboratory. Samples were then injected into vials containing sterilized clastic or carbonate grains, or glass beads, immersed in a solution that simulated the lake water. Falls in the levels of sulphate and rising pH in positive vials were interpreted as indicating active bacterial sulphate reduction accompanied by increased concentrations of carbonate. Within 2 months, sub-spherical, sub-micron-size crystals of dolomite identical to those of lake sediments were precipitated. It is concluded that bacterial sulphate reduction overcomes kinetic constraints on dolomite formation by removing sulphate and releasing magnesium and calcium ions from neutral ion pairs, and by generating elevated carbonate concentrations, in a hypersaline, strongly electrolytic solution. The results demonstrate that bacterial sulphate reduction controls dolomite precipitation in both the laboratory experiments and lake sediments. It is proposed that dolomite formation, through bacterial sulphate reduction, provides a process analogue applicable to thick platformal dolostones of the past, where benthic microbial communities were the sole or dominant colonizers of shallow marine environments.

Precipitation of dolomite using sulphate‐reducing bacteria from the Coorong Region, South Australia: significance and implications

Microbialites in a modern lagoonal environment: nature and distribution, Tikehau atoll (French Polynesia)

Natural populations of benthic cyanobacteria in the lagoon of Tikehau Atoll in French Polynesia were studied using a polyphasic approach that combined field observations, LM, culturing, and molecular sequencing. The approach assessed their phenotypic (morphotypic and ecological) and genotypic diversity. Partial sequences (approximately 450 bp long) of the PCR-amplified 16S rRNA gene were obtained from both natural and cultured populations using cyanobacteria-specific primers. The sequences obtained clustered into six separate phylogenetic clusters in relation to the complete set of 16S rRNA sequences available in public databases. Phylogenetic clustering correlated in many cases with some morphological characteristics. For example, Spirulina subsalsa could be identified to the morphospecies level using both molecular and microscopic approaches. Morphotypes identified as Symploca hydnoides (Kutz. ex Gomont) TK22, Phormidium sp. TK1, and P. laysanense (Lemmerman) TK14 formed a distinct cluster. The morphogenus Hydrocoleum (Blennothrix) clustered interestingly close to the morphologically similar, but planktonic, marine cyanobacterium Trichodesmium, suggesting a relationship. Other sequences belonging to morphotaxa with very narrow trichomes were found to be polyphyletic. Enrichment cultures, with inoculum obtained from field populations, were followed over a period of 18 months. Dominance in all cultures shifted over time in favor of a set of cyanobacterial strains with narrow trichomes, which were phenotypically and phylogenetically different from natural populations dominating the original samples. Sequences from strains enriched by cultivation clustered into two well-defined phylogenetic groups, possibly identifying new taxa. These clusters may represent a niche of opportunistic species, evolved to exploit short-term nutrient spikes in the environment.

Characterization of microbialite‐forming cyanobacteria in a tropical lagoon: tikehau atoll, tuamotu, french polynesia

Hemispheroidal domes (microbialites) produced by natural populations of filamentous cyanobacteria belonging to four distinct Phormidium species, and one probable new species of Schizothrix were collected alive from 0-25 m depth habitates in the lagoon of Tikehau atoll (Tuamotu, French Polynesia). This study aims to establish the biochemical control on in-situ carbonate precipitation processes ("organomineralization" processes) occuring merely in the alveolate network of non-coalescent microfibrils that characterizes the degraded parts of the microbialite domes. The comparison between amino acid and monosaccharide composition of purified cyanobacterially produced organic matter and that of intramineral (soluble and insoluble) organic matrices associated with carbonate precipitates emphasizes the importance of dicarboxylic (aspartic and glutamic) acids, released by the decay of cyanobacterial sheaths, in CaCO3 formation and demonstrates that the in situ precipitation of ultra-fine micrites is a highly selective process regarding the available external organic matter. This diagenetic process is thought to result from incipient hydrolysis of cyanobacterial S-layer proteins attached to extracellular polysaccharide fibrils composing the sheath. Taxonomic affinity of cyanobacterial populations responsible for microbialite construction is one of the major factors allowing biochemical discrimination of in-situ precipitated carbonates, indicating that specific mucilages or their degradational products are guiding forces for the calcification processes. Another possible source for the formation of carbonate-associated organic matrices is derived from metabolites (e.g. mucus) released in water by lagoonal dwelling benthic organisms.

Sophie Sprachta

Papers

Microbialites in a modern lagoonal environment: nature and distribution, Tikehau atoll (French Polynesia)

Characterization of microbialite‐forming cyanobacteria in a tropical lagoon: tikehau atoll, tuamotu, french polynesia

Biochemical Control of Calcium Carbonate Precipitation in Modern Lagoonal Microbialites, Tikehau Atoll, French Polynesia