Microbial control of mineralization processes in non-marine biofilms

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Contrary to calcified biofilms from hardwater creeks where calcification occurs in the top layer of the biofilm due to high cyanobacterial and algal photosynthetic activity calcification in microbial mats from hypersaline lakes on Kiritimati is observed in the deeper mat parts while phototrophic mat layers remain largely uncalcified. Until now it is unclear which non-phototrophic prokaryotes inhibit calcification in the upper parts of the mat or promote biochemical processes leading to exopolymer degradation at the anoxic bottom layer and CaCO3 precipitation in the basal mat parts. Identification of microorganisms inhabiting different zones of hypersaline microbial mats will allow clarifying the prokaryotic contribution to fabric formation via exopolymer production and degradation in dependency of external physicochemical parameters (pCO2, pH buffering, mineral saturation). This addresses to the hypothesis that certain microbialite fabrics traceable in the fossil record only occur as a result of heterotrophic activity, while a photosynthesis-controlled precipitation results in tubular cyanobacterial microfossils.In this following application period we first of all focus on the identification of phototrophic prokaryotic communities in different sub layers of microbial mats, i.e. Cyanobacteria and Purple Bacteria with a main focus on EPS producing organisms. We will characterize a selection of obtained strains which have no known close relative, in order to determine their taxonomic status. Genome sequences should be determined especially for isolates with ecological relevance. One main goal of the future work will be to clarify the origin and identity of the so-called „Black Pebbles“ in the transparent jelly top mat layer ("Froschlaich”) using microbial and chemical analyses. Furthermore, we would like to identify present decomposer communities in different sub layers of microbial mats, i.e. aerobic and anaerobic microorganisms (e.g. sulfate reducing bacteria, methanogens) to understand calcification processes within the mats. After identification of microorganisms we will demonstrate their in situ abundances and will reveal their spatial distribution pattern in relation to carbonate precipitates in thin mat sections and biofilm-veneered microbialite parts by fluorescence in situ hybridizations. In addition, we will compare microbial populations and in situ activities (using available pure cultures) to understand fundamental relationships between microbial exopolymer production/degradation, microbialite formation and environmental parameters. This will be done by stable isotope probing (SIP) which allows linking the phylogeny of organisms to their function. In addition, microcosm experiments with defined artificial mixtures of isolated bacteria will be used to study biomineralization in vitro. A further way to get an impression of the metabolic active fraction of the microbial community in distinct layers of the mat will be obtained by a metaproteomic analysis of mat samples which is planned in cooperation with the Helmholtz Centre for Infection Research in Braunschweig. As a culture-independent approach to identify functions characteristic for the different layers of the microbial mats metagenomic (bar-coded pyrosequencing of 16S rRNA genes) and metatranscriptomic approaches will be employed as well. In addition, investigation of changes in prokaryotic diversity and community composition will be conducted as well as the identification of taxon-specific patterns associated with different layers in microbial mats and biogeographical effects on bacterial community structures by analyzing mats from different lakes. The analyses of ooid-like spherulites, which form in oxygenic mats parts prior to exopolymer degradation and aragonite microbialite formation, may provide significant information with respect to ooid formation. The ooid-like spherulites migrate from their place of nucleation to lower mat parts, where they from part of friable sediments and microbialites. Microfacies, petrographic, isotopic (including 14C-dating), and biogeochemical characterization of the microbialites will constitute a fundamental task to assess the relationship between microbial activity, organic matter and mineral products, also in order to enlarge the understanding of fabrics and facies of ancient microbialites. Analysis of hydrochemical parameters of lake, seepage, microbial mat, and sedimentary pore waters on Kiritimati will be continued in addition to the ongoing investigation of karst water creeks to reveal the general hydro-chemical prerequisites for mineral precipitation, relevance of seepage influxes, and the effect of mi-crobial activities with regard to mineral saturation and growth.