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Calcium dynamics in microbialite‐forming exopolymer‐rich mats on the atoll of K iritimati, R epublic of K iribati, C entral P acific
Author(s) -
Ionescu D.,
Spitzer S.,
Reimer A.,
Schneider D.,
Daniel R.,
Reitner J.,
Beer D.,
Arp G.
Publication year - 2015
Publication title -
geobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.859
H-Index - 72
eISSN - 1472-4669
pISSN - 1472-4677
DOI - 10.1111/gbi.12120
Subject(s) - exopolymer , microbial mat , aragonite , cyanobacteria , chemistry , environmental chemistry , precipitation , sulfide , phototroph , oceanography , mineralogy , geology , calcite , photosynthesis , bacteria , biochemistry , paleontology , physics , meteorology , organic chemistry
Microbialite‐forming microbial mats in a hypersaline lake on the atoll of Kiritimati were investigated with respect to microgradients, bulk water chemistry, and microbial community composition. O 2 , H 2 S, and pH microgradients show patterns as commonly observed for phototrophic mats with cyanobacteria‐dominated primary production in upper layers, an intermediate purple layer with sulfide oxidation, and anaerobic bottom layers with sulfate reduction. Ca 2+ profiles, however, measured in daylight showed an increase of Ca 2+ with depth in the oxic zone, followed by a sharp decline and low concentrations in anaerobic mat layers. In contrast, dark measurements show a constant Ca 2+ concentration throughout the entire measured depth. This is explained by an oxygen‐dependent heterotrophic decomposition of Ca 2+ ‐binding exopolymers. Strikingly, the daylight maximum in Ca 2+ and subsequent drop coincides with a major zone of aragonite and gypsum precipitation at the transition from the cyanobacterial layer to the purple sulfur bacterial layer. Therefore, we suggest that Ca 2+ binding exopolymers function as Ca 2+ shuttle by their passive downward transport through compression, triggering aragonite precipitation in the mats upon their aerobic microbial decomposition and secondary Ca 2+ release. This precipitation is mediated by phototrophic sulfide oxidizers whose action additionally leads to the precipitation of part of the available Ca 2+ as gypsum.