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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. O2, H2S, 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.

Two different cyanobacterial biofilms from German karstwater creeks were investigated with respect to their photosynthetic effect on Ca 2C removal and potential CaCO 3 precipitation in artificial creek waters of different CO 2 partial pressures at a given, constant calcite supersaturation. CO 2 partial pressures were adjusted to 350 ppmV, 2200 ppmV and 8700 ppmV respectively, covering the range of Phanerozoic atmospheric CO 2 partial pressures inferred from palaeosoils, stomatal indices and model calculations. Microsensor measurements of calcium, pH and oxygen revealed differences in the potential to precipitate CaCO 3 between the two model organisms Tychonema-relative strain SAG 2388 and Synechococcus sp. strain SAG 2387. Whereas a strong removal of Ca 2C from the solution was measured at Tychonema-relative biofilm, the Synechococcus sp. biofilm exercised a much lower Ca 2C removal during photosynthesis. Photosynthesis was enhanced in both organisms with increasing CO 2 and HCO 3 ¡ , as indicated by enhanced O 2 production, but only for the motile filamentous taxon Tychonema-relative a concomitantly increasing calcium removal was measured. However, model calculations indicate that this short-term Ca 2C binding in the Tychonema-relative is due to complexation to exopolymers or oscillin, with no immediate CaCO 3 precipitation. In contrast, Ca 2C and pH measurements at Synechococcus sp. biofilm could be consistent with immediate CaCO 3 precipitation at the cells. In both biofilms, pH gradients increase with increasing pCO 2 from 350 to 2200 ppmV due to enhanced photosynthesis, but decrease at a pCO 2 of 8700 ppmV despite of further enhanced photosynthesis. This observation, regardless whether CO 2 or HCO 3 ¡ is used by the cyanobacteria, is in accordance with hydrochemical modeling demonstrating an increased DIC buffering at high pCO 2 conditions. These results indicate that the potential of cyanobacteria to form spatially defined calcification pattern via pH gradients at their cell envelopes ('calcified cyanobacteria') increases at elevated pCO 2 , while at high pCO 2 conditions Ca 2C binding and lowered pH microgradients lead to spatially diffuse calcification without defined cell envelope precipitates.

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