Effect of VariablepCO₂on Ca²⁺Removal and Potential Calcification of Cyanobacterial Biofilms —An Experimental Microsensor Study

Abstract

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.

Spatial Distribution and Preservation of Carbon Isotope Biosignatures in Freshwater Microbialite Carbonate

“…Ancient laminated microbialites (e.g., stromatolites) ubiquitous throughout the geologic record are cited as some of the earliest records of life on Earth. − Modern microbialites are organo-sedimentary structures with purported biogenic origins that are considered analogues of ancient microbialite and stromatolite forming systems. Microbialites are thought to form via several processes, including microbial trapping and binding of authigenic grains, , creation of substrates for crystal nucleation, , and/or induction of changes in local geochemical conditions resulting in carbonate precipitation. , The latter mechanism is of primary interest as the induced changes in geochemical conditions have the potential to generate nonequilibrium carbon isotope effects that can be recorded within the precipitated carbonates as biosignatures . Detection of these biosignatures can provide insight into the mechanisms by which the microbialites were formed and the activities of associated biological systems.…”

“…Microbialites are thought to form via several processes, including microbial trapping and binding of authigenic grains, 6,7 creation of substrates for crystal nucleation, 8,9 and/or induction of changes in local geochemical conditions resulting in carbonate precipitation. 10,11 The latter mechanism is of primary interest as the induced changes in geochemical conditions have the potential to generate nonequilibrium carbon isotope effects that can be recorded within the precipitated carbonates as biosignatures. 12 Detection of these biosignatures can provide insight into the mechanisms by which the microbialites were formed and the activities of associated biological systems.…”

Biomineralization Patterns of Intracellular Carbonatogenesis in Cyanobacteria: Molecular Hypotheses

“…Altogether, these transports of H + and Ca 2+ increase the saturation of the extracellular solution with carbonates. As a result, various carbonate phases such as calcite (CaCO 3 ), Mg-calcite ((Mg,Ca)CO 3 ), aragonite (CaCO 3 ) or hydromagnesite (Mg 5 (CO 3 ) 4 (OH) 2¨4 H 2 O) can precipitate depending on the chemical conditions prevailing in the cyanobacterial environment [22,23].…”

Authigenic clays as precursors to carbonate precipitation in saline lakes of Salar de Llamara, Northern Chile

“…In some environments, EPS has been shown to bind up to 8 times more Ca 2+ ions than Mg 2+ ions 25 . This removal of Ca 2+ ions from the surrounding microenvironment would inhibit the initial precipitation of CaCO 3 [25][26][27][28][29][30][31] , while enriching the microenvironment in Mg 2+ sensu 2 . Additionally, elevated pH increases the solubility of silica 23 .…”