Calcium carbonate precipitation by anoxygenic phototrophic bacteria.
Résumé
Carbonate biomineralization is considered as one of the main natural processes controlling CO2 levels in the atmosphere both in the past and at present time. In contrast to extensive studies of cyanobacterial calcification, biomineralization of anoxygenic phototrophic bacteria (APB) remained largely underestimated, despite their potentially important role on CaCO3 precipitation in the biomats, notably in the past. Haloalcaliphilic Rhodovulum steppense A-20s and halophilic neutrophilic Rhodovulum sp. S-17-65 were examined with respect to their ability to precipitate CaCO3 under controlled laboratory conditions. To characterize the link between the rate of bacterial growth (biomass production) and the rate of CaCO3 precipitation, batch kinetic experiments with live, dead and inactivated bacteria both in nutrient solution and in inert electrolyte were performed and produced precipitates were examined by SEM, TEM and XRD techniques. Active strains A-20s and S-17-65 precipitated calcite from initially supersaturated solutions (Ωcalcite = 10 to 40) via increasing Ωcalcite to 80–120 before the precipitation. The amount of precipitated CaCO3 (mole) was directly correlated with the amount of organic C in bacterial biomass produced with a slope of dependence ranging from 0.3 to 0.6 and from 0.1 to 0.3 for A-20s and S-17-65, respectively, depending on the initial solution composition. For both bacterial strains, only live actively photosynthetizing bacteria were capable of effectively decreasing Ca concentration and form CaCO3 with apparent bulk precipitation rates ranging from 0.001 to 0.0150 mmol/h at 10–20 gwet/L of biomass, similar to rates reported for other bacteria. SEM and XRD analyses of precipitates reveal the dominance of calcite with some amount of vaterite and monohydrocalcite forming spheres up to 100 μm diameter. The TEM analysis of bacterial suspension at the end of precipitation experiments did not demonstrate the presence of CaCO3 at the surface or in the vicinity of live cells. This suggests the existence of certain cell protection mechanism against carbonate precipitation at the cell surface. Given the lower efficiency of photoheterotrophic APB, compared to photoautotrophic cyanobacteria, to precipitate CaCO3 in natural conditions, it is possible that the overall potential of phototrophic community to form massive carbonate deposits was strongly limited before the appearance of oxygenic phototrophs.