Microbial influences on tufa deposition in a tropical climate

Abstract

Tufas are freshwater carbonate deposits that attracted attention as paleoclimatic archives and analogs of microbial carbonates. Recent geomicrobiological studies revealed that cyanobacterial photosynthesis significantly contributed to the precipitation of CaCO3 (photosynthesis-induced CaCO3 precipitation; PICP), and that the chemical properties of extracellular polymeric substances (EPS) secreted by cyanobacteria significantly influenced on the depositional fabrics. These depositional processes were revealed from tufas formed in cool and temperate climates; however, their applicability to tufa deposits in other climate conditions remains unclear. This study, therefore, investigated tufa deposits that formed in a tropical climate in Brazil and applied geomicrobiological techniques, including microelectrode measurements and lectin-binding analysis, to clarify their depositional processes. The surfaces of eight investigated tufa samples were dominated by cyanobacteria. Microelectrode measurements indicated the occurrence of PICP in all the samples. However, five samples additionally exhibited a Ca2+ concentration decrease at the tufa surface, even in dark conditions, which was difficult to explain by abiotic CaCO3 precipitation and light-independent microbial metabolisms. Based on the fluxes calculated from the results of microelectrode measurements, the contribution of PICP to the formation of CaCO3 precipitates was estimated as 100% for three samples, similar to tufa deposits in cool and temperate climates. For the remaining five deposits, the average contributions of PICP, Ca2+ adsorption to organic matter, and other processes were estimated as 36%, 3%, and 61%, respectively. Among the phototrophs living the tufa surface, Phormidium and Leptolyngbya secreted EPS abundantly containing carboxyl groups, and were heavily calcified. In contrast, Phormidesmis, Calothrix, Pleurocapsa, and diatoms secreted EPS without detectable amount of carboxyl groups, and they were largely not calcified and formed pore spaces. Such influence of microbial EPS on the depositional fabrics were common to cool and temperate tufas. Nonetheless, some features would be specific to tropical tufas, including the contribution of unidentified light-independent processes, conical and columnar surface morphologies, and unclear annual lamination.