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Abstract

We investigated the hypothesis that sulfate reduction rather than oxygenic photosynthesis promotes calcification in a hypersaline microbial mat by increasing the ion concentration product: ICP 5 [Ca2+] X [CO32-]. Pore‐water 3 calcium concentration profiles directly measured with microsensors show that calcium concentration in the photic zone decreased in illuminated mats and increased slightly in dark mats. High pH values in the photic zone of illuminated mats resulted in higher carbonate concentrations (2.25 mmol L-1) than in dark mats (0.75 mmol L-1), although the dissolved inorganic carbon (DIC) pore‐water concentration in the former was much lower (5.9 mmol L-1) than in the latter (9.9 mmol L-1). The pH‐induced rise in carbonate concentration in the light was the main factor influencing the ICP, while changes in Ca-1 concentration played a subsidiary role. Sulfate reduction did not result in a net pH increase in these mats, as rates in the photic zone were comparable between illuminated and dark mats (4 and 5 nmol cm-2 h-1, respectively), and pH increased in illuminated mats but not in dark mats. Calcium carbonate precipitation in the photic zone of these hypersaline mats is primarily controlled by photosynthesisinduced pH and carbonate concentration increases. However, heterotrophic bacteria, including sulfate reducers, play an important complementary role in calcification because they maintain high concentrations of DIC in the mat pore water.