Artificial cyanobacterial mats: cycling of C, O, and S

Cycling of carbon, oxygen and sulphur was studied in cyanobacterial mats growing on defaunated sediments. Like in natural stromatolitic mats, fluxes of O-2 in the dark and in the light and estimates of gross photosynthesis suggest that respiration is much higher in the Light than in the dark. It is shown here that the rate of organic C mineralisation is only moderately higher in the light. Mineralisation is entirely anaerobic in the dark and the reoxidation of reduced end-products (notably reduced sulphur) is incomplete due to temporary sulphide immobilisation. The sediments therefore develop an O-2 debt during darkness and the enhanced O-2 uptake in the Light to a large extent reflects the reoxidation of reduced end-products accumulated during darkness. This also explains the mismatch between fluxes of inorganic C and O-2. Oxygen fluxes between the mat and the overlying water are therefore not a measure of net photosynthesis or mineralisation rates, but cause underestimations of these processes by a factor of about 2. About 11% of the net influx of inorganic C during illumination (or about 5 % of gross photosynthesis) is permanently accumulated as organic C and about 0.5 % accumulates as carbonate minerals.