Microsensor measurements of hydrogen gas dynamics in cyanobacterial microbial mats

We used a novel amperometric microsensor for measuring hydrogen gas production and consumption at high spatio-temporal resolution in cyanobacterial biofilms and mats dominated by non-heterocystous filamentous cyanobacteria (Microcoleus chtonoplastes and Oscillatoria sp.). The new microsensor is based on the use of an organic electrolyte and a stable internal reference system and can be equipped with a chemical sulfide trap in the measuring tip; it exhibits very stable and sulfide-insensitive measuring signals and a high sensitivity (1.5–5 pA per μmol L(-1) H(2)). Hydrogen gas measurements were done in combination with microsensor measurements of scalar irradiance, O(2), pH, and H(2)S and showed a pronounced H(2) accumulation (of up to 8–10% H(2) saturation) within the upper mm of cyanobacterial mats after onset of darkness and O(2) depletion. The peak concentration of H(2) increased with the irradiance level prior to darkening. After an initial build-up over the first 1–2 h in darkness, H(2) was depleted over several hours due to efflux to the overlaying water, and due to biogeochemical processes in the uppermost oxic layers and the anoxic layers of the mats. Depletion could be prevented by addition of molybdate pointing to sulfate reduction as a major sink for H(2). Immediately after onset of illumination, a short burst of presumably photo-produced H(2) due to direct biophotolysis was observed in the illuminated but anoxic mat layers. As soon as O(2) from photosynthesis started to accumulate, the H(2) was consumed rapidly and production ceased. Our data give detailed insights into the microscale distribution and dynamics of H(2) in cyanobacterial biofilms and mats, and further support that cyanobacterial H(2) production can play a significant role in fueling anaerobic processes like e.g., sulfate reduction or anoxygenic photosynthesis in microbial mats.