In situ hydrogen dynamics in a hot spring microbial mat during a diel cycle
- Revsbech, Niels Peter [ Department of Bioscience, Aarhus University, Aarhus, Denmark ]
- Trampe, Erik [ Marine Biological Section, Department of Biology, University of Copenhagen, Helsingør, Denmark ]
- Lichtenberg, Mads [ Marine Biological Section, Department of Biology, University of Copenhagen, Helsingør, Denmark ]
- Ward, David M. [ Montana State University: Land Resources & Environmental Sciences ]
- Kuhl, Michael [ Plant Functional Biology and Climate Change Cluster, University of Technology Sydney ]
Microbes can produce molecular hydrogen (H2) via fermentation, dinitrogen fixation or direct photolysis, yet the H2 dynamics in cyanobacterial communities has only been explored in a few natural systems and mostly in the laboratory. In this study, we investigated the diel in situ H2 dynamics in a hot spring microbial mat, where various ecotypes of unicellular cyanobacteria (Synechococcus sp.) are the only oxygenic phototrophs. In the evening, H2 accumulated rapidly after the onset of darkness reaching peak values of up to 30 μmol H2 L(-1) at about 1 mm depth below the mat surface, slowly decreasing to about 11 μmol H2 L(-1) just before sunrise. Another pulse of H2 production reaching a peak concentration of 46 μmol H2 L(-1) was found in the early morning under dim light conditions too low to induce accumulation of O2 in the mat. The light stimulation of H2 accumulation indicated nitrogenase activity to be an important source of H2 during the morning. This is in accordance with earlier findings of a distinct early morning peak in N2 fixation and expression of Synechococcus nitrogenase genes in mat samples from the same location. Fermentation might have contributed to the formation of H2 during the night, where accumulation of other fermentation products lowered the pH in the mat to