In Situ Hydrogen Dynamics in a Hot Spring Microbial Mat during a Diel Cycle

Abstract

ABSTRACT Microbes can produce molecular hydrogen (H 2 ) via fermentation, dinitrogen fixation, or direct photolysis, yet the H 2 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 H 2 dynamics in a hot spring microbial mat, where various ecotypes of unicellular cyanobacteria ( Synechococcus sp.) are the only oxygenic phototrophs. In the evening, H 2 accumulated rapidly after the onset of darkness, reaching peak values of up to 30 μmol H 2 liter −1 at about 1-mm depth below the mat surface, slowly decreasing to about 11 μmol H 2 liter −1 just before sunrise. Another pulse of H 2 production, reaching a peak concentration of 46 μmol H 2 liter −1 , was found in the early morning under dim light conditions too low to induce accumulation of O 2 in the mat. The light stimulation of H 2 accumulation indicated that nitrogenase activity was an important source of H 2 during the morning. This is in accordance with earlier findings of a distinct early morning peak in N 2 fixation and expression of Synechococcus nitrogenase genes in mat samples from the same location. Fermentation might have contributed to the formation of H 2 during the night, where accumulation of other fermentation products lowered the pH in the mat to less than pH 6 compared to a spring source pH of 8.3. IMPORTANCE Hydrogen is a key intermediate in anaerobic metabolism, and with the development of a sulfide-insensitive microsensor for H 2 , it is now possible to study the microdistribution of H 2 in stratified microbial communities such as the photosynthetic microbial mat investigated here. The ability to measure H 2 profiles within the mat compared to previous measurements of H 2 emission gives much more detailed information about the sources and sinks of H 2 in such communities, and it was demonstrated that the high rates of H 2 formation in the early morning when the mat was exposed to low light intensities might be explained by nitrogen fixation, where H 2 is formed as a by-product.