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

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 mol H 2 liter ؊1 at about 1-mm depth below the mat surface, slowly decreasing to about 11 mol H 2 liter ؊1 just before sunrise. Another pulse of H 2 production, reaching a peak concentration of 46 mol 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. Cyanobacterial mats are found in various limnic, marine, and hypersaline environments. The thickest and most homogeneous mats are found in extreme environments such as hypersaline habitats and geothermal springs with a scarcity or total absence of grazers. Such microbial mats are important natural model systems for studying microbial interactions and the fundamental links between structure, diversity, and function in microbial communities (1, 2). Another reason for the extensive interest in these mats is their strong apparent similarity to ancient microbial mats inhabiting the early Earth before grazers evolved and now preserved as stromatolites (3). Knowledge about the functioning of modern microbial mats may thus give insights into the functioning of early microbial ecosystems on planet Earth. Hydrogen (H 2 ) presumably played a major role in the metabolism of primitive microbial communities (4), and outgassing of H 2 originating or escaping from these microbial c...

Hydrogen Dynamics in Cyanobacteria Dominated Microbial Mats Measured by Novel Combined H2/H2S and H2/O2 Microsensors

“…The high H 2 concentrations in a dark-incubated mat might, however, make methanogenesis locally and temporally favorable ( Hoehler et al, 2002 ). The effect of BES in the sliced mat provides evidence that methanogens like sulfate reducers could survive long periods of oxic conditions in the mat, as previously described for a hot spring microbial mat ( Revsbech et al, 2016 ) and soils ( Angel et al, 2012 ).…”

“…Even in anoxic environments the H 2 concentration is normally kept at few nanomolar levels due to the tight coupling between production and consumption ( Lovley et al, 1982 ; Hoehler et al, 1998 ; Stams and Plugge, 2009 ). A substantial H 2 accumulation has, however, been observed in the cyanobacterial layer of both hypersaline ( Burow et al, 2012 ), marine ( Nielsen et al, 2015 ) and hot spring ( Anderson et al, 1987 ; Revsbech et al, 2016 ) microbial mats during dark incubations following periods with illumination. The H 2 production in photosynthetic mats has been suspected to be due to nitrogenase activity, but the H 2 production in hypersaline mats seems to be mainly due to fermentation, where the photosynthetic bacteria ferment the photosynthate fixed during the day as a nighttime metabolism ( Hoehler et al, 2001 ; Burow et al, 2012 ; Lee et al, 2014 ).…”

Biogeography of American Northwest Hot Spring A/B′-Lineage Synechococcus Populations

“…Light is quickly attenuated in the microbial mat environment, altering the intensity, and quality of light available for photosynthesis for subsurface PEs. The concentration of dissolved minerals, and other ions or chemicals that are important inputs or outputs of Synechococcus metabolism also changes with depth over a diel cycle (Revsbech et al, 2016). In the present study we were unable to study depth as a parameter associated with distribution, since analyses were performed on bulk mat samples that had been collected before PEs adapted to different light environments had been discovered.…”

Insights into the ecological roles and evolution of methyl-coenzyme M reductase-containing hot spring Archaea

“…Also, mcr -containing H 2 -dependent methanogens identified in other thermal springs have been suggested to possess a methane-forming metabolism similar to Methanomassiliicoccales methanogens 22,24 . The inferred metabolism of these thermophilic Verstraetearchaeota organisms is not surprising as methylated compounds 25 and hydrogen 26 are key metabolic intermediates in thermal spring environments.

Fig. 2Overview of metabolic potentials in mcr -containing MAGs.

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