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

Karen Maegaard,Niels P Revsbech,Lars P Nielsen

doi:10.3389/fmicb.2017.02022

Abstract

Hydrogen may accumulate to micromolar concentrations in cyanobacterial mat communities from various environments, but the governing factors for this accumulation are poorly described. We used newly developed sensors allowing for simultaneous measurement of H2S and H2 or O2 and H2 within the same point to elucidate the interactions between oxygen, sulfate reducing bacteria, and H2 producing microbes. After onset of darkness and subsequent change from oxic to anoxic conditions within the uppermost ∼1 mm of the mat, H2 accumulated to concentrations of up to 40 μmol L-1 in the formerly oxic layer, but with high variability among sites and sampling dates. The immediate onset of H2 production after darkening points to fermentation as the main H2 producing process in this mat. The measured profiles indicate that a gradual disappearance of the H2 peak was mainly due to the activity of sulfate reducing bacteria that invaded the formerly oxic surface layer from below, or persisted in an inactive state in the oxic mat during illumination. The absence of significant H2 consumption in the formerly oxic mat during the first ∼30 min after onset of anoxic conditions indicated absence of active sulfate reducers in this layer during the oxic period. Addition of the methanogenesis inhibitor BES led to increase in H2, indicating that methanogens contributed to the consumption of H2. Both H2 formation and consumption seemed unaffected by the presence/absence of H2S.

Highlights

Photosynthetic microbial mats are highly diverse communities both physiologically and phylogenetically
A white layer of colorless sulfur bacteria was observed at the surface after several hours of dark incubation, indicating that the O2/H2S interface was at the very surface of the mat (Jørgensen and Revsbech, 1983)
We investigated the possible role of methanogens as hydrogenotrophs in the mat by sequentially adding molybdate and bromoethanesulfonic acid (BES) to both intact (Figure 5) and sliced mat

Summary

Introduction

Photosynthetic microbial mats are highly diverse communities both physiologically and phylogenetically. The mats are stratified communities where different functional groups exploit the microniches created by steep gradients in light and chemistry. They form on top of solid substrates where the photosynthetic primary producers fuel heterotrophic and chemolithoautotrophic bacteria. Photosynthetic microbial mats are autotrophic and reduce CO2 to organic matter while oxidizing an electron donor. Under certain conditions they may, produce H2. Hydrogen production by photosynthetic bacteria has been reported numerous times (Gest et al, 1962; Benemann and Weare, 1974; Warthmann et al, 1992; Moezelaar and Stal, 1997; Hoehler et al, 2001; Burow et al, 2012; Lee et al, 2014; Nielsen et al, 2015), and such microbial H2

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Conclusion