Hydrogen export from intertidal cyanobacterial mats: sources, fluxes and the influence of community composition

Microbial mats from marine intertidal settings have been reported to release significant quantities of H₂, in a unique trait among other mats and microbial communities. However, the H₂ source and ecophysiological mechanisms that enable its export are not well understood. We examined H₂ accumulation and export in three types of greenhouse-reared mats, from the intertidal region of Guerrero Negro, Mexico, and kept under natural light–dark conditions and wetting and drying cycles simulating low-, mid- and high-tidal height periodicity. All mats released H₂ reproducibly and sustainably for 1.5 years. Net H₂ export took place in a pulsed daily manner, starting after dusk, and waning in the morning, as photosynthesis resumed. Mid- and low-tidal mats developed high concentrations, capable of sustaining export fluxes that represented 2–4% of the water split through primary productivity. Neither N₂ fixation nor direct photolytic hydrogenogenesis was significant to this H₂ export, which was fermentative in origin, variable among mats, originating from cyanobacterial photosynthate. Analyses of community composition by pyrosequencing of 16S rRNA and hoxH genes indicate that filamentous non-heterocystous cyanobacteria (e.g. Lyngbya, Microcoleus) were important in the process of H₂ export, as was the relatively low abundance and activity of methanogens and sulfate reducers.