Interaction between ferruginous clay sediment and an iron-reducing hyperthermophilic Pyrobaculum sp. in a terrestrial hot spring

Green-coloured sediments in low-temperature geothermal surface features are typically indicative of photosynthetic activity. A near-boiling (89-93°C), alkali-chloride spring in the Taupō Volcanic Zone, New Zealand, was observed to have dark green sediments despite being too hot to support any known photosynthetic organisms. Analysis of aqueous and sediment microbial communities via 16S rRNA amplicon sequencing revealed them to be dominated by Aquifex spp., a genus of known hyperthermophilic hydrogen-oxidisers (69%-91% of operational taxonomic units (OTUs)), followed by groups within the Crenarchaeota (3%-20%), including the known iron-reducing genus Pyrobaculum. Cultivation experiments suggest that the green colouration of clay sediments in this spring may be due in part to ferruginous clays and associated compounds serving as substrates for the iron-reducing activity of low-abundance Pyrobaculum spp. These findings demonstrate the dynamic nature of microbe-mineral interactions in geothermal environments, and the potential ability of the rarer biosphere (1%-2% of observed sequences, cell densities of 450-33 000 g-1 sediment) to influence mineral formation at a macro-scale.