Microbiology meets geochemistry: geothermal flux shapes different microbial communities at the same exhalative area

Volcanic/geothermal areas are sites of complex interactions between geosphere and biosphere. Pantelleria island (Southern Mediterranean Sea) hosts a high enthalpy geothermal system characterized by high CH4 and low H2S fluxes. Two sites, FAV1 and FAV2, located a few meters apart at the main exhalative area of the island (Favara Grande), recorded similar physical conditions (soil temperature 60°C, soil gas composition enriched in CH4, H2 and CO2). However, while high methanotrophic activity (59.2 nmol g-1 h-1) and high diversity of methanotrophs was detected at FAV2, FAV1 was not active and appeared deprived of methanotrophs (1). Our aim was to investigate the main factors influencing methanotrophy as a result of biotic and abiotic interactions in this geothermal site. To this aim, soil bacterial and archaeal communities of the two sites were analysed by 16S rDNA MiSeq Illumina sequencing and the results related to geochemical data. At phylum level, both FAV1 and FAV2 bacterial sequences were mainly assigned to four phyla (Proteobacteria, Firmicutes, Actinobacteria and Chloroflexi) being Chloroflexi much more represented at FAV1 and Actinobacteria at FAV2. High abundance of thermo-acidophilic chemolithotrophs was detected in site FAV1, while FAV2 was dominated by methanotrophs (40% of the reads). The most represented species at FAV1 was Acidithiobacillus ferrooxidans (25%), while methanotrophs at FAV2 were dominated by Methylocaldum (31%). Archaea were almost exclusively represented, at both sites, by the chemolithotrophic ammonia-oxidating candidate species Nitrososphaera gargensis (Thaumarchaeota). Our results suggest that methanotrophs are not hampered at FAV1 by harsher conditions but by a previously unrecognized competition between chemolithotrophy and methanotrophy due to energetic convenience for chemolithotrophic lifestyle derived from higher availability of electron donors (NH4+, H2). (1) Gagliano et al., 2014 Biogeosciences, 11, 5865–5875