Massive methane-fueled microbial biofilms in a unique iodine-rich spring cavern ecosystem.

Massive microbial biofilms were discovered in the cavern of a former medicinal spring in southern Germany, where iodine-rich formation water reaches the surface. The substantial biomass produced completely covers the walls and ceilings of the cave, the latter bearing snottites of over 10 cm length. Yet, the nature of these microbiota, their ecophysiology and the putative chemolithoautotrophic processes fuelling the biofilms have not been elucidated. We hypothesize that methane emerges with the formation water, and that microaerophilic methane oxidation is a major driver of biofilm formation, possibly also involving a novel coupling of methane and halide cycling. To address these questions, we have sampled biofilms from horizontal and vertical transects in the cave, for a detailed dissection of biofilm composition by 454 pyrotag sequencing. Gas emissions in the spring water itself contained methane (>50%) of fossil origin, as confirmed by stable isotope analysis (-41‰ delta 13C/12C). Sequencing revealed distinct methanotrophic and methylotrophic populations within the Alpha- and Gammaproteobacteria, and potentially even the Verrucomicrobia to dominate the biofilms. Methylotenera and Methylobacter spp. were abundant on the walls and especially in the cavern water, but surprisingly not so in ceiling biofilms. Here, ribosomal and functional gene analysis rather hinted at methylotrophic and potentially also iodine-cycling populations, supporting a postulated production and oxidation of iodomethane in this unique habitat. In conclusion, primary insights into a dominantly chemolithoautotrophic microbial ecosystem are revealed, largely independent from phototrophic carbon an energy inputs, but just a several meters under our feet