Microbial and Geochemical Iron Redox Cycling in Chocolate Pots Hot Springs, Yellowstone National Par

Hydrothermal vent systems, both terrestrial and oceanic, are important environments for astrobiological research because of the hypothesized origin of life on Earth occurring at such environments. Recent and increasing evidence for relic vent deposits on Mars has further piqued the interest of astrobiologists and have become the target for future investigations for potential Martian life. While the origin of life is still highly debated, the redox gradients formed near hydrothermal vents and the energetic advantage this gives life living in such environments is undeniable. Hyperthermophilic prokaryotic organisms are phylogenetically deeply rooted, which supports the notion of originating near hydrothermal vents. Furthermore, many of these deeply rooted organisms encode Fe redox cycling based metabolic pathways suggesting dissimilatory Fe reduction (DIR) and Fe(II) oxidation are ancient microbial metabolisms. Chocolate Pots hot springs (CP) are a collection of Fe-rich circumneutral-pH hydrothermal springs located in northwestern Yellowstone National Park. For the past two decades, one of the more prominent features has been investigated with interest in how oxygenic phototrophs (e.g. cyanobacteria) may have contributed to banded iron formation deposition in the Archean. Here we expand on previous enrichment culture based investigations of the putative Fe cycling microbial community by conducting Fe(III)-reducing incubation experiments and collecting sediment and spring water samples directly from CP to gain a better understanding of the composition of the microbial community and its metabolic potential in situ. High DIR activity was observed in samples collected near the hot spring vent, and diminished further downstream. Results from 16S rRNA gene amplicon and shotgun metagenomic sequencing revealed taxa related to Thermodesulfovibrio and Ignavibacteria which encoded putative extracellular electron transfer pathways as potential indication of the in situ Fe(III)-reducing microbial community. Fe isotope fractionation that occurs as a result of DIR has been recognized as a potential biomarker of microbial activity in the rock record and in modern environments. Although natural variability obfuscated results, samples collected from the vent pool and sediment cores revealed fractionation suggestive of DIR. These studies provide constraint on the potential pathways and signatures of both extant and ancient Fe-based microbial life on Earth, Mars, and other rocky planets.