Abstract
Iron (Fe) redox-based metabolisms likely supported life on early Earth and may support life on other Fe-rich rocky planets such as Mars. Modern systems that support active Fe redox cycling such as Chocolate Pots (CP) hot springs provide insight into how life could have functioned in such environments. Previous research demonstrated that Fe- and Si-rich and slightly acidic to circumneutral-pH springs at CP host active dissimilatory Fe(III) reducing microorganisms. However, the abundance and distribution of Fe(III)-reducing communities at CP is not well-understood, especially as they exist in situ. In addition, the potential for direct Fe(II) oxidation by lithotrophs in CP springs is understudied, in particular when compared to indirect oxidation promoted by oxygen producing Cyanobacteria. Here, a culture-independent approach, including 16S rRNA gene amplicon and shotgun metagenomic sequencing, was used to determine the distribution of putative Fe cycling microorganisms in vent fluids and sediment cores collected along the outflow channel of CP. Metagenome-assembled genomes (MAGs) of organisms native to sediment and planktonic microbial communities were screened for extracellular electron transfer (EET) systems putatively involved in Fe redox cycling and for CO2 fixation pathways. Abundant MAGs containing putative EET systems were identified as part of the sediment community at locations where Fe(III) reduction activity has previously been documented. MAGs encoding both putative EET systems and CO2 fixation pathways, inferred to be FeOB, were also present, but were less abundant components of the communities. These results suggest that the majority of the Fe(III) oxides that support in situ Fe(III) reduction are derived from abiotic oxidation. This study provides new insights into the interplay between Fe redox cycling and CO2 fixation in sustaining chemotrophic communities in CP with attendant implications for other neutral-pH hot springs.
Highlights
Environments containing high concentrations of redox active elements, such as iron (Fe), are important areas of study because of the potential for these elements support the energy metabolism of microbial cells
The Chocolate Pots are a series of vent features along and within the Gibbon River ∼5 km south of the Norris Geyser Basin (Allen and Day, 1935; McCleskey et al, 2010)
71.2c aLetters in parentheses indicate taxonomic level: k, kingdom; p, phylum; c, class; o, order; f, family; g, genus. bAverage percent read abundance across all Chocolate Pots (CP) core libraries (n = 42). cTotal percent of reads comprising OTUs with >1% average read abundance. dAs determined by NCBI BLASTn. e16S rRNA gene sequences recovered from Metagenome-assembled genomes (MAGs) using CheckM, and aligned to amplicon sequences using BLASTn. f Not applicable; no 16S rRNA gene sequences from MAGs aligned to this OUT
Summary
Environments containing high concentrations of redox active elements, such as iron (Fe), are important areas of study because of the potential for these elements support the energy metabolism of microbial cells. Mineralogical analyses of the Martian surface have identified deposits indicative of circumneutral-pH (Arvidson et al, 2014), and relic hot spring environments (Squyres et al, 2008; Ruff and Farmer, 2016) Together, this makes CP a suitable analog environment in terms of gaining insight into metabolic processes that could have supported life on early Earth and possibly Mars. The potential for lithoautotrophic Fe(II) oxidation has been considered as well, after unsuccessful culturing of putative FeOB (Emerson and Weiss, 2004) and little experimental evidence to support their activity in the microbial mats (Trouwborst et al, 2007), research has not been continued in this area. Shotgun metagenomic sequences were obtained from the top 1 cm of three of these sediment cores as well as filtered vent pool water biomass in order to identify abundant taxa containing genes involved in extracellular electron transfer (EET) and CO2 fixation. Our genomic data supports the metabolic potential for lithoautotrophic FeOB, they do not appear to be prominent members of the microbial community
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