Microbial and viral roles in hydrothermal vent iron mat elemental cycling and ecology at the Loihi Seamount
Date
2019
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Publisher
University of Delaware
Abstract
Iron microbial mats at hydrothermal vents mediate fluxes of iron (Fe) and other elements to the oceans. These mats are formed by Zetaproteobacteria, which are the only apparent Fe-oxidizers in the mats. Most studies have focused on this dominant class, and their roles in carbon fixation and Fe biomineral formation. Iron mats host a diverse group of non-Zetaproteobacteria microorganisms, including viruses, and we do not yet know how these organisms contribute to biogeochemical cycling and interact with the Zetaproteobacteria. To address this, we sequenced and analyzed paired metagenomes and metatranscriptomes from three samples of Fe mats at Lō’ihi Seamount, Hawaii. These included a small, discrete sample of surface mat, as well as two large, bulk samples that included deeper regions of the mat. One deep mat sample was additionally used for shipboard Fe(II) and O2 addition experiments. Our results suggest that mat depth influences the diversity of the microbial community. Our shallow mat sample was almost entirely Zetaproteobacteria while the deeper mat samples had greater diversity, and were composed of taxa such as Zetaproteobacteria, Deltaproteobacteria, Gammaproteobacteria, Chloroflexi, Planctomycetes and Marinimicrobia. The shallow mat sample also had higher expression of carbon fixation genes and aerobic terminal oxidase genes; while the bulk samples had a larger number of polysaccharide degradation enzymes and higher expression of anaerobic processes. Within the deep mat samples, the Deltaproteobacteria were the second most abundant taxon. We identified an abundant novel clade of Deltaproteobacteria in an unknown order called DTB120, which contains genomic evidence for iron oxidation coupled to nitrate reduction. Overall, the mat microbes showed significant expression of genes involved in oxygen, nitrogen, carbon, and iron cycles. In addition to the microbes, we found evidence for viral contigs and complete viral genomes. CRISPR spacer identification revealed a history of infection among members of this microbial community. Together, the previously uncharacterized DTB120 and the viruses help link iron, nitrogen and carbon biogeochemistry in Lō’ihi Seamount hydrothermal vent Fe mats. This suggests that the flanking community is contributing to the chemical cycles and ecological success of the Fe mats.
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Keywords
Biogeochemistry, DTB120, Iron, Lō’ihi, Nitrogen, Zetaproteobacteria