Seafloor iron oxide deposits are a common feature of submarine hydrothermal systems. Morphological study of these deposits has led investigators to suggest a microbiological role in their formation, through the oxidation of reduced Fe in hydrothermal fluids. Fe-oxidizing bacteria, including the recently described Zetaproteobacteria, have been isolated from a few of these deposits but generally little is known about the microbial diversity associated with this habitat. In this study, we characterized bacterial diversity in two Fe oxide samples collected on the seafloor of Volcanoes 1 and 19 on the South Tonga Arc. We were particularly interested in confirming the presence of Zetaproteobacteria at these two sites and in documenting the diversity of groups other than Fe oxidizers. Our results (small subunit rRNA gene sequence data) showed a surprisingly high bacterial diversity, with 150 operational taxonomic units belonging to 19 distinct taxonomic groups. Both samples were dominated by Zetaproteobacteria Fe oxidizers. This group was most abundant at Volcano 1, where sediments were richer in Fe and contained more crystalline forms of Fe oxides. Other groups of bacteria found at these two sites include known S- and a few N-metabolizing bacteria, all ubiquitous in marine environments. The low similarity of our clones with the GenBank database suggests that new species and perhaps new families were recovered. The results of this study suggest that Fe-rich hydrothermal sediments, while dominated by Fe oxidizers, can be exploited by a variety of autotrophic and heterotrophic micro-organisms.

Interest in extracting mineral resources from the seafloor through deep-sea mining has accelerated in the past decade, driven by consumer demand for various metals like zinc, cobalt, and rare earth elements. While there are ongoing studies evaluating potential environmental impacts of deep-sea mining activities, these focus primarily on impacts to animal biodiversity. The microscopic spectrum of seafloor life and the services that this life provides in the deep sea are rarely considered explicitly. In April 2018, scientists met to define the microbial ecosystem services that should be considered when assessing potential impacts of deep-sea mining, and to provide recommendations for how to evaluate and safeguard these services. Here, we indicate that the potential impacts of mining on microbial ecosystem services in the deep sea vary substantially, from minimal expected impact to loss of services that cannot be remedied by protected area offsets. For example, we (1) describe potential major losses of microbial ecosystem services at active hydrothermal vent habitats impacted by mining, (2) speculate that there could be major ecosystem service degradation at inactive massive sulfide deposits without extensive mitigation efforts, (3) suggest minor impacts to carbon sequestration within manganese nodule fields coupled with potentially important impacts to primary production capacity, and (4) surmise that assessment of impacts to microbial ecosystem services at seamounts with ferromanganese crusts is too poorly understood to be definitive. We conclude by recommending that baseline assessments of microbial diversity, biomass, and, importantly, biogeochemical function need to be considered in environmental impact assessments of deep-sea mining.With increasing demand for rare and critical metals-such as cobalt, copper, manganese, tellurium, and zinc-there is increasing interest in mining these resources from the seafloor (Hein et al. 2013; Wedding et al. 2015;Thompson et al. 2018). The primary mineral resources in the deep sea that attract attention fall into four categories (Figs. 1, 2): (1) massive sulfide deposits created at active high-temperature hydrothermal vent systems along mid-ocean ridges, back-arc spreading centers, and volcanic arcs, from the mixing of mineral-rich, advecting hydrothermal fluids with bottom seawater; (2) similar deposits at inactive hydrothermal vent sites, where fluid advection has ceased but mineral deposits remain; (3) polymetallic nodules that form on the seafloor of the open ocean

Abstract. Little is known about nitrogen (N) transformations in general, and the elimination of N in particular, at diffuse vents where anoxic hydrothermal fluids have mixed with oxygenated crustal seawater prior to discharge. Oceanic N sinks that remove bio-available N ultimately affect chemosynthetic primary productivity in these ecosystems. Using 15N paired isotope techniques, we determined potential rates of fixed N-loss pathways (denitrification, anammox) and dissimilative nitrate reduction to ammonium (DNRA) in sulfidic hydrothermal vent fluids discharging from the subsurface at several sites at Axial Volcano and the Endeavour Segment on the Juan de Fuca Ridge. We also measured physico-chemical parameters (i.e. temperature, pH, nutrients, H2S and N2O concentrations) as well as the biodiversity and abundance of chemolithotrophic nitrate-reducing, sulfur-oxidizing γ-proteobacteria (SUP05 cluster) using sequence analysis of amplified small subunit ribosomal RNA (16S rRNA) genes in combination with taxon-specific quantitative polymerase chain reaction (qPCR) assays. Denitrification was the dominant N-loss pathway in the subsurface biosphere of the Juan de Fuca Ridge, with rates of up to ~1000 nmol N l−1 day−1. In comparison, anammox rates were always <5 nmol N l−1 day−1 and below the detection limit at most of the sites. DNRA rates were up to 152 nmol N l−1 day−1. These results suggest that bacterial denitrification out-competes anammox in sulfidic hydrothermal vent waters. Taxon-specific qPCR revealed that γ-proteobacteria of the SUP05 cluster sometimes dominated the microbial community (SUP05/total bacteria up to 38%). Significant correlation existed between fixed N-loss (i.e., denitrification, anammox) rates and in-situ nitrate and dissolved inorganic nitrogen (DIN) deficits in the fluids, indicating that DIN availability may ultimately regulate N-loss in the subsurface. Based on our rate measurements, and on published data on hydrothermal fluid fluxes and residence times, we estimated that, on average, ~10 Tg N yr−1 could globally be removed in the subsurface biosphere of hydrothermal vents systems, and could thus represent a small, but significant, fraction of the total marine N loss (240–400 Tg N yr−1).

Summary Elucidation of the potential roles of single‐celled eukaryotes (protists) in ecosystem function and trophodynamics in hydrothermal vent ecosystems is reliant on information regarding their abundance, distribution and preference for vent habitats. Using high‐throughput 18S rRNA gene sequencing on a diverse suite of hydrothermally influenced and background water samples, we assess the diversity and distribution of protists and identify potential vent endemics. We found that 95% of the recovered sequences belong to operational taxonomic units (OTUs) with a cosmopolitan distribution across vent and non‐vent habitats. Analysis of ‘vent only’ OTUs found in more than one vent sample and co‐occurrence network analysis comparing protist groups to extremophilic reference organisms suggest that the most likely vent endemics are infrequently encountered, potentially in low abundance, and belong to novel lineages, both at the phylum level and within defined clades of Rhizaria and Stramenopila. Potential endemism is inferred for relatives of known apusomonads, excavates and some clades of Syndiniales. Similarity in community composition among samples was low, indicating a strong stochastic component to protist community assembly and suggesting that rare endemics may serve as a reservoir poised to respond to changing environmental conditions in these dynamic systems.

32Interest in extracting mineral resources from the seafloor through deep-sea mining has 33 accelerated substantially in the past decade, driven by increasing consumer demand for various 34 metals like copper, zinc, manganese, cobalt and rare earth elements. While there are many on-35going discussions and studies evaluating potential environmental impacts of deep-sea mining 36activities, these focus primarily on impacts to animal biodiversity. The microscopic spectrum of 37 life on the seafloor and the services that this microbial realm provides in the deep sea are rarely 38 considered explicitly. In April 2018, a community of scientists met to define the microbial 39 ecosystem services that should be considered when assessing potential impacts of deep-sea 40 mining, and to provide recommendations for how to evaluate these services. Here we show that 41 the potential impacts of mining on microbial ecosystem services in the deep sea vary 42 substantially, from minimal expected impact to complete loss of services that cannot be remedied 43 by protected area offsets. We conclude by recommending that certain types of ecosystems should 44 be "off limits" until initial characterizations can be performed, and that baseline assessments of 45 microbial diversity, biomass, and biogeochemical function need to be considered in 46environmental impact assessments of all potential instances of deep-sea mining. 47 48 49KEYWORDS (5-8): deep-sea mining, ecosystem services, hydrothermal vents, inactive 50 sulfides, ferromanganese nodules, cobalt crusts, seamounts, marine microbiology 51 52