Complex subsurface hydrothermal fluid mixing at a submarine arc volcano supports distinct and highly diverse microbial communities.

Hydrothermally active submarine volcanoes are mineral-rich biological oases contributing significantly to chemical fluxes in the deep sea, yet little is known about the microbial communities inhabiting these systems. Here we investigate the diversity of microbial life in hydrothermal deposits and their metagenomics-inferred physiology in light of the geological history and resulting hydrothermal fluid paths in the subsurface of Brothers submarine volcano north of New Zealand on the southern Kermadec arc. From metagenome-assembled genomes we identified over 90 putative bacterial and archaeal genomic families and nearly 300 previously unknown genera, many potentially endemic to this submarine volcanic environment. While magmatically influenced hydrothermal systems on the volcanic resurgent cones of Brothers volcano harbor communities of thermoacidophiles and diverse members of the superphylum "DPANN," two distinct communities are associated with the caldera wall, likely shaped by two different types of hydrothermal circulation. The communities whose phylogenetic diversity primarily aligns with that of the cone sites and magmatically influenced hydrothermal systems elsewhere are characterized predominately by anaerobic metabolisms. These populations are probably maintained by fluids with greater magmatic inputs that have interacted with different (deeper) previously altered mineral assemblages. However, proximal (a few meters distant) communities with gene-inferred aerobic, microaerophilic, and anaerobic metabolisms are likely supported by shallower seawater-dominated circulation. Furthermore, mixing of fluids from these two distinct hydrothermal circulation systems may have an underlying imprint on the high microbial phylogenomic diversity. Collectively our results highlight the importance of considering geologic evolution and history of subsurface processes in studying microbial colonization and community dynamics in volcanic environments.

Deep-sea hydrothermal vent metagenome-assembled genomes provide insight into the phylum Nanoarchaeota.

Ectosymbiotic Nanoarchaeota live on the surface of diverse archaeal hosts. Despite being broadly distributed in global geothermal systems, only three Nanoarchaeota have been successfully co-cultivated with their hosts, and until now no nanoarchaeotal cultures or genomes have been described from deep-sea hydrothermal vents. We recovered three nanoarchaeotal metagenome-assembled genomes (MAGs) from deep-sea hydrothermal vent sites at the Eastern Lau Spreading Center (M10-121), Guaymas Basin (Gua-46) and the Mid-Cayman Rise (MC-1). Based on average amino acid identity analysis, M10-121 is a novel species in the candidate genus Nanoclepta, while the other two MAGs represent novel genera in the Nanoarchaeota. Like previously sequenced Nanoarchaeota, each MAG encodes at least one split protein-coding gene. The MAGs also contain a mosaic of key nanoarchaeotal features, including CRISPR repeat regions and marker genes for gluconeogenesis and archaeal flagella. MC-1 also encodes the pentose bisphosphate pathway, which may allow the nanoarchaeote to bypass several steps in glycolysis and produce ATP.

A new symbiotic nanoarchaeote (Candidatus Nanoclepta minutus) and its host (Zestosphaera tikiterensis gen. nov., sp. nov.) from a New Zealand hot spring.

Three thermophilic Nanoarchaeota-Crenarchaeota symbiotic systems have been described. We obtained another stable anaerobic enrichment culture at 80°C, pH 6.0 from a New Zealand hot spring. The nanoarchaeote (Ncl-1) and its host (NZ3T) were isolated in co-culture and their genomes assembled. The small (∼200nm) flagellated cocci were often attached to larger cocci. Based on 16S rRNA gene similarity (88.4%) and average amino acid identity (52%), Ncl-1 is closely related to Candidatus Nanopusillus acidilobi. Their genomes both encode for archaeal flagella and partial glycolysis and gluconeogenesis pathways, but lack ATP synthase genes. Like Nanoarchaeum equitans, Ncl-1 has a CRISPR-Cas system. Ncl-1 also relies on its crenarchaeotal host for most of its biosynthetic needs. The host NZ3T was isolated and grows on proteinaceous substrates but not on sugars, alcohols, or fatty acids. NZ3T requires thiosulfate and grows best at 82°C, pH 6.0. NZ3T is most closely related to the Desulfurococcaceae, Ignisphaera aggregans (∼92% 16S rRNA gene sequence similarity, 45% AAI). Based on phylogenetic, physiological and genomic data, Ncl-1 and NZ3T represent novel genera in the Nanoarchaeota and the Desulfurococcaceae, respectively, with the proposed names Candidatus Nanoclepta minutus and Zestosphaera tikiterensis gen. nov., sp. nov., type strain NZ3T (=DSMZ 107634T=OCM 1213T).

Comparative community gene expression analysis of Aquificales-dominated geothermal springs.

Members of Sulfurihydrogenibium are often observed as visible filamentous biomass in circumneutral hot springs and play roles in sulfur-cycling, hydrogen oxidation and iron mineralization. To gain insight into the ecophysiology of Sulfurihydrogenibium populations, we conducted preliminary metatranscriptomic analysis of three distinct thermal springs; Calcite Springs (YNP-CS) and Mammoth Springs (YNP-MHS) in Yellowstone National Park, USA, and Furnas Springs (AZ) in Azores, Portugal. Genes to which transcripts were assigned revealed commonly expressed functions among the sites, while several differences were also observed. All three sites, Sulfurihydrogenibium spp. dominate and are obtaining energy via metabolism of sulfur compounds under microaerophilic conditions. Cell motility was one of the expressed functions in two sites (YNP-CS and AZ) with slower stream flow rates and thicker well-formed biofilms. The transcripts from YNP-CS and -MHS exhibited varying levels of sequence divergence from the reference genomes and corresponding metagenomes, suggesting the presence of microdiversity among Sulfurihydrogenibium populations in situ. Conversely, the majority of the AZ transcripts were identical to the S. azorense genome. Our initial results show that the metatranscriptomes in these similar Aquificales-dominated communities can reveal community-level gene function in geochemically distinct thermal environments.

Microbial community structure of hydrothermal deposits from geochemically different vent fields along the Mid-Atlantic Ridge.

To evaluate the effects of local fluid geochemistry on microbial communities associated with active hydrothermal vent deposits, we examined the archaeal and bacterial communities of 12 samples collected from two very different vent fields: the basalt-hosted Lucky Strike (37°17'N, 32°16.3'W, depth 1600-1750 m) and the ultramafic-hosted Rainbow (36°13'N, 33°54.1'W, depth 2270-2330 m) vent fields along the Mid-Atlantic Ridge (MAR). Using multiplexed barcoded pyrosequencing of the variable region 4 (V4) of the 16S rRNA genes, we show statistically significant differences between the archaeal and bacterial communities associated with the different vent fields. Quantitative polymerase chain reaction (qPCR) assays of the functional gene diagnostic for methanogenesis (mcrA), as well as geochemical modelling to predict pore fluid chemistries within the deposits, support the pyrosequencing observations. Collectively, these results show that the less reduced, hydrogen-poor fluids at Lucky Strike limit colonization by strict anaerobes such as methanogens, and allow for hyperthermophilic microaerophiles, like Aeropyrum. In contrast, the hydrogen-rich reducing vent fluids at the ultramafic-influenced Rainbow vent field support the prevalence of methanogens and other hydrogen-oxidizing thermophiles at this site. These results demonstrate that biogeographical patterns of hydrothermal vent microorganisms are shaped in part by large scale geological and geochemical processes.