Emergent "core communities" of microbes, meiofauna and macrofauna at hydrothermal vents.

Assessment of ecosystem health entails consideration of species interactions within and between size classes to determine their contributions to ecosystem function. Elucidating microbial involvement in these interactions requires tools to distil diverse microbial information down to relevant, manageable elements. We used covariance ratios (proportionality) between pairs of species and patterns of enrichment to identify "core communities" of likely interacting microbial (<64 µm), meiofaunal (64 µm to 1 mm) and macrofaunal (>1 mm) taxa within assemblages hosted by a foundation species, the hydrothermal vent tubeworm Ridgeia piscesae. Compared with samples from co-located hydrothermal fluids, microbial communities within R. piscesae assemblages are hotspots of taxonomic richness and are high in novelty (unclassified OTUs) and in relative abundance of Bacteroidetes. We also observed a robust temperature-driven distinction in assemblage composition above and below ~25 °C that spanned micro to macro size classes. The core high-temperature community included eight macro- and meiofaunal taxa and members of the Bacteroidetes and Epsilonbacteraeota, particularly the genera Carboxylicivirga, Nitratifractor and Arcobacter. The core low-temperature community included more meiofaunal species in addition to Alpha- and Gammaproteobacteria, and Actinobacteria. Inferred associations among high-temperature core community taxa suggest increased reliance on species interactions under more severe hydrothermal conditions. We propose refinement of species diversity to "core communities" as a tool to simplify investigations of relationships between taxonomic and functional diversity across domains and scales by narrowing the taxonomic scope.

Bacterial diversity in Fe-rich hydrothermal sediments at two South Tonga Arc submarine volcanoes.

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.

Sulfide binding in the body fluids of hydrothermal vent alvinellid polychaetes.

Dissolved H2S is a major environmental factor in hydrothermal vent ecosystems. In a study of adaptations to sulfide by alvinellid polychaetes, the sulfide-binding capacity of body fluids was examined in Paralvinella palmiformis from northeast Pacific ridges and Alvinella species from the East Pacific Rise. Sulfide concentrations in vascular blood and coelomic fluid of freshly collected animals were notably variable. Separation of P. palmiformis body-fluid components revealed that most sulfide (ca. 77%) was accumulated in the dissolved fraction. In P. palmiformis, both vascular blood and coelomic fluid could reversibly bind sulfide in vitro with a low affinity, saturating only at high dialysate concentrations (ca. 2 mmol L-1). No sulfide-binding activity was observed in the vascular blood from Alvinella species. A dissolved protein component of greater than 90 kDa appears to be involved in sulfide binding in Paralvinella, probably a vascular extracellular high-molecular-weight hemoglobin. Some sulfide may also adsorb onto a 15.38-kDa intracellular hemoglobin present in the coelomic erythrocyte fraction. In the absence of epibiotic bacteria, Paralvinella body fluids may function as a sulfide buffer to protect tissues from deleterious effects of sulfide exposure.