Amy Gartman

Evidence for the role of endosymbionts in regional-scale habitat partitioning by hydrothermal vent symbioses

Deep-sea hydrothermal vents are populated by dense communities of animals that form symbiotic associations with chemolithoautotrophic bacteria. To date, our understanding of which factors govern the distribution of host/symbiont associations (or holobionts) in nature is limited, although host physiology often is invoked. In general, the role that symbionts play in habitat utilization by vent holobionts has not been thoroughly addressed. Here we present evidence for symbiont-influenced, regional-scale niche partitioning among symbiotic gastropods (genus Alviniconcha) in the Lau Basin. We extensively surveyed Alviniconcha holobionts from four vent fields using quantitative molecular approaches, coupled to characterization of high-temperature and diffuse vent-fluid composition using gastight samplers and in situ electrochemical analyses, respectively. Phylogenetic analyses exposed cryptic host and symbiont diversity, revealing three distinct host types and three different symbiont phylotypes (one ε-proteobacteria and two γ-proteobacteria) that formed specific associations with one another. Strikingly, we observed that holobionts with ε-proteobacterial symbionts were dominant at the northern fields, whereas holobionts with γ-proteobacterial symbionts were dominant in the southern fields. This pattern of distribution corresponds to differences in the vent geochemistry that result from deep subsurface geological and geothermal processes. We posit that the symbionts, likely through differences in chemolithoautotrophic metabolism, influence niche utilization among these holobionts. The data presented here represent evidence linking symbiont type to habitat partitioning among the chemosynthetic symbioses at hydrothermal vents and illustrate the coupling between subsurface geothermal processes and niche availability.

The uptake and excretion of partially oxidized sulfur expands the repertoire of energy resources metabolized by hydrothermal vent symbioses

Symbiotic associations between animals and chemoautotrophic bacteria crowd around hydrothermal vents. In these associations, symbiotic bacteria use chemical reductants from venting fluid for the energy to support autotrophy, providing primary nutrition for the host. At vents along the Eastern Lau Spreading Center, the partially oxidized sulfur compounds (POSCs) thiosulfate and polysulfide have been detected in and around animal communities but away from venting fluid. The use of POSCs for autotrophy, as an alternative to the chemical substrates in venting fluid, could mitigate competition in these communities. To determine whether ESLC symbioses could use thiosulfate to support carbon fixation or produce POSCs during sulfide oxidation, we used high-pressure, flow-through incubations to assess the productivity of three symbiotic mollusc genera—the snails Alviniconcha spp. and Ifremeria nautilei, and the mussel Bathymodiolus brevior—when oxidizing sulfide and thiosulfate. Via the incorporation of isotopically labelled inorganic carbon, we found that the symbionts of all three genera supported autotrophy while oxidizing both sulfide and thiosulfate, though at different rates. Additionally, by concurrently measuring their effect on sulfur compounds in the aquaria with voltammetric microelectrodes, we showed that these symbioses excreted POSCs under highly sulfidic conditions, illustrating that these symbioses could represent a source for POSCs in their habitat. Furthermore, we revealed spatial disparity in the rates of carbon fixation among the animals in our incubations, which might have implications for the variability of productivity in situ. Together, these results re-shape our thinking about sulfur cycling and productivity by vent symbioses, demonstrating that thiosulfate may be an ecologically important energy source for vent symbioses and that they also likely impact the local geochemical regime through the excretion of POSCs.

An Introduction to the Major Chemical Components Released from Hydrothermal Vents

Hydrothermal fluid forms as seawater are modified through interactions with heat and the earths crust. These fluids emanate back into ocean water at hydrothermal vents on the seafloor. A major goal of the study of hydrothermal chemistry is to better constrain the impact that hydrothermal emissions have on the chemistry of the broader ocean, and how this chemistry differs between the varied vents found around the worlds oceans. Here, processes and factors affecting the chemistry of hydrothermal fluid at high-temperature black smoker vents and the surrounding lower temperature diffuse flow zones are discussed. Chemical reactions that occur at vents are also considered, including organic synthesis and the formation of metal sulfides. The relationship between metals, sulfide, depth, spreading rate, and type of host system is explored through existing data. Because of the ongoing nature of this review, figures will be updated as more hydrothermal data are collected. This will provide both a basic introduction to hydrothermal chemistry and a data-driven review of the chemistry of high-temperature hydrothermal vents.