White Paper Title: 
The role of primary consumers in models of ecosystem processes at hydrothermal vents

Primary consumers play a critical role in ecosystem processes at deep-sea hydrothermal vents through the top-down regulation of chemoautotrophic bacteria, which produce biomass with the energy from the oxidation of reduced chemicals in hydrothermal fluids.  Morphological descriptions and geochemical analyses indicate that grazers, such as gastropod limpets, are primary consumers that feed on free-living bacteria at hydrothermal vents.  Gastropod limpets are some of the most abundant and diverse endemic vent species and can comprise as much as 85% of the heterotrophic biomass in hydrothermal vent communities.  Thus, the biomass of primary consumers reflects, to some degree, the biomass produced by free-living chemoautotrophic bacteria and could be used to constrain or validate models of net ecosystem productivity at hydrothermal vents.  

We aim to continue to resolve the source of variation and trophic dynamics at the base of the food web to refine models of energy transfer and nutrient cycling in hydrothermal vent ecosystems.  We used a combined approach of molecular identification of gut contents and tissue stable isotope analyses to examine the variation in the diets of four co-occurring gastropod limpets at five diffuse-flow hydrothermal vents at the East Pacific Rise.  Our results indicate that limpets feed on chemoautotrophic bacteria, but the diets were different among species and some diffuse-flow vents.  However, the differences in the carbon stable isotope values fit with the expected carbon fixation pathways of the different bacteria identified in the gut contents and provide new insights to explain the variation in stable isotope values among primary consumers at hydrothermal vents on other mid-ocean ridges.  The next steps will be to determine the rate of consumption (i.e. grazing) and the proportion of the diet formed by primary producers (i.e. chemoautotrophic bacteria).  Together with measurements of the flow and composition of hydrothermal fluids and the characterization of microbial communities, a better understanding of primary consumers will further improve models of ecosystem processes at hydrothermal vents.