Variation in the diets of hydrothermal vent grazers reveals different sources of chemosynthetic primary production in a diffuse-flow community

B. Govenar¹* & T.M Shank¹

Corresponding author: bgovenar@whoi.edu
¹Woods Hole Oceanographic Institution, Biology Department, Woods Hole, MA 02543

Abstract:
Trophic relationships and food webs are fundamental to ecological processes that regulate community dynamics and ecosystem function. We employed a combination of molecular genetic techniques and stable isotope analyses to examine the variation in the diets of four species of gastropod limpets, Lepetodrilus elevatus, L. pustulosus, L. ovalis and Eulepetopsis vitrea, collected from a aggregation of bathymodiolin mussels at a diffuse-flow hydrothermal vent site on the East Pacific Rise. The ¹⁵N values for all individuals were < 11 ‰, as expected for endemic hydrothermal vent fauna. However, the ¹³C values ranged from -24 to -8 ‰, suggesting more than one source of local primary production for these functionally similar species. The two gastropod species, L. elevatus and L. pustulosus that had gut contents that matched only with epsilon-proteobacteria also had ¹³C values that were the most enriched in ¹³C. The gastropod E. vitrea had gut contents that matched with alpha-proteobacteria also had ¹³C values that were the least enriched in ¹³C. The ¹³C values of these grazers reflect the presumed carbon-fixation pathway of the consumed bacteria. In hydrothermal vent ecosystems, the dominant carbon-fixation pathways of chemoautotrophic bacteria are the reductive tricarboxylic acid (rTCA) cycle and the Calvin-Benson-Bassham (CBB) cycle. All autotrophic epsilon-proteobacteria studied to date use the rTCA cycle for carbon fixation, rather than the CBB cycle, which is used by most autotrophic gamma- and alpha-proteobacteria. Differences in the isotopic fractionation during carbon fixation via the rTCA cycle is considerably less than the CBB cycle and would result in biomass enriched in ¹³C relative to biomass produced via the CBB cycle. Results of this work indicate that hydrothermal vent grazers feed on different bacteria and that resource partitioning may facilitate species coexistence and contribute to the maintenance of species diversity in hydrothermal vent communities.

Contributions to Integration and Synthesis:
The transfer of energy in hydrothermal vent ecosystems is a primary theme for the Ridge 2000 Program and multi-disciplinary integrated studies at mid-ocean ridges. At hydrothermal vents, chemoautotrophic bacteria that use the energy from the oxidation of reduced chemicals to fix inorganic carbon form the base of the food web. With the exception of a few species that harbor chemoautotropic bacterial symbionts, most of the vent-endemic macrofauna are heterotrophs that feed on free-living bacteria, protists, and other invertebrates. We used a combined approach of stable isotope analyses and molecular identification of gut contents has to identify the source of the variation in the diets of hydrothermal vent grazers. Building on the past two decades of ecological research of the spatial and temporal variability in the structure of macrofaunal communities at the East Pacific Rise, results of this work will enable us to develop models of energy transfer through food webs in hydrothermal vent communities, by integrating with additional data that is being collected and analyzed by other researchers, including the δ¹³C of the inorganic carbon in diffuse hydrothermal fluids, the diversity of Archaea and Eubacteria, the diversity of protists, and the community composition of the meiofauna.