A temperate bivalve-seagrass symbiosis mediated by sulfide-oxidizing bacteria

Background/Question/Methods

Sulfide accumulation in sediments may hinder the growth and survival of seagrasses, especially in concert with other stressors such as high temperature, low light, or low oxygen. The metabolic activities of infaunal bivalves that host sulfur-oxidizing chemoautotrophic bacteria in their gills are one mechanism by which sulfide stress may be reduced within coastal seagrass beds. While several studies have addressed this potential role for lucinid clams in subtropical or tropical seagrass beds, similar investigations have not been conducted for northern temperate eelgrass (Zostera marina) and the common Atlantic awning clam (Solemya velum). We hypothesized that these two species are facultatively mutualistic: eelgrass benefits because S. velum reduces potentially phytotoxic sulfide concentrations in sediment, and S. velum benefits because high levels of organic matter and sulfate reduction in eelgrass sediments provide ample energy supplies for S. velum’s chemosymbionts. In pilot tests of our hypothesis, we conducted two mesocosm studies in a two-way factorial design based on the presence or absence of eelgrass and S. velum in treatments. We monitored sediment porewater sulfide concentrations, measured eelgrass photosynthetic efficiency, and used several measures of morphometry and biomass for both species to compare the condition of eelgrass and S. velum cultured separately or together.

Results/Conclusions

We found that sediment porewater sulfide concentrations were significantly reduced and eelgrass was similarly or slightly less photosynthetically stressed in mesocosms with S. velum. However, our data also suggest greater aboveground eelgrass productivity in the summer trial in treatments with S. velum but lesser productivity in the fall trial under the same conditions. In addition, eelgrass appeared to negatively affect S. velum condition in the fall trial. We suggest that the relationship between eelgrass and S. velum is not mutualistic when based solely on sulfide oxidation, although we do not exclude the possibility that eelgrass provides other benefits (e.g. predation refuge) to S. velum in the field. We also tentatively conclude that, in the fall trial, reduced sediment sulfide concentrations in treatments with S. velum allowed eelgrass to allocate more resources to belowground biomass in preparation for overwintering, through either reduced sulfur stress or simple resemblance of sediment sulfide concentrations to typical winter values. Thus, these experiments provide equivocal evidence that S. velum could be usefully incorporated into the design of ongoing restoration efforts in temperate eelgrass beds as a facilitator for eelgrass growth. Upcoming work will assess the consistency of the short-term patterns observed this year.