Energy acquisition and allocation In vesicomyid clam symbioses

Shana K.Goffredi, Ph.D.
Monterey Bay Aquarium Research Institute

Wednesday, May 30, 2001
3:00 p.m.—Pacific Forum

Symbioses involving sulfide-oxidizing bacteria and various metazoan phyla dominate megafaunal assemblages at cold seeps and hydrothermal vents worldwide. Vesicomyid clams are often the dominant chemosynthetic megafauna of seep and vent environments and, due to their size and abundance, are integral components of these deep-sea benthic communities. Although much progress has been made toward understanding basic biological processes enabling species to inhabit the deep sea, many aspects of the physiological ecology, diversity, and community dynamics of these organisms remain poorly understood.

The predominant species found living at cold seeps in Monterey Bay are the vesicomyid clams Calyptogena kilmeri and C. pacifica. The growth and survival of these clams depend directly upon the productivity of their chemoautotrophic endosymbionts, which is fueled by the oxidation of sulfide. For this reason, sulfide (energy) availability and sulfide physiology are thought to constrain symbiont and host production. Here we describe research concerning the productivity of two common clam species in relation to sulfide-related environmental and physiological parameters. Both of these species inhabit sulfide-rich sediments and depend nutritionally on their symbionts, however, many aspects of their life styles differ considerably. Results indicate that C. pacifica is physiologically poised for the uptake and transport of sulfide, as measured by increased sulfide consumption rates, sulfide binding ability, and internal sulfide levels, as well as symbiont energy turnover, as measured by sulfide oxidation potential, sulfur metabolism enzymes, and bacterial densities.

Growth rates of C. pacifica, however, are considerably slower than C. kilmeri (3% vs. 15% per year). This is surprising given that C. pacifica possesses a seemingly greater potential to process sulfide. This contradicts the idea that sulfide limits productivity in these two systems and, for this reason, we believe them to be constrained by factors other than sulfide. This research represents the first comparative investigation of the physiological functioning of closely-related species in chemosynthetic symbioses and elucidates the constraints and advantages posed by different modes of sulfide (energy) uptake and assimilation in these, and other, symbiotic organisms.