Habitat characterization and nutritional strategies of the endosymbiont-bearing bivalve Lucinoma aequizonata

Abstract

A population of the lucinid bivalve Lucinorna aequizonata, with sulfur-oxidizing endosym-biotic bacteria in the gills, is restricted to a narrow depth range (500-f-10m) on the slope of the Santa Barbara Basin, California, USA. In this zone, the seawater just above the substratum is sub-oxic ([ 0 2 ] < 2 0 F M). The organically rich sediments in which these clams live are well mixed by bioturbation, which appears to maintain a redox condition Limiting the extensive accumulation of hydrogen sulfide. However, thiol levels in the blood of the clams indicate an exposure to significant amounts of sulfide and/or thiosulfate apparently from randomly &spersed short-lived pockets of sulfidic mud that can b e reached by the clam's burrowing vermiform foot. When the bivalves are incubated in the presence of hydrogen sulfide, thiosulfate is concentrated in the blood and apparently utilized by the bacteria for metabohc energy and the production of intracellular elemental sulfur. Laboratory growth experiments demonstrated that sulfide concentrations greater than 10 F M were detrimental to the host, even though the bacteria continued to accumulate elemental sulfur. The utilization of thiosulfate under near anaerobic conditions and the accumulation of intracellular elemental sulfur by the endosymbiotic bacteria coupled with the high availability of environmental nitrate and low molecular oxygen suggests a metabolic strategy analogous to the free-living sulfur oxidizer Thlobacillus denjtrificans. The 6 ' 3 ~ values of the purified endosymbiont bacteria (-34.0 5 0.8%) were significantly lighter than those of the host tissue (-29.0 + 0.7 %) suggesting that in addition to the nutrition provided by the bacteria, over 25 % of the host carbon may b e attributed to exogenous dissolved carbon which is in high concentrations in its natural habitat.