Influence of incubation conditions on the anoxic survival of marine bivalves. Static and semi-static incubations

In a comparative study of 4 bivalve species we show that the apparent widel y different tolerances in survival time observed in a closed system filled with N-2-gassed seawater is mainly due to the experimental conditions. Both a high dose of cadmium and the antibiotic chloramphenicol increase surviva l time 2- to 4-fold. Without precautions for bacterial growth, the survival time of the most tolerant species, Scapharca inaequivalvis, is about 4 tim es longer than that of the most sensitive species, Venus gallina. The other bivalves Tapes philippinarum and Mytilus galloprovincialis, have intermedi ate survival times. When proliferation of bacteria is prevented by chloramp henicol, the survival time of S, inaequivalvis remains the highest; however under these conditions the other 3 species show similar survival times. Ba cteria cause a drop in pH of the incubation medium and the accumulation of sulphide. S. inaequivalvis exhibits a significant decrease in survival time in the presence of exogenous sulphide. The antibiotic postpones the accumu lation of sulphide, but a significant population of sulphate-reducing bacte ria is able to survive in its presence. In the case of V; gallina and M. ga lloprovincialis, exogenous sulphide (up to 400 muM) does not affect the imp roved survival time due to the antibiotic. This holds also for a decrease i n pH. Bacterial toxins other than sulphide or direct action on the integrit y of the organism must be responsible for the adverse effects on survival. The responsible micro-organisms are associated with the bivalves, and anoxi a induces their proliferation. For this reason, accumulation of bacterial n oxious products cannot be avoided by frequent renewal of the medium.