Microbial utilization of the neurotoxin domoic acid: blue mussels (Mytilus edulis) and soft shell clams (Mya arenaria) as sources of the microorganisms

Abstract

The neurotoxin domoic acid is produced in quantity by the diatom Pseudo-nitzschia multiseries and is released to the environment directly and indirectly via food chains. Presumably there is a mechanism for the biodegradation and disposal of domoic acid and as bacteria are logical candidates for such an activity, a search for bacteria competent to carry out biodegradation of domoic acid was initiated. Extensive trials with a wide variety of bacteria isolated mainly from muds and waters taken from the marine environment showed that the ability to grow on or degrade domoic acid was rare; in fact, domoic acid was inhibitory to resting cells or growing cultures of most of these bacteria. In contrast, using enrichment techniques, it was possible to isolate from molluscan species that eliminate domoic acid readily, i.e., blue mussels (Mytilus edulis) and soft-shell clams (Mya arenaria), bacteria that exhibited growth with and biodegradation of domoic acid when supplemented with low concentrations of growth factors. The species that retain domoic acid for lengthy periods, such as sea scallops (Placopecten magellanicus) and red mussels (Modiolus modiolus), only occasionally yielded bacteria with this capability. The differences may be a result of the mechanisms used by the different shellfish in dealing with domoic acid, i.e., freely available in the blue mussels and soft-shell clams but likely sequestered in the digestive glands of sea scallops and red mussels and thus, largely unavailable for bacterial utilization. The results show that Mytilus edulis and Mya arenaria, almost uniquely, are prime and reliable sources of domoic acid utilizing bacteria. These findings suggest a strong possibility that autochthonous bacteria may be significant factors in the elimination of the neurotoxin in these two species of shellfish.