DEGRADATION OF BACTERIA BY MYTILUS-EDULIS

The uptake of several species of bacteria by the common mussel M. edulis (L.) and the subsequent fate of some polymers of the bacteria were investigated in a study carried out during 1981. Bacteria (Escherichia coli, Micrococcus luteus, M. roseus, Bacillus cereus, Staphylococcus aureus and a marine pseudomonad, 1-1-1) were radiolabeled by growth in medium containing 3H-thymidine and the uptake of bacteria by Mytilus edulis was monitored. Labeled and unlabeled bacteria, at initial concentrations of 0.5 to 1 .times. 107 bacteria ml-1, were cleared at similar, exponential rates with no significant difference in the rates for different bacteria; 90% of bacteria were cleared in a mean time of 1.93 .+-. 0.12 h (SEM, n = 63). Those bacteria with cell walls which were sensitive to M. edulis lysozyme were rapidly degraded by the mussel and 3H-labeled DNA was released in a form not precipitable by 10% trichloroacetic acid. Lysozyme-resistant bacteria (Micrococcus roseus and S. aureus) were cleared from suspension by Mytilus edulis but most were rejected intact. By measuring the rate of release of 3H-thymidine-labeled material from the mussel the rate of degradation of lysozyme-sensitive bacteria by M. edulis was found. For different bacteria the degradation rate varied from .apprx. 2 .times. 108 to 27 .times. 108 bacteria h-1 with an overall mean of 10 .times. 108 bacteria h-1. A thymidine- and diaminopimelic acid-requiring auxotroph of E. coli was radiolabeled with 3H-thymidine, 3H-diaminopimelic acid or 14C-glucose and fed to M. edulis. Bacteria were cleared and degraded by the mussel; 3H-diaminopimelic acid-labeled or 14C-glucose-labeled polymers were retained; 3H-thymidine-labeled polymers were released into the surrounding water. Extracts of the digestive gland of M. edulis degraded lysozyme-sensitive bacteria to release 3H-thymidine-labeled material, but did not release 3H-thymidine-labeled material from lysozyme-resistant bacteria. M. edulis can select lysozyme-sensitive bacteria for subsequent processing and discriminate between bacterial polymers to reject DNA. Also, bacteria could provide a substantial fraction of the C requirement of the mussel.