Factors involved in the (near) anoxic survival time of Cerastoderma edule: associated bacteria vs. endogenous fuel

The effect of several antibiotics, molybdate and hydrogen sulfide was tested on anoxic tolerance of the cockle Cerastoderma edule, as well as utilisation of glycogen. The aim was to evaluate the role of fuel depletion and growth of bacteria as a cause of mortality. The exponential increase of sulfide and ammonium occurred in anoxic natural seawater incubations and to a lesser extend in artificial, sulfate free, seawater. This could be strongly decreased by antibacterial agents, which led to improved survival time by approximately two-fold. Molybdate suppressed sulfide formation also, but did not affect survival time. Exogenous sulfide showed a negative effect on survival time at pH 6.8 and induced stronger accumulation of free glucose, D-lactate and L-alanine. This was not the case at pH 8.2. Fifty percent (LT50) of cockles in anoxic seawater died after 3.5 days still with half the initial glycogen concentration present. However, in the presence of chloramphenicol (LT50 7.9 days), the cockles utilised their endogenous fuel almost completely. In both incubations there was initially a strong increase of D-lactate and L-alanine. The D-lactate levels subsequently decreased again, probably due to bacterial consumption. This study strongly indicates that in anoxic closed systems, infection by pathogenic bacteria is the first cause of death and not exhaustion of endogenous fuel depots.