Anoxic survival of Macoma balthica: the effect of antibiotics, molybdate and sulphide.

In anoxic semi-closed systems, the survival time of the clam Macoma balthica was compared to clams which were incubated in the presence of several antibiotics (chloramphenicol, 5-oxytetracycline hydrochloride, penicillin, streptomycin, a mix of penicillin and streptomycin and a mix of chloramphenicol, polymyxin, neomycin and penicillin), sulphide and chloramphenicol at pH 6.8 and 8.2 and molybdate (specific inhibitor of the process of sulphate reduction). The aim was to detect maximum survival times of this clam and indications for the cause of mortality under the conditions tested. Median survival time (LT(50)) of the clam was 4.8 days (at 19 degrees C) in incubations without any addition. Added sulphide (200 µM) decreased survival time. At pH 8.2, LT(50) decreased by 20.8% and at pH 6.8 by 35.2%. However, added molybdate, which suppressed biotic sulphide formation, did not improve survival time (LT(50)=4.4 days). Biotic sulphide probably did not speed up mortality rate, but indicated excessive growth of sulphate reducing bacteria once mortality started. The presence of different antibiotics increased significantly survival time (LT(50)) from 8.9 to 14.9 days. Qualitative estimations were made of the numbers of bacteria present in the systems. Compared to a seawater control, highest numbers were observed in the incubation of clams without additions and in the presence of molybdate. Nevertheless, due to the presence of molybdate, bacteria numbers were significantly lower. However, very low numbers of bacteria were observed in the incubations of clams in the presence of chloramphenicol. These data demonstrated that the presence and proliferation of bacteria was probably the cause of death of the clams.