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

In anoxic semi-closed systems, the survival time of the clam Macoma balthic a was compared to clams which were incubated in the presence of several ant ibiotics (chloramphenicol, 5-oxytetracycline hydrochloride, penicillin, str eptomycin. a mix of penicillin and streptomycin and a mix of chloramphenico l, polymyxin, neomycin and penicillin), sulphide and chloramphenicol at pH 6.8 and 8.2 and molybdate (specific inhibitor of the process of sulphate re duction). The aim was to detect maximum survival times of this clam and ind ications for the cause of mortality under the conditions tested. Median sur vival time (LT50) of the clam was: 4.8 days (at 19 degreesC) in incubations without any addition. Added sulphide (200 muM) decreased survival time. At pH 8.2, LT50 decreased by 20.8% and at pH 6.8 by 35.2%. However. added mol ybdate, which suppressed biotic sulphide formation, did not improve surviva l time (LT50 = 4.4 days). Biotic sulphide probably did not speed up mortali ty rate, but indicated excessive growth of sulphate reducing bacteria once mortality started. The presence of different antibiotics increased signific antly survival time (LT50) from 8.9 to 14.9 days. Qualitative estimations w ere made of the numbers of bacteria present in the systems. Compared to a s eawater control, highest numbers were observed in the incubation of clams w ithout 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. (C) 2001 Elsevier Science B.V. All rights reserved.