Differences in susceptibility of marine bacterial communities to metal pyrithiones, their degradation compounds and organotin antifouling biocides

Abstract

The susceptibility of marine bacterial communities to copper pyrithione (CuPT2), zinc pyrithione (ZnPT2) and their degradation product is described and toxicities of these relatively new antifouling biocides compared with those of their harmful organotin (OT) predecessors, tributyltin (TBT) and triphenyltin (TPT). These biocides were added to agar at concentrations of 0, 0.01, 0.1, 1 and 10 mg l-1 and coastal seawater including indigenous bacteria added to each batch of agar solution. The number of bacterial colony forming units (CFU) was measured after 7 days culture. Relative CFU (as a percentage of control) was more than 80% at a concentration of 0.01 mg l-1 of each compound, except for TBT. Relative CFU decreased as a function of dose of each biocide, although concentration-dependent changes in rate of CFU were relatively low during exposure to degradation products of CuPT2 and ZnPT2, pyridine N-oxide (PO) and pyridine-2-sulphonic acid (PSA). Based on comparisons of EC50, TBT was the most bacterio-toxic of the tested compounds (0.2 mg l-1), marginally more so than CuPT2 (0.3 mg l-1). Interestingly, EC50 values of degradation products of CuPT2 and ZnPT2, 2-mercaptopyridine N-oxide (HPT) and 2,2′-dithio-bispyridine N-oxide (PT2) were 0.8 and 0.5 mg l-1, respectively, lower than that of the parent chemical, ZnPT2 (1.4 mg l-1). The EC50 of PT2 was also lower than that of TPT (0.7 mg l-1), implying higher toxicity. Given the overlapping toxicity ranges, these results suggest that marine bacterial communities experience comparably high susceptibility to metal PTs and OTs during their life history.