ORGANOTIN COMPOUNDS AND AQUATIC BACTERIA - A REVIEW

Organotins are toxic to microorganisms. Trisubstituted organotins (R(3)SnX) are considered more toxic than disubstituted (R(2)SnX(2)) or monosubstituted (RSnX(3)) compounds, and tetrasubstituted compounds (R(4)Sn) are not considered toxic. In the R(3)Sn series propyl-, butyl-, pentyl-, phenyl- and cyclohexyltins are the most toxic to microorganisms. Toxicity towards aerobes in the R(3)Sn series is related to total molecular surface area and to the octanol:water partition coefficient Kow, which is a measure of hydrophobicity. Care must be taken when testing the toxicity of tin compounds in the laboratory, for a number of biological, chemical and physical factors can influence the apparent toxicity. Although TBT is generally the most toxic of the butyltins, there are instances where monobutyltin (MBT) is as toxic, or more toxic, than TBT to microorganisms. Thus, debutylation in the sequence TBT --> DBT --> MBT --> Sn does not detoxify TBT for all microorganisms. Some microorganisms can methylate inorganic or organic tins under aerobic or anaerobic conditions. Methylation can also occur by chemical means and the relative contributions of biotic and abiotic mechanisms are not clear. It is difficult to isolate a pure culture which can methylate tin compounds aerobically, and it is difficult to isolate a pure culture which degrades TBT, suggesting that microbial consortiums may be involved in transformations of organotins in the aquatic environment. Methylation and debutylation alter the adsorbtivity and solubility of tin compounds; thus, microorganisms can influence the environmental mobility of tin. TBT-resistant microorganisms can be isolated, and in some of them resistance to TBT can be plasmid-mediated.