Effects of Tri-, Di- and monobutyltin on heterotrophic nitrifying bacteria from surficial estuarine sediments

Four heterotrophic nitrifying bacteria—two Bacillus spp., an unidentified gram-positive rod and a gram-negative rod tentatively identified as a Pseudomonas sp.—were isolated from surficial sediments from Boston Harbor, Massachusetts. None of the organisms produced a detectable level of NO₃^–. Nanomolar levels of tributyltin (TBT) and its degradation products dibutyltin (DBT) and monobutyltin (MBT) were examined for their effects on growth, NH₄⁺- uptake and N-oxidation by the organisms. TBT inhibited growth of all four isolates. DBT inhibited growth of Bacillus sp. SC-2 and the unidentified gram-positive rod. Growth of the Pseudomonas sp. was inhibited by exposure to 100 nM MBT. NH₄⁺uptake was inhibited, even by butyltin species that did not inhibit growth. Free NH₂OH production and NO₂^–production were inhibited by TBT but varied from being inhibited to being stimulated with different DBT or MBT exposure regimes. Nitrite production was affected differently from free NH₂OH production for Bacillus sp. SC-2 and the unidentified rod. DBT appears to be particularly toxic to NH₂OH oxidation by these four organisms. At the concentration used, TBT was more toxic than DBT or MBT. The results suggest that butyltins have the potential to affect nitrification by heterotrophic bacteria in the aquatic environment at nanomolar levels and that degradation of TBT to DBT and MBT does not necessary detoxify it.