Seasonal dynamics of dimethylarsinic-acid-decomposing bacteria dominating in Lake Kahokugata

Abstract

Decomposition processes of organoarsenic compounds significantly influence arsenic cycles in aquatic environments, and such processes depend on bacterial activity. However, the bacterial characteristics in these environments are obscure. Accordingly, we observed seasonal variations of arsenic species and the bacterial population decomposing dimethylarsinic acid (DMAA) in Lake Kahokugata from April 2002 to January 2003. Monitoring of bacterial biomass involving DMAA decomposition using the most probable number procedure showed that the bacterial cell densities ranged from 36 to 3600 ml -1 . On the other hand, methylated arsenic was not detected during the experimental period, although the inorganic arsenic concentration was over 4 nM. This suggests that bacteria remineralized methylated arsenic species to inorganic arsenic. Furthermore, the composition of bacterial communities involving DMAA decomposition was examined by restriction-fragment-length polymorphism analysis of the 16S rDNA nucleotide. As a result, a total of 49 isolates were classified into 10 type groups, and 32 of these isolates belonged to three dominant type groups. Phylogenetic analysis using 16S rDNA partial sequences (ca 320 bp) suggests that the representative isolates of the dominant type groups are specific to the summer or winter season. Moreover, as a result of the culture experiments to examine DMAA decomposition activity, the representative isolates decomposed 1 μM DMAA at a decomposition percentage of below 80%. In conclusion, some bacterial communities in a specific season can decompose DMAA to varying degrees, contributing to the annual cycle of arsenic species.