Seasonal Dynamics of Bacterial Population Degrading Dimethylarsenic Acid in Lake Kahokugata

The degradation processes of organoarsenic compounds in aquatic environments would depend on the bioactivities of microorganisms and significantly influence the cycles of the arsenic species. The concentrations of arsenic species were determined in Lake Kahokugata. During the all season, the inorganic arsenic was detected, while the dimethylarsinic acid (DMAA) appeared in only winter seasons, and MMAA was not detected. Moreover, when DMAA was added to the water samples of Lake Kahokugata collected at every month from June in 2005 to November in 2006, the DMAA in the sample water of all seasons decreased and was converted to inorganic arsenic until the 28th day of incubation. According to the Most Probable Number (MPN) procedure, the DMAA-degrading bacteria were detected at cell densities ranged from 120 cells/mL to 2100 cells/mL during the all season. These facts suggested that the lake water of all season in Lake Kahokugata possessed the microbial activities for DMAA decomposition. To determine the composition of DMAA-degrading bacteria, the total 352 isolates obtained as dominated bacterial species were analyzed by the restriction-fragment-length polymorphism (RFLP) analysis of 16S rDNA genes. As a result, total 352 isolates were classified into 10 types of which the rates of total isolate numbers indicated seasonal changes. The diverse compositions of DMAA-degrading bacteria would seasonally change and control the organoarsenic degradation contributing to the seasonal arsenic cycles in Kahokugata.