Role and Behavior of Benthic Microbes Able to Degrade Herbicide Atrazine in Naturally Derived Water/sediment Microcosm

Abstract

In a simple laboratory microcosm constituted of riverbed sediment and its associated water, rapid degradation of herbicide atrazine mainly to cyanuric acid occurred through microbial degradation. To elucidate the role and behavior of microbes involved in atrazine degradation, comparative microcosms constituted of different sediment/water conditions and constitutions were employed. Fluorescence microscopic observation and direct bacterial count by DAPI (4',6-diamidino-2-phenylindole)-staining and CFU (colony forming unit) count by the plate method were also conducted to supplement the comparative study. As a result, planktonic microbes in the aqueous phase appeared not to be responsible for the initial degradation of atrazine, at least during the present experimental period of 70 days. The sediment was found to be important as a source of degrading microbes. However, once the degradation was initiated, the degrading activity in the aqueous phase was maintained without the sediment. Although CFU in aqueous phase increased with time, this increase in cell number seemed not to reflect the degrading activity. Large, glittering and whole particles, supposed to be microbial cells, were observed by DAPI-staining in the aqueous phase under a microscopic field only when active atrazine-degradation occurred. Therefore, they might be involved in the degrading activity.