Aerobic microbial transformation of pesticides in surface water

Abstract

Aerobic transformation by microbial organisms is a dissipation process of pesticides in surface water, but the corresponding information is much less available as compared with their microbial degradation in soil. Bacteria freely floating in water or associated with suspended particles are the key microorganisms degrading pesticides; their species and populations, however, depend on sites and seasons. The various factors related to pesticide properties, experimental conditions and characteristics of surface water are involved in the complex control of microbial processes. Bottom sediment and macrophytes with associated biofilms not only act as sink for pesticides but also provide the habitat for both bacteria and fungi that degrade pesticides. More understanding of each factor is necessary to utilize laboratory biodegradation data for the refined assessment of pesticide behavior in surface water.

Pesticide behavior in modified water-sediment systems

“…7,13) Furthermore, any of a water flow, the presence of phototrophic biofilm at the interface, and the shape of sediment surface may affect the transport of pesticide between water and sediment. The pond water-sediment interface covered with microbial mat was used to examine the effect of a water flow on the diffusion of molecular oxygen.…”

“…7,13) It is frequently necessary to derive the DT 50 value of pesticide at a reference temperature from the experimentally obtained one at a different temperature, not only as the input parameter to simulate its PEC, but also for the normalization of the field data obtained at daily fluctuating temperatures. The Arrhenius relationship was generally taken for this purpose, and the activation energy (E a ) of 65.4 kJ/mol is recommended for an aerobic microbial metabolism in soil through the statistical analysis of the accumulated data.…”

“…15,27) By the way, the diurnal pH change of water in the field is known to occur under sunlight as a result of photosynthesis by aquatic algae and macrophytes, possibly leading to photosynthetically-driven alkaline hydrolysis of some pesticides. 7,13) There are many kinds of phototrophic bacteria and algae in natural water, and biofilms not only at the water-sediment interface but also on the surface of aquatic macrophytes are known to be productive environments for many types of microbes and algae. 7,13) Therefore, these organisms most likely participate in the removal of contaminants, including pesticides from the water-sediment system under illumination.…”

“…5) An aerobic mineralization study (OECD 309) is utilized alternatively to simulate the behavior of pesticide in the surface water of an open water body, and the effect of diffuse light may be examined if necessary. 6,7) The applicability of these standardized tests has also been discussed from the viewpoint of the PBT (persistent, bioaccumulative and toxic) assessment of pesticide. 8) The usage of a radio-labeled pesticide in these tests gives much information such as mass distribution, dissipation or degradation rates, profiles of metabolites, and the extent of bound formation and mineralization, but such a study is very expensive with a long experimental period of up to about 3 months.…”

“…In a sunlit environment, unique products may be formed by photolysis via rearrangement and radical reactions, 13,15) and the presence of aquatic macrophytes and phototrophic microorganisms including algae not only gives an additional phase for adsorption but also causes further metabolic degradation of the dissolved pesticide. 7,13) By the way, the transformation of pesticide proceeds in both overlying water and heterogeneous sediment, and concurrently pesticide and its degradates are transported between two phases and diffuse in pore water under adsorption/desorption with sediment.…”