Assessing environmental persistence: balancing pragmatism with realism
Gary Bending 1 , Agnieszka Kowalczyk 1 , Hendrik Schafer 1 , Chris Finnegan 2 , Roger van Egmond 2 , Oliver Price 2 1 School of Life Sciences, University of Warwick, UK; 2 Unilever Safety and Environment Centre, UK
Standardised laboratory degradation tests provide data which is used to determine chemical behaviour and risk in the environment. The advantage of laboratory tests is that they are conducted with defined environmental substrates under standardised conditions, which generally makes them reproducible, and ensures that test results are readily understood in terms of their regulatory significance. However, use of laboratory test systems inevitably results in loss of complexity, and the extrapolation of laboratory test results to the natural environment can be problematic. For most chemicals, biodegradation represents the major route for dissipation in the environment. A variety of factors which affect pollutant bioavailability and microbial community diversity and functioning differ between natural systems and the laboratory, and could affect biodegradation rates. These include chemical concentration, scale, light / dark cycles, redox and temperature variation and interactions between the water column and sediment. Furthermore there can be great variability in the physico-chemical and biological properties of materials within and between environmental compartments (e.g. environmental heterogeneity) which could affect test outcome. The results of work which investigated the effect of adding complexity and greater environmental realism to degradation screening tests was presented. River biofilms generated on glass slides were found to provide greater inoculum density than unconcentrated river water while preserving diversity. It was shown that bacterial diversity in biofilms and river water showed seasonal variation, and that this was a greater determinant of bacterial community composition than proximity to the outflow of a sewage treatment plant (STP). River water collected from the STP outflow showed more consistent degradation of p-nitrophenol than water collected from upstream and downstream of the STP. River biofilms provided similar rates of biodegradation to river water, despite the larger amounts of biomass applied in degradation assays. In a number of river water samples, biodegradation of p-nitrophenol did not occur. This could not be attributed to reduced biomass or bacterial diversity in these samples. Furthermore quantitative PCR showed that these samples contained genes in the biodegradative pathway of p-nitrophenol, indicating that factors controlling bacterial proliferation, rather than absence of catabolic potential was responsible for the lack of biodegradation. Introduction of natural light to river water biodegradation tests with p-nitrophenol resulted in the inhibition of biodegradation. This was shown to result from growth of algae, which increased pH, preventing growth of degraders. Similarly use of p-nitrophenol concentrations at levels approaching those found in the environment resulted in reduced biodegradation rates, and at the lowest concentrations, variable results between replicates, including the introduction of test failures. The addition of complexity into test systems may therefore affect the outcome of biodegradation tests in a manner which is hard to predict.