Inclusion of seasonal variation in river system microbial communities and phototroph activity increases environmental relevance of laboratory chemical persistence tests

Rebecca V Southwell,Sally L Hilton,Jonathan M Pearson,Laurence H Hand,Gary D Bending

doi:10.1016/j.scitotenv.2020.139070

Rebecca V Southwell, Sally L Hilton + Show 3 more

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https://doi.org/10.1016/j.scitotenv.2020.139070

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Abstract

Regulatory tests assess crop protection product environmental fate and toxicity before approval for commercial use. Although globally applied laboratory tests can assess biodegradation, they lack environmental complexity. Microbial communities are subject to temporal and spatial variation, but there is little consideration of these microbial dynamics in the laboratory. Here, we investigated seasonal variation in the microbial composition of water and sediment from a UK river across a two-year time course and determined its effect on the outcome of water-sediment (OECD 308) and water-only (OECD 309) biodegradation tests, using the fungicide isopyrazam. These OECD tests are performed under dark conditions, so test systems incubated under non-UV light:dark cycles were also included to determine the impact on both inoculum characteristics and biodegradation. Isopyrazam degradation was faster when incubated under non-UV light at all collection times in water-sediment microcosms, suggesting that phototrophic communities can metabolise isopyrazam throughout the year. Degradation rate varied seasonally between inoculum collection times only in microcosms incubated in the light, but isopyrazam mineralisation to 14CO2 varied seasonally under both light and dark conditions, suggesting that heterotrophic communities may also play a role in degradation. Bacterial and phototroph communities varied across time, but there was no clear link between water or sediment microbial composition and variation in degradation rate. During the test period, inoculum microbial community composition changed, particularly in non-UV light incubated microcosms. Overall, we show that regulatory test outcome is not influenced by temporal variation in microbial community structure; however, biodegradation rates from higher tier studies with improved environmental realism, e.g. through addition of non-UV light, may be more variable. These data suggest that standardised OECD tests can provide a conservative estimate of pesticide persistence end points and that additional tests including non-UV light could help bridge the gap between standard tests and field studies.

Highlights

Crop protection products (CPPs) are used widely to improve crop productivity and food security (Hazell, 2002), but they have the potential to cause detrimental effects to both the environment and human health (Carter, 2000)
Both dark watersediment and dark water-only microcosms had DegT50s between 101.0 and 2960.0 days and there were no significant differences in degradation rate between the two microcosm treatments, showing that sediment addition only had an impact in illuminated microcosms
As there were no clear links between specific microbial taxa and degradation or mineralisation rates, a clear identification of the microbial species responsible for isopyrazam metabolism was not possible; these results suggest that the microbial species capable of isopyrazam metabolism were present at each collection time, and that either phototrophic taxa which contribute to degradation varied over time, or that the metabolic capability of specific microbial phototrophs differed from season to season

Summary

Introduction

Crop protection products (CPPs) are used widely to improve crop productivity and food security (Hazell, 2002), but they have the potential to cause detrimental effects to both the environment and human health (Carter, 2000). Microbial degradation is considered to be the most important process determining the environmental fate of CPPs. A number of tests have been developed by the OECD to predict degradation of CPPs and other chemicals in the environment, including OECD test 308, which determines the aerobic and anaerobic transformation of a chemical in aquatic sediment systems (OECD, 2002), and OECD test 309 which determines the aerobic mineralisation of a chemical in surface water (OECD, 2004). A number of tests have been developed by the OECD to predict degradation of CPPs and other chemicals in the environment, including OECD test 308, which determines the aerobic and anaerobic transformation of a chemical in aquatic sediment systems (OECD, 2002), and OECD test 309 which determines the aerobic mineralisation of a chemical in surface water (OECD, 2004) These tests provide advantages of consistency, high throughput, and low variability (Carpenter, 1996). Tests are conducted under laboratory conditions which lack environmental realism, and may not give an accurate assessment of the chemical fate and transformation seen in nature (Kowalczyk et al, 2015)

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