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

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 14 CO 2 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.

Linking biodegradation kinetics, microbial composition and test temperature – Testing 40 petroleum hydrocarbons using inocula collected in winter and summer

“…A possible explanation for this observation is the reduced microbial diversity in the laboratory tests compared to eld conditions. 19 Southwell et al 20 found that both the season of collection and incubation affected the microbial community composition, in a setup where biodegradation was investigated in the OECD 308 and 309 standard tests using inoculum from the same site eight times during a two year period. However, the microbial changes could not clearly be linked to degradation potential of the pesticide isopyrazam.…”

“…However, the microbial changes could not clearly be linked to degradation potential of the pesticide isopyrazam. 20 The inoculum used in laboratory biodegradation tests is oen only characterized based on microbial density (e.g. CFU per mL), whereas the initial microbial composition or its dynamics during tests are rarely investigated.…”

Separation of Photochemical and Non-Photochemical Diurnal In-Stream Attenuation of Micropollutants

“…Biotransformation could also cause a diurnal pattern, for instance, by differences in the temperature 27 and water chemistry, for example, pH or a lightdependent metabolic activity. 28 To determine for which TrOCs diurnal attenuation, expressed as a correlation with k att,DIC , can be explained mostly by a photochemical process, k att,TrOC can be expressed as the sum of rate constants of a diurnal photochemical process k photo and a time-independent rate constant (k night ), that is identical during day and night:…”

“…Second, the diurnal pattern could be caused by other reactive processes besides photochemistry such as biotransformation. Biotransformation could also cause a diurnal pattern, for instance, by differences in the temperature and water chemistry, for example, pH or a light-dependent metabolic activity …”

Moving persistence assessments into the 21st century: A role for weight‐of‐evidence and overall persistence

“…Light availability controls processes such as photodegradation or photooxidation. It can also affect the microbial population by encouraging the growth of photosynthesizers and increasing the organic content of a system (Pastore et al, 2018 ; Southwell et al, 2020 ).…”