Effects of global change on the ability of stream biofilm to dissipate the herbicide glyphosate.

The herbicide glyphosate is contaminating a large number of freshwater ecosystems worldwide and its fate and effects remains uncertain in light of the effects of global change. The present study examines how variations in water temperature and light availability relative to global change affect the ability of stream biofilms to degrade the herbicide glyphosate. Biofilms were exposed in microcosms to two levels of water temperature simulating global warming (Ambient = 19-22 °C and Warm = 21-24 °C) and three levels of light representative of riparian habitat destruction due to land use change (Dark = 0, Intermediate = 600, High = 1200 µmol photons m-2 s-1). Biofilms were acclimated to six different experimental treatments, namely i) ambient temperature without light (AMB_D), ii) ambient temperature and intermediate light (AMB_IL), iii) ambient temperature and high light (AMB_HL), iv) warm temperature without light (WARM_D), v) warm temperature and intermediate light (WARM_IL) and vi) warm temperature and high light (WARM_HL). The ability of biofilms to degrade 50 µg L-1 of glyphosate was tested. Results showed that water temperature increase, but not light availability increase, significantly increased aminomethyl phosphonic acid (AMPA) production by biofilms. However, the combined increase of temperature and light generated the shortest time to dissipate half of the glyphosate supplied and/or half of the maximum AMPA produced (6.4 and 5.4 days, respectively) by biofilms. Despite light had a major effect in modulating biofilm structural and functional descriptors, the response of certain descriptors (i. e. chlorophyll-a concentration, bacterial density and diversity, nutrient content and PHO activity) to light availability increase depended on water temperature. Specifically, the biofilms in the WARM_HL treatment displayed the highest Glucosidase: Peptidase and Glucosidase: Phosphatase enzyme activity ratios and the lowest biomass C: N molar ratios compared to the other treatments. According to these results, warmer temperatures and high light availability could have been exacerbating the decomposition of organic C compounds in biofilms, including the use of glyphosate as a C source for microbial heterotrophs. This study shows that ecoenzymatic stoichiometry and xenobiotic biodegradation approaches can be combined to better understand the functioning of biofilms in pesticide-polluted streams.

Dissipation of pesticides by stream biofilms is influenced by hydrological histories.

To evaluate the effects of hydrological variability on pesticide dissipation capacity by stream biofilms, we conducted a microcosm study. We exposed biofilms to short and frequent droughts (daily frequency), long and less frequent droughts (weekly frequency) and permanently immersed controls, prior to test their capacities to dissipate a cocktail of pesticides composed of tebuconazole, terbuthylazine, imidacloprid, glyphosate and its metabolite aminomethylphosphonic acid. A range of structural and functional descriptors of biofilms (algal and bacterial biomass, extracellular polymeric matrix (EPS) concentration, microbial respiration, phosphorus uptake and community-level physiological profiles) were measured to assess drought effects. In addition, various parameters were measured to characterise the dynamics of pesticide dissipation by biofilms in the different hydrological treatments (% dissipation, peak asymmetry, bioconcentration factor, among others). Results showed higher pesticide dissipation rates in biofilms exposed to short and frequent droughts, despite of their lower biomass and EPS concentration, compared to biofilms in immersed controls or exposed to long and less frequent droughts. High accumulation of hydrophobic pesticides (tebuconazole and terbuthylazine) was measured in biofilms despite the short exposure time (few minutes) in our open-flow microcosm approach. This research demonstrated the stream biofilms capacity to adsorb hydrophobic pesticides even in stressed drought environments.