Holding poison: Retention and biodegradation of pesticides by freshwater biofilms

Biofilms play a central role in the self-cleaning capacity of freshwater ecosystems and bioremediation of chemical contaminants. In this study, we evaluated the contribution of biofilms developing on organic and inorganic substrates to the retention and degradation of pesticides in freshwater streams under controlled laboratory conditions. Experiments were conducted at two temperatures (16 and 20°C) and using a mixture of ten pesticides at three concentrations (0, 2.5 and 35 µg/L). Our results confirmed a significant contribution of biofilms to pesticide retention, as evidenced by reduced concentrations in the water column being partially more than a factor of two higher than in absence of biofilms. This is supported by pesticide-specific sorption-factors to biofilms between 0.3 and 28734. Biofilm origin had a significant effect on microbial taxonomic composition and enzyme profiles (PERMANOVA, p < 0.001), which in turn influenced interaction mechanisms and the efficiency of pesticide removal. Biofilms associated with inorganic substrates primarily functioned as sinks, retaining pesticides, whereas those associated with organic substrates predominantly acted as bioremediators, promoting pesticide degradation. In contrast, temperature and pesticide concentration had no significant effects, indicating comparatively minor influence on the retention and degradation efficiency of pesticides by biofilms under the tested conditions. Therefore, this study highlights the important role of biofilms in reducing xenobiotic concentrations in aquatic environments, with this function being resilient to temperature and pesticide concentration. Moreover, we are - to the best of our knowledge - the first to document functional differences in pesticide retention and reduction between biofilms associated to organic and inorganic substrate, respectively.