Changes in tolerance to herbicide toxicity throughout development stages of phototrophic biofilms

• We investigate changes in tolerance to 10 μg L−1alachlor toxicity throughout development stages of phototrophic biofilms.
• The effect of alachlor was limited to biomass reduction without changes of bacterial and diatom communities.
• Phototrophic biofilms were most strongly influenced by the temporal dynamic of microbial community development.
• The intact three-dimensional structure of the biofilm did not appear to confer protection from the effects of alachlor.
• The results confirm the influence of maturation processes or community age when investigating herbicide effects.

Ecotoxicological experiments have been performed in laboratory-scale microcosms to investigate the sensitivity of phototrophic biofilm communities to the alachlor herbicide, in relation to the stages of phototrophic biofilm maturation (age of the phototrophic biofilms) and physical structure (intact biofilm versus recolonization). The phototrophic biofilms were initially cultivated on artificial supports in a prototype rotating annular bioreactor (RAB) with Taylor–Couette type flow under constant operating conditions. Biofilms were collected after 1.6 and 4.4 weeks of culture providing biofilms with different maturation levels, and then exposed to nominal initial alachlor concentration of 10 μg L−1 in either intact or recolonized biofilms for 15 days in microcosms (mean time-weighted average concentration – TWAC of 5.52 ± 0.74 μg L−1).At the end of the exposure period, alachlor effects were monitored by a combination of biomass descriptors (ash-free dry mass – AFDM, chlorophyll a), structural molecular fingerprinting (T-RFLP), carbon utilization spectra (Biolog) and diatom species composition. We found significant effects that in terms of AFDM, alachlor inhibited growth of the intact phototrophic biofilms. No effect of alachlor was observed on diatom composition or functional and structural properties of the bacterial community regardless of whether they were intact or recolonized. The intact three-dimensional structure of the biofilm did not appear to confer protection from the effects of alachlor. Bacterial community structure and biomass level of 4.4 weeks – intact phototrophic biofilms were significantly influenced by the biofilm maturation processes rather than alachlor exposure. The diatom communities which were largely composed of mobile and colonizer life-form populations were not affected by alachlor.This study showed that the effect of alachlor (at initial concentration of 10 μg L−1 or mean TWAC of 5.52 ± 0.74 μg L−1) is mainly limited to biomass reduction without apparent changes in the ecological succession trajectories of bacterial and diatom communities and suggested that carbon utilization spectra of the biofilm are not damaged resulting. These results confirmed the importance of considering the influence of maturation processes or community age when investigating herbicide effects. This is particularly important with regard to the use of phototrophic biofilms as bio-indicators.