Trophic transfer of microcystins through the lake pelagic food web: evidence for the role of zooplankton as a vector in fish contamination.

An in situ study was performed to investigate the role of zooplankton as a vector of microcystins (MCs) from Planktothrix rubescens filaments to fish during a metalimnic bloom of P. rubescens in Lake Hallwil (Switzerland). The concentrations of MCs in P. rubescens and various zooplanktonic taxa (filter-feeders and predators) were assessed in different water strata (epi-, meta- and hypolimnion) using replicated sampling over a 24-hour survey. The presence of P. rubescens in the gut content of various zooplanktonic taxa (Daphnia, Bosmina and Chaoborus) was verified by targeting the cyanobacterial nucleic acids (DNA). These results highlighted that cyanobacterial cells constitute a part of food resource for herbivorous zooplanktonic taxa during metalimnic bloom periods. Furthermore, presence of MCs in Chaoborus larvae highlighted the trophic transfer of MCs between herbivorous zooplankton and their invertebrate predators. Our results suggest that zooplanktonic herbivores by diel vertical migration (DVM) act as vectors of MCs by encapsulating grazed cyanobacteria. As a consequence, they largely contribute to the contamination of zooplanktonic predators, and in fine of zooplanktivorous whitefish. Indeed, we estimated the relative contribution of three preys of the whitefish (i.e. Daphnia, Bosmina and Chaoborus) to diet contamination. We showed that Chaoborus and Daphnia were the highest contributor as MC vectors in the whitefish diet (74.6 and 20.5% of MC-LR equivalent concentrations, respectively). The transfer of MCs across the different trophic compartments follows complex trophic pathways involving various trophic levels whose relative importance in fish contamination might vary at daily and seasonal scale.

Enhanced co-tolerance and co-sensitivity from long-term metal exposures of heterotrophic and autotrophic components of fluvial biofilms.

Understanding the interactive effects of multiple stressors on ecosystems has started to become a major concern. The aim of our study was therefore to evaluate the consequences of a long-term exposure to environmental concentrations of Cu, Zn and As on the pollution-induced community tolerance (PICT) of lotic biofilm communities in artificial indoor channels. Moreover, the specificity of the PICT was assessed by evaluating the positive and negative co-tolerance between these metals. Photosynthetic efficiency and substrate-induced respiration (SIR), targeting the autotrophic and heterotrophic communities respectively were used in short-term inhibition bioassays with Cu, Zn and As to assess sensitivities of pre-exposed biofilms to the metals tested. Diversity profiles of a phototrophic, eukaryotic and prokaryotic community in biofilms following the different treatments were determined and analyzed with principal component analysis. The results demonstrated that pre-exposure to metals induced structural shifts in the community and led to tolerance enhancements in the phototrophic and heterotrophic communities. On the other hand, whatever the functional parameter used (i.e. photosynthesis and SIR), communities exposed to Cu were more tolerant to Zn and vice versa. Furthermore, only phototrophic communities pre-exposed to As developed tolerance to Cu but not to Zn, whereas no co-tolerance between Cu and As was observed in the heterotrophic communities. Finally, phototrophic and heterotrophic communities exposed to Cu and Zn became more sensitive to As, reflecting a negative co-tolerance between these metals. Overall, our findings support the fact that although the mode of action of the different metals is an important driver for the structure and thus the tolerance of the communities, it appears that the detoxification modes are the most important factors for the occurrence of positive or negative co-tolerance.

Use of the MicroResp™ method to assess pollution-induced community tolerance to metals for lotic biofilms.

Understanding the ecological status of aquatic ecosystems and the impact of anthropogenic contamination requires correlating exposure to toxicants with impact on biological communities. Several tools exist for assessing the ecotoxicity of substances, but there is still a need for new tools that are ecologically relevant and easy to use. We have developed a protocol based on the substrate-induced respiration of a river biofilm community, using the MicroResp™ technique, in a pollution-induced community tolerance approach. The results show that MicroResp™ can be used in bioassays to assess the toxicity toward biofilm communities of a wide range of metals (Cu, Zn, Cd, Ag, Ni, Fe, Co, Al and As). Moreover, a community-level physiological profile based on the mineralization of different carbon substrates was established. Finally, the utility of MicroResp™ was confirmed in an in-situ study showing gradient of tolerance to copper correlated to a contamination gradient of this metal in a small river.