| Resumo: | Bivalves play a marked role in coastal marine ecosystems, impacting primary productivity with a strong top-down control on phytoplankton communities. In addition, they are an important food source for higher trophic levels, including humans. However, bivalves can be impaired by several external factors, either biotic or abiotic, natural or anthropogenic. Exposure to these stressors, coupled with intensive farming, raised the need for adequate management of the wild populations and farmed production. To improve the difficult balance between ecological and socio-economic interests, it must be investigated how bivalves cope with external stressors. For this reason, the effects caused by the following stressors were studied in the present thesis: (i) harmful algal blooms (HABs) and (ii) climate change drivers in the seawater, such as warming (W) and acidification (A). HABs are increasing in frequency, intensity, persistence and geographic distribution, but are ambiguous regarding their ecological impacts and physiological effects on bivalves. A strong relationship is emerging between HABs and climate change drivers, what may affect both shellfisheries and wild populations. Hence, the objectives of this work are: 1) To determine whether native and invasive species cope with exposure to HABs differently, providing new insights on species-specificities and ecosystem functioning fragilities in the presence of invasive species and HABs; 2) To evaluate how commercially valuable bivalve species cope with simultaneous exposure to several climate change drives and HABs, providing new insights on how environmental changes affects toxicokinetics, physiological and genotoxic bivalves responses, under HABs; 3) To assess, marine biotoxins bioaccessibility, contributing for new information relevant for risk assessment. Different approaches were used to achieve these goals. Complementary field and laboratory works were performed to assess toxicokinetics patterns of marine biotoxins in native vs. invasive bivalve species. Bivalves were exposed to toxic blooming algae species under climate change scenarios in laboratory controlled conditions to assess toxicokinetics as well as physiological and genotoxic responses. Naturally contaminated bivalves were used to investigate their role as vectors of marine biotoxins to humans through in vitro digestion methodology. Relevant new data were obtained regarding the effects of several marine biotoxins in bivalves. Assessing accumulation of regulated and non-regulated biotoxins in native (Ruditapes decussatus) and invasive (R. philippinarum) clam species from Aveiro Lagoon revealed that higher toxin content, particularly regarding OA-group, the most abundant and frequent toxins in the Portuguese coast, is reached by native species. Accumulation of lower toxin levels by invasive clam may then favour farmers interest for their production. The kinetics and genotoxicity study in native and invasive clams reinforced the existence of species-specific behaviours. Exposure to the dinoflagellate Prorocentrum lima, an OA and dinophysistoxin-1 producer, increases the pressure over native clams, with higher toxin accumulation and genetic damage, as well as early and increased induction of DNA repair activity. Invasive clams, on the other hand, are better adapted to cope with these challenges. Investigation of the effects of combined exposure to multiple stressors revealed that W, A and HABs alter the accumulation/elimination dynamics of Paralytic Shellfish Poisoning (PSP) toxins in mussels (Mytilus galloprovincialis). Lower accumulation levels and slower elimination rates were observed. The predicted climate change scenarios and exposure to HABs may then lead to lower contamination levels but to longer harvesting closures. Simultaneous exposure to altered environmental conditions and HABs also had significant impacts in the antioxidant system and DNA integrity, resulting in an organ-specific modulation of the antioxidant response, increasing genetic damage and preventing/retarding DNA damage repair. However, the DNA damage observed seems to be non-oxidative. While the investigation of factors enhancing the elimination of toxins was not a main objective of this study, it was shown that acidification might promote PSP toxins elimination in mussels. Finally, the bioaccessibility studies revealed a significant reduction in the OA group content available after in vitro digestion, suggesting an overestimation of exposure to these biotoxins and an over-conservative approach in safety levels definition and risk assessment studies. This was the first study assessing bioaccessibility of OA-group toxins, pointing to in vitro digestion as a promising tool to obtain accurate data regarding toxin ingestion from bivalves to the consumer. Overall, the complexity of the relation between HABs, bivalves and the environmental factors to consider in marine biotoxins management is increasing, and only continuous and extensive monitoring of environmental, biological and anthropogenic conditions may allow for a healthier balance between environmental and socio-economic interests.
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