• heavy metals
• dissolved gaseous mercury
• detoxification mechanisms
• autotrophic protists
• phytochelatins

For the comprehension of the processes of transport, transformation and accumulation of heavy metals in the marine environment an important contribute lies on the understanding of the diverse strategies developed during evolution by aquatic and terrestrial organisms in order to maintain an equilibrated relation with heavy metal ions present and available in the surrounding medium. The aim of the present study was to investigate the main mechanisms of detoxification acting in autotrophic protists to cope with heavy metal stress. Since a general response to metal stress in autotrophic organisms is represented by the synthesis of metal-binding peptides, named phytochelatins (PC), this study was focused on the induction of these peptides in different taxa of marine phytoplanktonic protists. Initially the induction of PC was investigated in cells of the marine diatom Phaeodactylum tricornutum exposed to environmentally relevant levels of dissolved Cd, Cu, Pb and Zn by performing short-term incubations both in EDTA-buffered artificial seawater and in natural seawater samples. Results showed that PC behave as a biomarker of exposure to the bioavailable metal fraction. Successively, experiments were carried out to examine the PC response in 5 taxa of autotrophic protists exposed to Cd, Cu and Pb. Results showed that the PC response depends on the particular species, the chemical form of the metal, the time of exposure and the metal concentration in the external medium. Successively, the research was devoted to develop new bioassays involving the presence of PC in phytoplankton as biomarkers of metal bioavailability in marine waters and sediments. The results obtained strongly support the feasibility of using this biochemical response in the assessment of toxicity of environmental systems. Besides the ability to synthesize metal binding peptides, the capability to produce Dissolved Gaseous Mercury (DGM) as another defence mechanism was investigated in Thalassiosira weissflogii exposed to potentially toxic concentration of mercury. Mercury exposure experiments needed a separate study given the chemical particularities of this metal in comparison to the other heavy metals and the peculiarity of the experimental methodology necessary for mercury determination. The results showed that the diatom responded to mercury exposure by synthesizing PC, besides to increase the intracellular pool of glutathione and -Glu-Cys. The time course of the non protein thiols pool and Hg intracellular concentration showed that PC, glutathione and -Glu-Cys represent a rapid cellular response to mercury exposure; however, at longer incubation times, their role in Hg detoxification seems to lose importance. At lower Hg concentration, at which the PC synthesis doesn’t seem to be involved, the occurrence of a process of reduction of the DGM production was investigated in the same diatom. The significant correlation between the cellular density in solution and the production of DGM, both in light and dark conditions, clearly showed that T. weissflogii is capable to directly produce DGM. This finding has been confirmed by the absence of DGM production in culture media containing formaldehyde-killed cells of T. weissflogii. This approach is part of a wider study regarding the contribution of the eukaryotic and prokaryotic microorganisms to the production of DGM in aquatic systems.