WQSTRESS · Biological trace metal stress indicators for the assessment of freshwater ecosytems

Final Activity Report Summary - WQSTRESS (Biological trace metal stress indicators for the assessment of freshwater ecosytems)

The development of human activities and industrialisation has led to an increased release of metals to the aquatic environment. Several metals (such as copper, zinc, and iron) are essential for many physiological processes of marine organisms but can be toxic at enhanced concentrations; others (such as cadmium, lead, mercury and arsenic) are not physiologically essential and are toxic at very low concentrations. To cope with the deleterious effects of metals, eukaryotic cells produce strong high-affinity metal-binding proteins and peptides involved in metal tolerance and detoxification mechanisms. Sulfhydryl compounds or thiols (e.g. glutathione and phytochelatins) are part of known metal-binding peptides in marine algae and form key molecules which take part in these mechanisms. Marine phytoplankton is also able to maintain the homeostasis of essential metal ions in different cellular compartments by interactions between metal transport, chelation, trafficking and sequestration activities, which regulate the uptake and distribution of these metal ions. As increasing worldwide concerns about water quality have resulted in targets to control metal contamination of aquatic ecosystems. The principal aim of this project was to establish the usefulness of the production, by marine phytoplankton, of extracellular and intracellular thiols, as trace metal stress indicators in environmental pollution monitoring. Two main questions were addressed: Is the induction of thiols dependent on metal species (i) and/or algal species (ii)? And two complementary approaches explored: (A) Occurrence of intra- and extracellular thiols in metal exposure experiments (single addition of element, and mixtures of metals) conducted under controlled laboratory incubations and (B) field experiments (in contrasting marine environments). The research has shown that thiols are produced in culture experiments under metal stress. In addition, we have observed thiols in oceanic environments (in the vicinity of the Cape Verde) with a mesotrophic nutrient status. The main conclusions of this work are that phytochelatins produced by marine phytoplankton are relevant components for environmental pollution monitoring and water quality risk assessment. However, before these thiols can be widely applied as part of a suite of biomarkers of metal stress, further multidisciplinary research work is required on the influence of the environmental conditions on both intracellular and extracellular thiol production. It has become evident that a complex network controls the regulation of intracellular metal ions in marine algae and that there is no a single and simple mechanism employed by phytoplankton for metal detoxification and tolerance. The role of glutathione in these activities is more complex than for phytochelatins, likely due to the multi-functionality of glutathione. In addition, the research has shown an increased expression of phytochelatin synthase under metal stress, in laboratory culture conditions but some more interesting questions relating to the roles of this key enzyme in metal activation of phytochelatin biosynthesis and regulation, remain to be answered. To further our knowledge, the research should be extended to the investigation of the full range of genes potentially involved in trace metal homeostasis, detoxification and hyperaccumulation. A combination of changes in expression of genes and key enzymes and phytochelatin production may be a better sensitive and useful response for metal exposure in natural assemblages. Moreover, while hyper-accumulators appear to offer beneficial solutions in a number of fields including bio- and phytoremediation and phytomining, a better understanding of their remarkable metal selectivity and metal accumulation pathways is needed to optimise their full potential. Some more key processes involved in this network and related components, have to be identified and several fundamental questions remain to be answered.

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