Autotrophic organisms to counteract excess of micronutrients and heavy metals in the agro-ecosystem: phycoremediation and biofortification perspectives

The bioconcentration and toxicity of micronutrients and heavy metals is becoming a problem of great concern in agro-ecosystems and aquatic environments. As water scarcity is increased by climate change, population growth and rapid urbanization, the reuse of treated wastewater and sewage sludge for crop irrigation and fertilization is expected to rapidly rise. Reused waters and compost products contain a considerable variety of macro-and micronutrients essential for plant growth. Among these, copper (Cu), zinc (Zn), iron (Fe) and manganese (Mn) are heavy metals, thus posing important aspects of ecological risk when the fast rate of reused waters and compost application has significant cumulative effects on nutrient soil availability and on leachate migration in run-off waters. Microalgae are very sensitive organisms to trace levels of various organic and inorganic pollutants including heavy metals, thanks to their unicellular nature, photosynthetic ability and fast growth rate. A wide range of publications shows the microalgae ability to take up, metabolize and accumulate heavy metals from several habitats as fresh- and seawater, domestic and industrial effluents, salt marshes and constructed wetlands. This ability is due to the capacity of microalgae cells to develop mechanisms of absorption (intracellular) and adsorption (extracellular) for coping with excess of micronutrients and heavy metals. The specific mechanisms underlying the microalgae capacity as biological sensors of potential toxic metals or accumulators of vitamins and minerals (food supplement) are still unknown. With the purpose of elucidating the mechanisms of absorption and adsorption with which microalgae can uptake and remove trace elements potentially toxic in the agro-environment such as Cu, Zn, Fe or Mn, the microalga Chlorella vulgaris was grown at a laboratory scale, in Pyrex flasks inoculated with the algae colonies in TAP medium (control) and TAP enriched with excess of micronutrients. The algae cultures were weekly monitored in three weeks-experiments, by checking markers of growth and health state, and measuring the photosynthetic activity (pigments and chlorophyll fluorescence) and the micronutrients´ contents in algal dry biomass and in the growth medium by ICP-OES. For the most micronutrients tested, significant decreases of their contents in the growth medium were associated with increases of concentrations in the algae dry biomass. This suggests a good ability of C. vulgaris in removing these heavy metals from polluted waters and/or accumulating them in the extra- or intra-cellular matrix. The results are discussed in the perspective of using microalgae for removing excess of metals from the environment ("phycoremediation") and/or for accumulating beneficial micronutrients in edible matrices ("biofortification").