Effects of copper on photosynthetic and physiological parameters of a freshwater microalga (Chlorophyceae)

Abstract

Abstract Copper is known for its essentiality for optimal metabolism, and damage caused in algae under high exposure is also well documented; however, some mechanisms of its toxicity are still not totally understood. In the present study, we exposed the freshwater microalga Selenastrum gracile to sublethal copper concentrations and evaluated responses in algal growth, chlorophyll production, photosynthetic performance, and lipid class and fatty acid production. Excess metal impaired algal growth, but did not affect the production of chlorophyll per cell, which is consistent with the reported substitution of Mg2+ by Cu2+ in the chlorophyll molecule. With regards to photosynthesis, there was a decrease in the maximum quantum yield (≈20%) and the operational (effective) quantum yield was remarkably lower (≈60%) with excess of metal. The photochemical and non-photochemical quenchings were also affected with the presence of copper; however, there was no linear response according to the amount of metal, which suggests inactivation of reaction centers and damage to the photoprotection mechanisms. The amount of lipids and fatty acids increased with the increase of the metal, and the results indicate important roles of the proportions of structural lipids (sterol and phospholipids) and mono- and polyunsaturated fatty acids to maintain the photosynthetic structure; however, this was not enough to avoid damage of copper in the photoprotective mechanisms. As an outcome, the algae could not cope with excess of light under high amounts of metal, affecting the photochemical reactions and consequently photosynthetic activity.