Exposure to metals and metallic mixtures at environmental concentrations impairs photosynthesis and microalgal biomass production: A lab study with Raphidocelis subcapitata (Chlorophyceae)

In the environment, microalgae are exposed to several stressors, including metals. These stressors affect the algal metabolism (e.g., growth, photosynthesis, biochemical composition) and can alter the quality (mainly in terms of lipids, fatty acids, carbohydrates) and the quantity of algal biomass produced. These alterations can be beneficial, e.g., due to the presence of a stressor, algae can produce more lipids, which can be useful for biotechnological purposes. However, finding the exact combination and/or concentration of stressors that stimulates algal production is still a huge challenge. In the present study, we evaluated how environmental concentrations of cobalt (Co, essential metal), aluminum, and lead (Al and Pb, non-essential metals), isolated and in combination (Co + Al and Co + Pb), affect the growth and photosynthesis of the microalga Raphidocelis subcapitata. Isolated, Co and Al negatively affected growth and photosynthetic parameters, while Pb did not impact them. Growth was the most sensitive parameter evaluated under mixture, while the decrease of non-photochemical quenchings indicates damage to the photoprotection mechanisms of algae in the presence of multiple stressors, especially in Co + Al. Given that the evaluated concentrations of Co, Al, and metallic mixtures negatively impacted growth and compromised carbon assimilation by algae, we recommend conducting further studies to assess the biochemical composition of algae exposed to environmental concentrations of metals and mixtures. This will enhance our understanding of changes in algal metabolism and help define better strategies for increasing carbon assimilation and biomass production, which could be beneficial for biotechnological applications and phytoremediation efforts.