Copper affects photosynthetic parameters of N- or P-limited Ankistrodesmus densus

Algae require micro- and macronutrients for optimal growth and metabolism. Under limitation or excess of the nutrients in the environment, they can adapt their photosynthetic machinery to cope with the new concentrations available to decrease damage to their performance. In the present study, to evaluate a microalga's responses to a macronutrient's limitation and the excess to a micronutrient, we acclimated the well-distributed freshwater microalga Ankistrodesmus densus to N- or P-limited medium before exposing it to sublethal copper (Cu) concentrations. Our results indicate that Cu affected the chlorophyll a concentration in N- and P-replete conditions, while the N- or P-limitation affected chlorophyll a concentration, maximum and effective quantum yield of photosystem II (PS II). Within the time frame of 72 h, and the maximum Cu concentration used (1.26 µM Cu2+), the addition of Cu to N- or P-limited algae resulted in synergism in all of these parameters, except in chlorophyll concentration under P limitation. In addition, the combination of Cu with N- or P-limited algae decreased the photochemical quenching (qP) and increased the non-photochemical quenching (qN and NPQ). The values obtained in Y (NPQ) – i.e., the quenching of regulated energy loss in PS II – indicate that the combination of Cu and N- or P-limited algae induced the activation of photoprotective mechanisms. Under the highest Cu exposure, the changes obtained from N- or P-limited algae were similar, indicating that under low metal concentrations, the concentration of macronutrient is responsible for changing the chlorophyll concentration, qN, NPQ, and Y (NPQ); however, at higher concentrations of metal, Cu apparently drives these changes. All of the parameters evaluated were affected under N or P limitation and Cu combination, indicating a synergism. Based on the present study results, we suggest using Ankistrodesmus densus in ecotoxicological studies due to its sensitivity and adaptation to adverse scenarios