Changes in photosynthetic parameters and lipid classes of N-limited freshwater microalga under cadmium exposure

Abstract

Abstract In the environment, algae are exposed to non-optimal conditions and adapt their physiology to cope with adverse conditions, such as limiting nutrients or excess contaminants. The present study evaluated the changes in photosynthetic parameters and lipid classes of the freshwater green microalga Ankistrodesmus densus exposed to nitrogen (N) limitation and cadmium (Cd) exposure. While N is required for the algal metabolism, Cd does not present known functions in freshwater microalgae. Our results indicate that the growth rate was more affected by metal than by the nutrient. At the same time, the quenchings, i.e., fractions of light energy driven to photochemical or non-photochemical processes of photosynthesis, were not affected under the N limitation. However, in the combination of the two stressors, the changes were more pronounced, suggesting a synergism in the growth rate, chlorophyll a production, photochemical quenching (qP), and effective quantum yield ((Y(II)). While Cd did not affect any lipid classes, the percentage of storage lipids (triacylglycerol) was increased under N limitation and in some of the combinations of N limitation and Cd. However, in the combination of the stressors, there were some non-linear changes in structural lipids (phospholipids) and a decrease in acetone mobile polar lipids. Our data indicate that the combination of the two stressors affected algal physiology with synergism in several parameters. However, the algal photoprotective mechanisms effectively avoided photodamage, as indicated by non-photochemical quenchings (qN and NPQ) and in the quantum yield related to regulated energy loss Y (NPQ) values.