Response patterns of the freshwater green microalgae Kirchneriella obesa (Chlorophyta, Sphaeropleales) to free copper ions

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ABSTRACT Microalgae can be exposed to a gradual copper (Cu) increase over a wide concentration range in the environment, but most studies focus on punctual increases. This research investigates the effects of quasi-continuous Cu exposure on Kirchneriella obesa, providing new insights on thresholds of healthy and unhealthy cellular responses. Photosynthesis, cell density, and biomolecules were measured at 20 free Cu levels, from environmental to toxic (3x10−9 to 4 x 10−5 mol L−1). Using nonlinear Bayesian regression models, we established mathematical functions describing the relationship between measurements and Cu level and computed credibility intervals for the curves. Our findings revealed two distinct physiological response patterns dependent on Cu concentration and variable. Growth rate gradually decreased with Cu increase, whereas total proteins and carbohydrates increased. A steep transition from healthy to unhealthy condition was present in most photosynthetic variables, whereas others exhibited gradual variation. The Cu2+ concentration triggering these physiological changes differed, signalizing varying photosynthetic susceptibility. The most sensitive variables were rETRmax (120 to 50 μmol e− m−2 s−1) and Ek (180 to 80 μmol photons m−2 s−1) with a steep decrease between 6.5 x 10−8 and 5 x 10−7 mol L−1 Cu2+. Conversely, NPQ demonstrated resilience, increasing steeply from 0.2 at 4.1 x 10−7 mol L−1 Cu2+ to 1.1 at 5.0 x 10−7 mol L−1 Cu2+. Overall, photosynthesis was more resistant to Cu stress than growth rate. This research offers important insights into the responses of a common microalga exposed to a wide range of Cu concentrations, reflecting a scenario of gradual increase in the environment.