Cu and Cd affect distinctly the physiology of a cosmopolitan tropical freshwater phytoplankton

•Chlorolobion braunii was more sensitive to Cd than Cu.•Endpoints such as growth and cell viability were more sensitive than maximal and effective photosynthetic efficiencies.•Non-photochemical quenchings were severely damaged by both metals.•Average cell volume increased as metal concentrations increased.•Chlorophyll a (Chl a) content per cell increased too, but the Chl a content per unit volume decreased.

Copper and Cd are natural constituents of freshwater ecosystems, both cycling influenced by microbial communities. The present research examined the impacts of environmentally relevant concentrations of Cu and Cd on the growth, viability, cell size, chlorophyll a (Chl a) content and photochemical efficiency of the tropical freshwater phytoplankton Chlorolobion braunii. Cell growth was significantly impaired by Cu and Cd, with EC50 occurring at 33.6 and 1.6 µM, respectively. At sublethal levels (< EC50), cell death was already induced at 5 µM Cu and 1 µM Cd. Average cell volume significantly increased as metal concentrations increased, as did the Chl a content per cell, although the Chl a content per unit volume decreased. Copper did not affect both the photosystem II (PSII) maximum quantum yield (ΦM) or the operational quantum yield (ΦE), while Cd significantly impacted ΦE, with EC50 occurring at 18.4 µM. Different responses for Cu and Cd were obtained whether the photochemical fluorescence quenching (Qp) or non-photochemical quenching (Qn) were considered. Qp decreased after Cd addition, but was not altered after Cu addition. Qn values significantly increased after the addition of either metal. Non-photochemical quenching due to heat dissipation (NPQ) significantly increased in response to both metals, but it was more pronounced in the case of Cd. Overall, Cd was more toxic to C. braunii than Cu.