Combined effects of warming and benzophenone-3 exposure on growth and photosynthetic performance of temperate and polar Chlorella

Rising global temperature has significant implications for the survival and physiological performance of microalgae, which form the basis of aquatic food webs globally. In addition to thermal stress, ultraviolet (UV) filter contaminants such as benzophenone-3 (BP-3) are increasingly detected in freshwater environments, raising concerns about their combined impacts on microalgal physiology and productivity. This study investigated the combined effects of BP-3 and elevated temperature on two Chlorella strains, Chlorella UMACC 248 (temperate) and Chlorella UMACC 237 (polar), by evaluating growth, photosynthetic performance, pigment content, biomass production and biochemical composition. The temperate strain was cultured at 18, 23 and 28 °C, while the polar strain was grown at 4, 9 and 14 °C. Each strain was exposed to BP-3 concentrations of 0, 0.1, 10, 100 and 200 mg L-1. Exposure to low BP-3 concentrations (0.1–10 mg L⁻1) primarily demonstrated a threshold of robust stress tolerance, with chlorophyll-a, carotenoid contents, Fv/Fm and rETRmax closely tracking or remaining statistically comparable to unchallenged controls. Under specific warming scenarios, this acclimation response translated into localized hormetic overcompensation, sustaining high specific growth rates (up to 0.252 day⁻1 in temperate Chlorella and 0.772 day⁻1 in polar Chlorella) and optimizing photosynthetic performance. Conversely, high concentrations (100–200 mg L⁻1) caused dose-dependent inhibition, reducing growth rate (e.g., 0.004 day⁻1 in polar Chlorella), pigment contents, PSII efficiency and biomass, with near-complete photosynthetic collapse at the highest doses. Biochemical responses revealed stress-induced metabolic reallocation, with increased carbohydrate and lipid up to 50.00% dry weight (DW) accumulation in temperate Chlorella, while polar Chlorella exhibited elevated protein (up to 56.97% DW) and carbohydrate content but reduced lipid levels at high BP-3 exposure. These findings indicate that, if environmental concentrations of pollutants such as BP-3 increase alongside global warming, interactive stress effects may alter microalgal
productivity with potential consequences for freshwater food webs.