Chromium Removal by Dunaliella salina in High-Salinity Environments: An Investigation Based on Microalgal Cytotoxic Responses and Adsorption Capacity

Chromium (Cr) is a widespread heavy metal contaminant in aquatic environments, posing serious risks to phytoplankton due to its persistence, biotoxicity, and mutagenic potential. Microalgae have emerged as promising biological agents for Cr remediation. In this study, the Cr removal potential of living Dunaliella salina (D. salina) was evaluated by examining the toxic effects and adsorption behavior of trivalent Cr(III) and hexavalent Cr(VI) through short-term exposure experiments. This study elucidated the mechanisms by which Cr disrupts key photosynthetic metabolic pathways, quantified the short-term toxicity thresholds of Cr(III) and Cr(VI) to D. salina, and characterized the saturation adsorption capacity and adsorption kinetics of Cr on algal cells. The results showed that Cr(VI) at concentrations of 5–20 mg/L inhibited the growth of D. salina in a dose-dependent manner throughout the culture period, with inhibition rates ranging from 22.8% to 70.9%. After 72 h of exposure, the maximum growth inhibition rates caused by Cr(III) and Cr(VI) reached 42.5% and 52%, respectively. Interestingly, low concentrations of Cr(VI) (0.1–1 mg/L) slightly enhanced the growth of D. salina. However, Cr(VI) exhibited stronger biotoxicity than Cr(III). Exposure to both Cr species significantly reduced the levels of chlorophyll a (Chl a), chlorophyll b (Chl b), and carotenoids (Car), resulting in damage to the photosynthetic reaction centers and suppression of the photosynthetic electron transport system. The adsorption of Cr(VI) by D. salina followed a pseudo-second-order kinetic model, with a maximum adsorption capacity of 38.09 mg/g. The process was primarily governed by monolayer chemisorption. These findings elucidate the toxic mechanisms of Cr in D. salina and highlight its potential application as an effective bioremediation agent for heavy metal pollution, particularly Cr(VI), in marine environments.