Effects of Microplastics and Chromium in Marine and Freshwater Microalgae

<title>Abstract</title> <p> The concurrent presence of microplastics (MPs) and heavy metals such as hexavalent chromium (Cr⁶⁺) in aquatic ecosystems poses a significant ecotoxicological threat. This study investigates the individual and combined effects of polyethylene microplastics (10 and 50 mg/L) and chromium (0.02, 0.05, and 0.5 mg/L) on the physiological and biochemical responses of freshwater microalga <italic>Scenedesmus sp.</italic> and marine microalga <italic>Chlorella sp.</italic> Quantitative assessments included growth inhibition, chlorophyll reduction, lipid and protein content changes, and oxidative stress markers - catalase (CAT) and glutathione S-transferase (GST). The findings revealed concentration-dependent and species-specific stress responses. <italic>Scenedesmus</italic> sp. exhibited a maximum growth inhibition of 56% at 50 mg/L PE + 0.5 mg/L Cr, with a significant reduction in chlorophyll content (up to 48%). Lipid content reduced by 22%, and protein levels declined by 30% under high-dose combination exposure. Notably, CAT activity peaked at 2.36 U/mg under moderate Cr exposure (0.05 mg/L), while GST activity peaked at 1.89 U/mg but declined at higher pollutant levels, indicating enzymatic suppression. Conversely, <italic>Chlorella</italic> sp. showed enhanced tolerance, with only 35% growth inhibition and relatively stable pigment and protein levels under the same combined stressors. Scanning Electron Microscopy (SEM) revealed structural distortion and membrane roughness in <italic>Scenedesmus</italic> sp., whereas <italic>Chlorella</italic> sp. retained more intact cellular architecture. These results highlight the synergistic interactions between MPs and Cr⁶⁺, emphasizing the vulnerability of freshwater systems and the resilience of marine counterparts. The study justifies the use of microalgae as sensitive bioindicators and underscores the urgent need to regulate co-contaminant exposure in aquatic environments. </p>