Bioremediation of sewage water by microalga Graesiella emersonii for sewage-fed aquaculture in East Kolkata Wetlands, a Ramsar site in India

Increased nutrient load from industrial and domestic sewage has led to eutrophication and the gradual deterioration of East Kolkata wetland (EKW) fisheries. Thus, the present study investigated the nutrient removal efficiency and physiological responses of Graesiella emersonii MN877773, a native chlorophyte strain, cultivated in EKW sewage water. Microalgae inoculated in different sewage and lab grade water dilutions (50:50, 60:40, 80:20, and 100%). Physico-chemical characterization revealed elevated nutrient and organic loads in raw sewage, with progressive improvement in water quality following 60:40 dilution. G. emersonii demonstrated efficient removal of nitrate (up to 41.5%), ammonium (91.73%), and COD (54.2%) within 3–6 days of cultivation. The 60:40 dilution exhibited the highest algal growth rate (μ = 0.65 day−1), cell viability and chlorophyll-a concentration, accompanied by balanced antioxidant enzyme activity (SOD, CAT, GST, and Gt) and low malondialdehyde (MDA) levels. Principal component analysis indicated a strong association between antioxidant defence and nutrient removal. Post-cultivation analysis of sewage water showed reduced ionic load and COD, along with increased DO, indicating improved water quality for sewage-fed aquaculture. A separate experiment on sewage dilution with rain water (60:40) showed nutrient removal efficiency similar to lab grade water, highlights the potential for field applicability.