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Summary The production of bioethanol by cyanobacteria can become economically sustainable if other valuable commercial products, such as exopolysaccharides (EPS), are produced simultaneously. This study investigates the feasibility of producing bioethanol and the commercially important EPS from a non‐nitrogen fixing marine cyanobacterium, Synechococcus elongatus BDU 10144 in a novel low‐cost seawater‐fertilizer medium for the development of a biorefinery strategy. Herein, agricultural fertilizer diammonium phosphate (DAP) (nitrogen source), magnesium sulfate (MgSO4) and potassium were optimized by the central composite design to produce maximum biomass and carbohydrate accretion. The optimal physical conditions for attaining maximum growth and carbohydrate accumulation were determined to be pH 10, 50 μmol m−2 s−1 (light intensity) and 25°C. Further, different concentrations of seawater were mixed with the optimized fertilizer‐salts, and it was observed that 70% of seawater mixed with the optimized fertilizer‐salts (hereafter FSW medium) was an optimum condition for growing the test cyanobacterium, which ultimately reduced the cost of the medium by >40 times compared to the standard artificial seawater nutrient (ASN‐III) medium. Under the optimal conditions, the maximum biomass (1.79 g/L) and carbohydrate (702.4 mg/L) yield obtained were ~1.7 and 2.2 times higher than the ASN‐III. The yield of bioethanol was noted to be 318.1 mg/L, which was ~2.3 times greater than ASN‐III. The commercially important EPS was obtained from the discarded supernatant (0.28 g/L), which was ~1.6 times higher in the FSW medium. Thus, the present investigation paves a way forward for mass cultivation of the S. elongatus BDU 1044 in the novel FSW medium for the cost‐efficient and sustainable development of a biorefinery concept.

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The current research work was an innovative approach providing dual advantages of waste bioremediation and an effective biorefinery. The study attempted to exploit wastewater like aqua discharge and solid wastes like poultry litter/cow dung for cyanobacterial cultivation. Aqua discharge appended with 7.5 g L−1 poultry litter turned out as the best combination generating 46% higher carbohydrate yield than BG-11 control. A. variabilis cultivation in this waste-utilized medium also revealed its excellent bioremediation ability. While 100% removal was observed for nitrite, nitrate, and orthophosphate, a respective 74% and 81% reduction was noted for ammonium and total organic carbon. Chemical and biological oxygen demands were also reduced by 90%. This work was also novel in developing a sequential design for the production of bioethanol and co-products like exopolysaccharides, sodium copper chlorophyllin, C-phycocyanin, and poly-β-hydroxybutyrate from the same cyanobacterial biomass. The developed biorefinery implementing the waste-utilized medium was one of its kind, enabling biomass valorization of 61%. Therefore, the present study would provide a leading-edge for tackling the high production costs that limit the practical viability of biorefinery projects. The recyclability of the bioremediated wastewater would not only curtail freshwater usage, the waste disposal concerns would also be mitigated to a great extent.