Production of exopolymer substances from the thermophilic chlorophyte Graesiella : industrial and ecological applications

Microalgal extracellular polymeric substances (EPSs) are heteropolysaccharides that have characteristics suitable for industrial and biotechnological applications. Notably, they have strong anionic nature and high hydrophobicity. Nevertheless, systematic studies to demonstrate the viability of the production of EPSs on an industrial scale are still crucial. In this research, the chlorophyte Graesiella was grown on a raceway pond to view its EPS valorization. The biomass production achieved a maximum of 1.98 g L−1 and an EPS production of 1.6 g L−1 after six production days. The Graesiella EPSs with a molecular weight above 100 kDa are sulfated exopolymers containing mainly polysaccharide (70%) and protein (16%). The EPSs produced more stable emulsions with hydrocarbons and oils than Tween-20. The emulsification indices with n-hexane (88%) and maize oil (28%) indicate the EPSs’ strong emulsion-stabilizing capacity. The EPSs showed a peak flocculating percentage of 95% to kaolin suspension, with better flocculation performance than Al2(SO4)3 and alginate. Moreover, Graesiella EPSs had a significant effect on antimicrobial activity, significantly inhibiting fungal growth (71% for Botytis cinerea and 87% for Fusarium oxysporum), spore germination (100% of inhibition at a concentration of 1.8 g L−1), and mycelium growth (68% of inhibition). Also, Graesiella EPSs acted as a bactericide against Vibrio anguilaruim and Listonella anguilaruim (100% inhibition). EPSs were also found to have potent antioxidant activity compared with l-ascorbic acid. The obtained results open new perspectives to the further exploration of Graesiella sp. as a potential EPS producer, making it a promising candidate for numerous industrial applications.