Protease secreting Nannochloropsis limnetica can valorise invasive plants through circular water-use and accumulate neutral lipids

Nannochloropsis limnetica can be cultivated on a wide range of effluents and generate high value lipids. Using a fed-batch system, this microalgae was used to demonstrate circular reuse of water to valorise protein depleted fractions (PDF) generated from Gorse leaf protein concentrate (LPC) production. Protein, phenolics and sugar content in the PDF was measured over seven days and corresponding microalgal lipid content was profiled. Extracellular protease activity was also screened to understand how N. limnetica digests and utilises proteins in the medium. A signal peptide-based approach predicted seven proteases expected to be secreted.

N. limnetica could rapidly deplete nutrients (k _sugar =0.20 ± 0.06 day ⁻¹ and k _proteins =0.17 ± 0.04 day ⁻¹ ) from the wastewater which could be reused for subsequent LPC extraction processes without any impact on recovery or purity. Phenolics levels remained unaffected during microalgal growth and accumulated in the water across each extraction cycle. Microalgal neutral lipid content (∼132.8 mg _lipid ) was higher in the effluent compared to standard BBM (∼114.5 mg _lipid ). Casein agar plates and 1D zymogram confirmed extracellular protease production by N. limnetica , although their identities remains unclear owing to conflicting / fragmented data from the LC/MS analysis.

This work shows that N. limnetica is able to secrete proteases to digest and utilise residual proteins in the medium, and can be employed to valorise agricultural wastewater. Identification of the proteases will require further investigation.