Cryopreservation of Antarctic microalgae: the protective role of exopolysaccharides from Nostoc sp. ARAS18

Abstract Cryopreservation is vital for conserving microalgal biodiversity and biotechnological potential, but conventional cryoprotective agents (CPAs) like dimethyl sulfoxide and methanol are often cytotoxic. This study introduces exopolysaccharides (EPS) from the cyanobacterium Nostoc sp. ARAS18 as a natural alternative. The strain exhibited a high EPS production yield (90 ± 0.3 mg L −1 ) with a complex anionic structure, characterized by high carbohydrate content (91.8%) and functional groups like uronic acids, which facilitate vitrification and extracellular membrane stabilization. In cryopreservation trials with three Antarctic microalgae ( Nannochloris sp. ASYA12, Chlorella sp. ASYA31, Micractinium simplicissimum ASYA46), 10% EPS consistently yielded high post-thaw viability and growth recovery, outperforming conventional CPAs. Crucially, Trypan Blue staining and microscopy confirmed that EPS preserved full cellular integrity without toxicity, whereas traditional agents caused significant membrane damage. The extracellular mode of action of EPS eliminates the risks of intracellular toxicity and the microbial contamination associated with glycerol. This work establishes Nostoc -derived EPS as a highly effective, biocompatible, and sustainable CPA, offering a robust strategy for the long-term preservation of valuable polar microbiomes.