Novel biopolymer in extracellular substances from the filamentous cyanobacterium Limnothrix sp

The filamentous cyanobacterium Limnothrix sp. strain SK1-2-1 is capable of producing pentadecane (a C15 alkane) and phycocyanin. The aim of this study was to develop a method for the production and recovery of extracellular substances (skECS), containing a novel acidic extracellular polysaccharide (skEPS) as a biopolymer, and to analyze the properties and functions of the obtained skECS. Analyses showed that skECS had a high molecular weight of 1.46 × 10⁶ g/mol and comprised 28.2% (w/w) sugar and 30.2% (w/w) protein. By contrast, skEPS contained glucose (38.4 mol%), mannose (35.3 mol%), and rhamnose (14.8 mol%). skECS was characterized using native PAGE and atomic force microscopy. Aqueous skECS solutions at concentrations ranging from 0.25 to 1.00 mg mL^-1 showed antioxidant capacity. Additionally, blue-light irradiation of SK1-2-1 cells enhanced skEPS production and promoted simultaneous cell flocculation. The accumulation of candidate gene transcripts involved in skEPS biosynthesis in response to blue- and white-light irradiation showed that the expression of sesA (sk0924), which encodes the blue-light sensor substance cyclic-di-GMP, was induced by blue light. Furthermore, nine genes related to the Wzy-dependent pathway, which were inferred to act in an sesA-dependent manner, were non-uniformly induced under blue-light irradiation, with varying degrees of expression. These results reveal novel properties and functions of skECS, light-specific induction of production, and their potential as useful materials for biorefineries.