Characterization of the Nannochloropsis gaditana storage carbohydrate: A 1,3-beta glucan with limited 1,6-branching

As a result of a secondary endosymbiosis of a red alga, stramenopiles retained beta-1,3 glucans as the primary soluble storage carbohydrate, in contrast to the starch and glycogen of green plants and animals. This storage carbohydrate has been identified and characterized in representative diatoms, brown algae, and oomycetes, but has not been biochemically characterized in species of Nannochloropsis, an industrially relevant genus of algae favored for its lipid content and productivity. In this study, the soluble storage carbohydrate of Nannochloropsis is characterized by H-1 NMR, linkage analysis, and size exclusion chromatography. The putative genes encoding enzymes required for synthesis of the carbohydrate oligosaccharide from glucose 6-phosphate are identified. A glycogenin-like glycosyltransferase family 8 (GT8) protein was also identified to be conserved among all species of Nannochloropsis, despite the lack of glycogen in the genus. Homologs were identified in available genomes of brown algae, diatoms, and oomycetes, all documented to utilize beta-1,3 glucans for carbon storage. Finally, a set of three likely laminarinases are highlighted from the glycosyl hydrolases by phylogenetic analyses.