Transcriptomic analysis for phosphorus limitation-induced -glucans accumulation in Chlorella sorokiniana SCSIO 46784 during the early phase of growth

Algal beta-glucans have received attention in recent years, but little is understood for the pattern and molecular mechanism of beta-glucans accumulation in Chlorella, a more economically feasible microalga. In the present study, P-limited conditions were found to substantially inhibit the microalgal growth, reduce the lipid and protein production but induce the accumulation of beta-glucans and total carbohydrate of Chlorella sorokiniana SCSIO 46784 during the early phase of growth. Comparative transcriptomics analysis of the microalga for the specific time point under P-limited and P-replete conditions identified key genes involved in beta-glucans metabolism and revealed the probable molecular mechanism of the beta-glucans accumulation. The significant up- and down-regulation of genes encoding beta-glucans synthesis-and degradation-related enzymes coincided with a rapid in-crease in beta-glucans production. The reduced synthesis of certain sugars (starch, sucrose, UDP-L-rhamnose, and UDP-sulfoquinovose) might provide more carbon precursors for beta-glucans biosynthesis. The suppression of lipids (fatty acid, triacylglycerol, sulfoquinovosyl diacylglycerol, and monogalactosyldiacylglycerol) synthesis pre-sumably shunted carbon flux toward carbohydrates and beta-glucans biosynthesis. In addition, the down-regulation of multiple genes related to carbon fixation (CO2 concentrating mechanism and Calvin cycle) and central carbon metabolism (pentose phosphate pathway, glycolysis/gluconeogenesis, and tricarboxylic acid cycle) might contribute to the inferior growth of C. sorokiniana SCSIO 46784 under P-limited conditions. To our knowledge, this is the first report to reveal the metabolic insights into beta-glucans in Chlorella. This study provides not only the useful information for understanding the biosynthesis and accumulation of fi-glucans but also potential gene targets for genetic modification to improve the production of beta-glucans in Chlorella.