Production of high-amylose starch with low digestibility in a green marine microalga Tetraselmis subcordiformis by delaying high-bicarbonate induction.

A photoautotrophic green marine microalga Tetraselmis subcordiformis was applied to sustainably produce high-amylose starch (HAS) from CO2 via high-bicarbonate (HB) induction. Delaying the addition of HB from Day 0 to Day 3 overcome the HB stress and significantly improved photosynthetic activity, resulting in 7.2-times higher starch productivity and the production of HAS with the total starch content of 21.81 % and amylose/amylopectin ratio of 1.51 obtained on Day 6, which represented the highest HAS levels among the photoautotrophic microalgae reported hitherto. The HB treatment improved the activity of granule-bound starch synthase and decreased starch branching enzymes, soluble starch synthase and amylase activity, leading to the formation of HAS by enhancing amylose production with simultaneously attenuating amylopectin biosynthesis. HB induction mainly changed the structure of amylose with increased chain lengths rather than amylopectin, and the ordered structures and degree of branching were also reduced. The produced HAS under HB induction (H-MS) was a kind of very small granular starch with A-type crystalline. It was identified to have a high proportion of short-chain amylopectin and elongated amylose that enabled low gelatinization temperatures (56-69 °C) and digestibility (containing 55.58 % of resistant starch). These features manifested an excellent prospect for application in sustainable food industry.