Acetate Combined with CO₂ as Effective Carbon Sources for the Production of Resistant Starch in a Marine Microalga <i>Tetraselmis subcordiformis</i>

Abstract

Microalgae are considered as sustainable starch producers, yet the carbon sources for this process in terms of starch productivity and functionality require further elucidation. The present study investigated the roles of CO2 and acetate on the starch production in a marine microalga Tetraselmis subcordiformis, and the ordered structure and digestibility of the starches obtained were characterized. CO2 and acetate could serve as efficient carbon sources for T. subcordiformis to accumulate starch, with the maximum starch content, yield, and productivity reaching 66.0%, 2.16 g/L, and 0.71 g/L/day on day 3 and the maximum biomass productivity reaching 0.94 g/L/day on day 2, respectively, when 2.5 g/L sodium acetate and 2% CO2 were simultaneously applied. The addition of acetate under 2% CO2 improved the photosynthetic efficiency and enhanced the activity of ADP-glucose pyrophosphorylase, facilitating the biomass and starch production. The supply of CO2 and acetate changed the amylose/amylopectin ratio by affecting the activity of starch branching enzymes and isoamylases. FTIR and XRD analyzes showed that the supply of CO2 reduced the long- and short-range ordered structure of starch, while acetate promoted the production of additional B- and V-type starch, resulting in a reduced digestibility. The combined supply of 2% CO2 and 5 g/L sodium acetate enabled the most efficient production of functional resistant starch (RS) measured with Englyst’s method, with a maximum RS content and yield reaching 13.7%DW and 0.40 g/L, respectively, on day 3. This study provided novel insights into the efficient production of high value-added functional starch (RS) from microalgae.

Keywords

• microalgae
• mixotrophy
• amylose
• amylopectin
• enzyme activity
• morphology