Microalgal starch is considered as renewable and sustainable feedstock for biofuels and biorefinery. High cell density culture is favourable for photoautotrophic starch production in microalgae in the aspects of productivity and economy, but it often encounters low starch content or extra stress exposure that limits the production. This study aimed to economically enhance photosynthetic starch production from CO2 fixation in a green microalga Tetraselmis subcordiformis by regulating photosynthetic stress status with a signalling molecule γ-aminobutyric acid (GABA) combined with the application of high initial cell density culture. By increasing initial cell density (ICD) from the normal of 1.1 g L−1 (NICD) to as high as 2.8 g L−1 (HICD), the starch content, yield, and theoretical productivity were improved by 7%, 63%, and 42%, respectively. The addition of GABA under HICD resulted in 14%, 19%, and 26% of further enhancement in starch content, yield, and theoretical productivity, respectively. GABA exhibited distinct regulatory mechanisms on photosynthesis and stress status under HICD relative to NICD. GABA augmented excessive light energy absorption and electron transfer through photosystem II that reinforced the photoinhibition under NICD, while alleviated the stress reversely under HICD, both of which facilitated starch production by enabling a suitable stress status while simultaneously maintaining a sufficient photosynthetic activity. The increase of ICD and/or GABA supply particularly boosted amylopectin accumulation, leading to the changes in starch composition and was more favourable for fermentation-based biofuels production. Preliminary techno-economic analysis showed that the highest net extra benefit of 9.64 $ m−3 culture could be obtained under HICD with 2.5 mM GABA supply where high starch content (62%DW) and yield (2.5 g L−1) were achieved. The combined HICD-GABA regulation was a promising strategy for economic starch production from CO2 by microalgae for sustainable biomanufacturing. Graphical Abstract

Chrysolaminarin, a kind of water-soluble bioactive β-glucan produced by certain microalgae, is a potential candidate for food/pharmaceutical applications. This study identified a marine microalga Isochrysis zhangjiangensis, in which chrysolaminarin production was investigated via nutrient (nitrogen, phosphorus, or sulfur) deprivations (-N, -P, or -S conditions) along with an increase in light intensity. A characterization of the antioxidant activities of the chrysolaminarin produced under each condition was also conducted. The results showed that nutrient deprivation caused a significant increase in chrysolaminarin accumulation, though this was accompanied by diminished biomass production and photosynthetic activity. -S was the best strategy to induce chrysolaminarin accumulation. An increase in light intensity from 80 (LL) to 150 (HL) µE·m−2·s−1 further enhanced chrysolaminarin production. Compared with -N, -S caused more suitable stress and reduced carbon allocation toward neutral lipid production, which enabled a higher chrysolaminarin accumulation capacity. The highest chrysolaminarin content and concentration reached 41.7% of dry weight (%DW) and 632.2 mg/L, respectively, under HL-S, with a corresponding productivity of 155.1 mg/L/day achieved, which exceeds most of the photoautotrophic microalgae previously reported. The chrysolaminarin produced under HL-N (Iz-N) had a relatively competitive hydroxyl radical scavenging activity at low concentrations, while the chrysolaminarin produced under HL-S (Iz-S) exhibited an overall better activity, comparable to the commercial yeast β-glucan, demonstrating I. zhangjiangensis as a promising bioactive chrysolaminarin producer from CO2.

The performance of microalgae-based wastewater treatment processes for ammonium-N (NH4+-N) removal depends on the maintenance of a favorable pH that is critical for minimizing nitrogen escape in the form of free ammonia (NH3) and preventing high-NH3 or extreme-pH stress. This study developed a CO2-inorganic carbon (CO2-IC) buffering system that automatically stabilized pH with the supply of a carbon source for efficient photosynthetic reclamation of NH4+-N by a euryhaline microalga Tetraselmis subcordiformis. The soluble (NaHCO3) and insoluble (CaCO3 and MgCO3) ICs were compared for this purpose. The pH was well controlled in the range of 6.5~8.5 in the CO2-IC system, which was suitable for the photosynthetic growth of T. subcordiformis. The NH4+-N (100 mg/L) was almost completely removed in three days, with the maximum removal rate of 60.13 mg N/L/day and minimal N escape of 19.65% obtained in the CO2-NaHCO3 system. The CO2-IC system also restricted the release of extracellular organic matter by preventing stress conditions. The CO2-NaHCO3 system enabled the highest “normal” starch production suitable for fermentation, while the CO2-CaCO3/MgCO3 system facilitated high-amylose starch accumulation that was conducive to producing bio-based materials and health-promoting ingredients. The proteins accumulated in T. subcordiformis were of good quality for animal feeds.