Fine-tuned regulation of photosynthetic performance via γ-aminobutyric acid (GABA) supply coupled with high initial cell density culture for economic starch production in microalgae

Abstract

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

CO2-Inorganic Carbon Auto-Buffering System for Efficient Ammonium Reclamation Coupled with Valuable Biomass Production in a Euryhaline Microalga Tetraselmis subcordiformis

“…L38 when the pH increased from 7 or 8 to 9 during the NH 4 + -N removal, which was consistent with the present study [26]. The starch accumulation generally occurred on Day 4 and Day 2, respectively, in the ICs and CO 2 -ICs systems (Figure 4b), which correlated to the exhaustion of NH 4 + -N in the medium (Figure 2a,b), indicating a nitrogen limitation-triggered starch synthesis, as had been widely recognized in microalgae, including Tetraselmis subcordiformis [39,60]. Similar to the protein accumulation, the starch production was markedly improved in the CO 2 -IC cultures compared to the IC cultures.…”

“…The starch in the microalgal biomass was extracted with 45% perchloric acid and stained with I 2 -KI solution (1:2, v/v) at 25 • C for 15 min followed by a spectrophotometry analysis under 618 and 550 nm according to the previous study [39], which allowed the simultaneous determination of amylose (Am) and amylopectin (Ap) concentrations [40]. The total starch was estimated as the sum of amylose and amylopectin.…”

“…where TN bimass (t)/(0) (mg/L) is the total N element in the biomass at time t/0 (day), TN water (t)/(0) (mg/L) is the total N concentration in the water at time t/0 (day), TN (0) represents the initial total N (mg/L) in the culture system, ω N (w/w) is the N content in biomass, and C biomass (mg/L) is the apparent biomass concentration determined gravimetrically according to the previous study [39]. The total organic carbon (TOC) in the water was determined with a TOC analyzer (TOC-L CPH/CPN, Shimadzu, Japan).…”

Ultrasonication followed by aqueous two-phase system for extraction, on-site modification and isolation of microalgal starch with reduced digestibility

“…While the cultivation of microalgae for starch production has been focused on for investigation for years [10] , [13] , [14] , [15] , the downstream processing such as extraction, isolation and modification of microalgal starch for functional use is still in its infancy. Microalgae generally have cell walls, which needs cell disruption process to release the intracellularly stored starch granules before isolation [16] , [17] .…”

“…The aim of this study was to use ultrasonication for synchronous cell disruption, extraction and modification (termed as on-site modification) of starch from a starch-producing marine microalga Tetraselmis subcordiformis [15] , [26] , followed by a ATPS process to isolate and discriminate the starches with reduced digestibility. Different ultrasonic powers and durations were applied to dissect the influence of ultrasonic conditions on the cell disruption efficiency, starch repartition behavior and recovery efficiency on ATPS, and the morphological, structural, compositional and functional changes of the isolated starch.…”