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Microalgal starch, a potential feedstock for renewable fuels and chemicals, is usually induced under stress, which hinders the simultaneous enhancement of starch content and yield. This study applied γ-aminobutyric acid (GABA) to modulate cell metabolism for improving photosynthetic starch production in microalga Tetraselmis subcordiformisunder nitrogen limitation (±N) and depletion (−N). Under ±N, 2.5 mM GABA abated nitrogen metabolism but enhanced starch content by 27% with an uncompromised starch yield due to the moderate inhibition of cell growth and elevated photoprotection. Under −N, 10 mM GABA entitled improved stress tolerance and enhanced starch content by 23.4% with a simultaneous 28.6% promotion of starch yield. The maximum starch content and yield under this condition reached 69.0% DW and 1.84 g L–1, respectively, which represented the highest starch levels in this microalga and exceeded most of the starch production performance among the microalgae reported. GABA addition was promising to improve microalgal starch production.

Microalgae are potential starch producers as alternatives to agricultural crops. This study disclosed the effects and mechanism of phosphorus availability exerted on storage starch production in a starch-producing microalga Tetraselmis subcordiformisin nitrogen starvation conditions. Excessive phosphorus supply facilitated starch production, which differed from the conventional cognition that phosphorus would inhibit transitory starch biosynthesis in plants. Phosphorus enhanced energy utilization efficiency for biomass and storage starch production. ADP-glucose pyrophosphorylase (AGPase), conventionally known to be critical for starch biosynthesis, was negatively correlated to storage starch biosynthesis. Excessive phosphorus supply maintained large cell volumes, enhanced activities of starch phosphorylases (SPs) along with branching enzymes and isoamylases, and increased phosphoenolpyruvate and trehalose-6-phosphate levels to alleviate the inhibition of high phosphate availability to AGPase, all of which improved starch production. This work highlighted the importance of phosphorus in the production of microalgal starch and provided further evidence for the SP-based storage starch biosynthesis pathway.

ABSTRACTThe production of poly-3-hydroxybutyrate (PHB) by photosynthetic cyanobacteria is a potentially sustainable production method for the biodegradable plastics industry. β-Ketoacyl-ACP reductase (FabG), from the photosynthetic cyanobacterium Synechocystissp. PCC 6803 (SpFabG), is the first NADPH-dependent reductase in the fatty acid biosynthesis pathway. Its structure is similar to that of acetoacetyl-CoA reductase (SpPhaB), which is critical for PHB synthesis and can replace SpPhaB for acetoacetyl-CoA reductionin vitro. However, the specific function of SpFabG in fatty acid synthesis and whether SpFabG could participate in PHB synthesisin vivowere not yet clear. In this study, the role of SpFabG in fatty acid synthesis was first verifiedin vivoby knocking down and overexpressing of fabG. It was shown that SpFabG was essential yet not rate-limiting for fatty acid biosynthesis. The biochemical characterization of SpFabG using acetoacetyl-CoA as the substrate showed that the optimum temperature, optimum pH, Kmand kcatwere 30°C, 7, 2.30 mM, and 19.85 s−1, respectively, which exemplified the ability of SpFabG to reduce acetoacetyl-CoA with a relatively low affinity and weak catalytic efficiency. Functional analysis of SpFabGin vivoindicated that SpFabG was able to partially complement SpPhaB under nitrogen-deprived conditions, and overexpression of fabGled to the diversion of partial carbon flux from fatty acid toward PHB synthesis.