Proteomics reveals toxin tolerance and polysaccharide accumulation in Chlorococcum humicola under high CO₂...

Algae have great application prospects in excess sludge reclamation and recovery of high-value biomass. Chlorococcum humicola was cultivated in this research, using sludge extract (mixed with SE medium) with additions of 10%, 20%, and 30% CO₂ (v/v). Results showed that under 20% CO₂, the dry weight and polysaccharide yield reached 1.389 ± 0.070 g/L and 313.49 ± 10.77 mg/L, respectively. 10% and 20% CO₂ promoted the production of cellular antioxidant molecules to resist the toxic stress and the toxicity of 20% CO₂ group decreased from 62.16 ± 3.11% to 33.02 ± 3.76%. 10% and 20% CO₂ accelerated the electron transfer, enhanced carbon assimilation, and promoted the photosynthetic efficiency, while 30% CO₂ led to photosystem damage and disorder of antioxidant system. Proteomic analysis showed that 20% CO₂ mainly affected energy metabolism and the oxidative stress level on the early stage (10 d), while affected photosynthesis and organic substance metabolism on the stable stage (30 d). The up-regulation of PSII photosynthetic protein subunit 8 (PsbA, PsbO), A0A383W1S5 and A0A383VRI4 promoted the efficiency of PSII and chlorophyll synthesis, and the up-regulation of A0A383WH74 and A0A2Z4THB7 led to the accumulation of polysaccharides. The up-regulation of A0A383VDH1, A0A383VX37 and A0A383VA86 promoted respiration. Collectively, this work discloses the regulatory mechanism of high-concentration CO₂ on Chlorococcum humicola to overcome toxicity and accumulate polysaccharides.