Effect of elevated carbon dioxide and nitric oxide on the physiological responses of two green algae, Asterarcys quadricellulare and Chlorella sorokiniana

Algae have the potential to capture carbon dioxide (CO₂) and nitric oxide (NO) from flue gases. However, the effects of high concentrations of these gases on the photophysiology of algae are poorly understood. To that end, we used the techniques of chlorophyll fluorescence to study the effect of industrially relevant levels of CO₂ and NO on the photophysiology of two green microalgae, Asterarcys quadricellulare and Chlorella sorokiniana, that are tolerant to these gases. Measurements of maximum quantum yield (F_v/F_m) and maximum relative electron transport rate (rETR_max) show an enhanced performance of photosystem II (PSII) under high CO₂ levels. In C. sorokiniana, high CO₂ stimulated non-photochemical quenching (NPQ), while the opposite effect was observed in A. quadricellulare. Light-saturated photosynthetic rates (P_max) of both species were highest at 10% CO₂. Further, the tested levels of NO did not show adverse effect on the performance of PSII. OJIP chlorophyll fluorescence transients suggest that in C. sorokiniana, the energetic communication between PSII units declined at 15% CO₂. However, in A. quadricellulare, this decline was visible even at 10% CO₂ with complete inhibition of cell growth at 15% v/v. Overall, our results suggest that although photosynthesis was regulated differently in the two microalga, both species exhibited enhanced PSII performance under reasonably high levels of CO₂ and NO. Thus, the two species are potential candidates for bio-fixation of CO₂ and NO from flue gases.