Regulatory Mechanisms of Photosynthetic CO2 Fixation in Unicellular Microbes

Aquatic photoautotrophs, such as microalgae and cyanobacteria, contribute 50% of carbon assimilated into organic matter through their carbon fixation ability. The Calvin-Benson cycle, a complex metabolic pathway, reduce carbon dioxide (CO2) and drive glucose production. Efficiency of carbon sequestration depends on the uninterrupted supply of reducing equivalents from light reactions and RuBp regeneration capacity. Regulation of the Calvin cycle in photosynthetic microbes involves glyceraldehyde-3-phosphate dehydrogenase, phosphoribulokinase, and CP12 protein complex. Rubisco produces two 3-phosphoglycerate molecules when CO2 is fixed to the acceptor molecule, ribulose-1,5-bisphosphate (RuBP). The enzyme has competitive carboxylase and oxygenase activities that rely on the balance of oxygen and CO2 at its active site. The RUBISCO enzyme has evolved in various forms with varying kinetic characteristics in autotrophic organisms. The CO2 concentrating mechanism (CCM) is an efficient approach to carbon acquisition used by microalgae and other photosynthetic microbes to survive and propagate when CO2 concentration limits photosynthesis. Photorespiration is a crucial metabolic process that helps to cope with high oxygen levels and prevent damage to photosynthetic machinery. Iron-sulfur clusters, malate shuttles, and light-enhanced respiration (LER) play essential roles in photorespiration and photosynthesis. Various approaches have been employed to improve algal growth and productivity cost-efficiently. Genetic engineering tools have been utilized to reduce antenna size, improve photosynthetic efficiency, and increase carbon fixation. Overexpression of RuBisCO activase, sedoheptulose bisphosphatase, and fructose bisphosphate, aldolase has also been shown to improve growth and photosynthesis. Recent developments in metagenomics-based discoveries, CRISPR-based genome editing, and adaptive evolution methods could facilitate the development of efficient microalgal strains for CO2 fixation.