Photosynthetic energy networks for the functioning and regulation of the algal CCM – AlgalCCM

Microalgae account for nearly half of the CO2 annually fixed by the Earth’s ecosystems. Because CO2 diffusion is slow in water and the CO2-fixing enzyme of photosynthesis (Rubisco) has a low affinity for CO2, the efficiency of microalgal photosynthesis greatly depends on a mechanism that concentrates CO2 (CCM) at the catalytic site of the carboxylating enzyme Rubisco. During CCM, the transport of inorganic carbon (Ci) against a concentration gradient and across membrane bilayers depends on the energy supplied by photosynthesis. Recently, bioenergetics mechanisms involved in the energy supply to the CCM have started to be uncovered and an integrated vision of the energy supply and distribution networks used for a proper functioning of the CCM has been proposed. Since the energy of photosynthesis is also needed to drive metabolic reactions of CO2 fixation, a tight control of energy-producing and energy-consuming reactions is required to make these two mechanisms functioning at their optimum and in a fluctuating environment. Thiol-based redox post-translational modifications have emerged as important mechanisms of regulation in all organisms and thioredoxins (TRXs) have been shown to control the thiol-disulfide status of many proteins involved in photosynthetic CO2 fixation. Moreover, recent large scale proteomic analysis (so called “thioredoxome”) showed that several CCM components are targets of TRXs. The objective of the Algal-CCM project is to unveil molecular mechanisms regulating photosynthetic energy production, distribution and use for optimal functioning of the CCM by focusing on the redox control by thioredoxins.
We first plan to identify novel thioredoxin-regulated CCM components using proteomics. Then, we plan to determine the redox level of some key cysteine residues involved in CCM regulation under contrasted physiological conditions, particularly when the CCM is functioning under different CO2 regimes. In parallel, we aim at studying redox regulation of CCM components both in vitro and in vivo, particularly stromal components involved in the production/use of energy (such as electron carriers of alternative pathways) or CO2 recycling (such as carbonic anhydrases), as well as components involved in energy distribution (metabolic shuttles) or bicarbonate transport.
By combining the expertise of two laboratories specialized in the fields of photosynthesis bioenergetics/genetics, and metabolic redox regulation/proteomics, the Algal-CCM project will provide new insight into the functioning and regulation of CCM. The ultimate ambition of the project is to determine how CO2 fixation and CCM components are fine-tuned to allow optimal energy use and coordinated operation of both systems CO2 concentration and fixation systems in a fluctuating environment. Such knowledge will be of critical importance to better assess the efficiency of CO2 capture by algae in aquatic ecosystems subjected to climate change, as it will be for the implementation of a functional algal CCM in plants to improve crop productivity.