Investigating inorganic carbon transport in globally important algal lineages

Algae are responsible for up to 30% of all global photosynthesis, the process whereby light and CO2 are converted into O2 and chemical energy, in the form of sugars. This process provides energy for the bottom of the global food web, produces the O2 we breathe and plays an important role in removing the greenhouse gas CO2 from the atmosphere. Algae have evolved a mechanism to boost their photosynthesis making it very efficient, this mechanism is known as the CO2 concentrating mechanism (CCM). The CCM functions by taking up both CO2 and HCO3- from the environment to fuel photosynthesis. Algae take up HCO3- as well as the necessary CO2 because: (1) it can easily be converted to the CO2 needed for photosynthesis; (2) it is more readily available than CO2 in the aquatic environments algae live in; and (3) it doesn't leak out the cell like CO2 is prone to. The HCO3- is transported through the algal cell to a compartment called the thylakoid, which is found in the chloroplast and is where the photosynthetic process occurs. In the thylakoid the HCO3- is converted to CO2. The CO2 then is readily available for an enzyme called Rubisco which uses it to drive the photosynthesis process. Although we know HCO3- transport is very important in the CCM, until now we did not know how it travelled into the thylakoid to be converted to CO2. My research has recently identified a protein channel that enables HCO3- transport into the thylakoid in a species of algae called Chlamydomonas reinhardtii. I have also identified similar protein channels in other environmentally important groups of algae. I hypothesise that these similar protein channels may be a shared mechanism of HCO3- transport across algal groups. The Fellowship research I have proposed will extensively investigate these potential HCO3- channels in different algal groups to fully understand their function. Many algae have a CCM, but we know there are differences in the specific mechanism between different groups. Therefore, it is important to explore CCM components, like HCO3- channels, in different groups to be able to inform the research community accurately on the shared components in the algal CCM. This research is important because it strengthens our understanding of a process responsible for approximately a third of all global photosynthesis, influencing all life on Earth. The data generated in this project also helps researchers I collaborate with closely who are attempting to engineer a CCM into crop plants. If the CCM is successfully inserted into crop plants, it has been modelled to significantly increase photosynthetic efficiency and therefore increase crop yields by up to 60%. This yield increase could directly alleviate issues surrounding global food supply which have been increasing as a result of population increase and climate change.

Photosynthesis is fundamental to life on Earth. Up to 30% of global photosynthesis is conducted by algae, a diverse group of eukaryotes living in aquatic environments. Numerous lineages of algae have evolved CO2 concentrating mechanisms (CCMs) to enhance the efficiency of their photosynthesis. A crucial component of the CCM is HCO3- transport through the cell, but the protein mediated pathway the HCO3- takes remains unresolved. I have recently identified a bestrophin-like protein channel (BST1) in the green algae Chlamydomonas reinhardtii (hereafter Chlamydomonas). BST1 has been shown to localise to the thylakoid membrane; is involved in the CCM; and has been shown to be specifically permeable to HCO3- anions using a Xenopus expression system. The data conclude BST1 enables HCO3- transport across the thylakoid membrane in Chlamydomonas. I have subsequently identified BST1 homologues in broad algal lineages and hypothesise this may be a shared mechanism of inorganic carbon (Ci) transport. Although Chlamydomonas is the most well characterised algal CCM, the majority of algal primary productivity is conducted by marine algal outside the green lineage. Therefore, this project will characterise the bestrophin-like proteins in Chlamydomonas and wider algal lineages, including diatoms and coccolithophores, to fully understand potential shared components of HCO3- transport through the algal CCM and the environmental relevance of these channels. To study these proteins I will (1) conduct extensive bioinformatics analysis; (2) investigate the protein structure using biochemical and cryoEM approaches; (3) use a range of molecular biology techniques including fluorescent tagging for localisation studies and quantitative gene expression analysis to investigate the regulation of these proteins in response to CCM induction; (4) generation of CRISPR knock-out mutants and their characterisation; and (5) functional screening through a heterologous expression system.