Photosynthesis optimization in Nannochloropsis oceanica through genetic engineering of Photosystem I photoprotection mechanisms

In the last decades, the population on Earth has increased dramatically. This phenomenon, together with climate change, requires powerful and sustainable innovations in the production methods to meet the enormous energy and food demand without increasing the greenhouse effect. The exploitation of the microalgae biomass is a sustainable solution that can contribute to biomass production, thanks to their high growth rate, the production of valuable compounds and the lack of competition with crops for arable land. In photosynthetic organisms, the biomass production depends on the optimal functionality of the photosynthetic process, converting light energy into chemical energy. Several photoprotection mechanisms have been evolved by microalgae in order to modulate photosynthesis in response to variable environmental conditions and they have a major impact on biomass productivity. Among these are the cyclic electron flow and the alternative electron flow, which play a role in relieving the risk of over-reduction in the photosystem I (PSI). Nannochloropsis oceanica is a microalga with promising industrial applications, but its productivity on industrial scale is limited also by the irreversible photodamage at PSI in cultivation in photobioreactors. In this work we explored two strategies to improve PSI photoprotection in this organism by targeting the PGR5/PGRL1-driven cyclic electron flow and on the reduction of oxygen to water catalysed by the flavodiiron proteins (FLVs). In the former case, the presence of endogenous PGR5 and PGRL1 genes in N. oceanica was verified, and vectors for their overexpression were generated. Given the absence of the FLV proteins in N. oceanica, overexpressors strains of FLVA and FLVB proteins from the moss Physcomitrium patens were generated and analysed. Nevertheless, the presence and activity of the FLV proteins in the overexpressor strains could not be confirmed. At the same time, the selection of a strain of N. oceanica more suitable for industrial production was initiated, aiming at reducing its chlorophyll content. A strain with lower chlorophyll content could improve photosynthetic productivity by optimizing light capture and reducing the effect of the light gradient in dense cultures, typical of photobioreactors. After random mutagenesis and an initial screening, 15 promising clones were isolated for further analysis.