Study on the molecular mechanisms at the basis of photosynthesis toward microalgae domestication

Oxygenic photosynthesis sustains life on Earth, being the process through which sunlight is used to drive carbon fixation into biomass releasing oxygen as by-product. Photosynthetic organisms convert light energy into chemical energy thanks to the activity of pigment-binding multiproteic complexes known as photosystems and their relative light-harvesting antenna complexes. More than often, light availability is non-optimal, either too scarce or in excess compared to the photosynthetic capacity of the organism operating photosynthesis, to the point it may result dangerous. Among the most sensitive complexes of the photosynthetic chain, photosystem II is the main site of Reactive Oxygen Species (ROS) formation. These detrimental compounds may lead to a compromised growth and losses in biomass productivity. For this reason, photosynthetic organisms evolved different photoprotection mechanisms, among which Non-Photochemical Quenching (NPQ) is one of the most important and investigated. Microalgae dissipate most of the absorbed energy through this safe-valve mechanism, even when conditions are not dangerous, determining a consequent reduction of biomass accumulation. Moving towards an optimized photosynthetic yield is one of the main objectives in microalgae domestication, which itself is an ever-growing interest, because sustainable microalgal industrial applications have the potential to satisfy many global demands. Among the most important, is the use of algal biomass as food and feed, bio-fuels production, extraction of high-value nutraceuticals and pharmaceuticals, recovery of wastewaters, and carbon capture. However, realization of this potential requires a decrease of the current production costs. To achieve profitability, identification of limiting factors is fundamental. Particularly, light-to-biomass conversion efficiency is a key bottleneck that needs to be addressed to achieve domestication. With this focus, the aim of this thesis was to study microalgal photosynthetic behavior upon different growth conditions and the relative activated energy dissipation processes as a possible target to improve productivity, thus enabling the use of microalgae as green cell factories. All the studies herein presented were conducted on the model organism for green algae, Chlamydomonas reinhardtii, for which extensive literature, mutants’ libraries, and genetic tools are available.