Light- and substrate-driven variations in microalgal metabolism for biotechnological applications

Microalgae are photosynthetic organisms of great interest as they represent a promising and sustainable resource with a wide range of potential biotechnological applications, such as nutraceuticals, cosmeceuticals, pharmaceuticals, and biostimulants. Light and nutrient availability are key factors influencing the growth and production of bioactive molecules in microalgae. Light quality and intensity, as well as the availability of organic carbon sources, can significantly modulate the physiology of these unicellular organisms. In particular, mixotrophy, a nutritional strategy that combines photosynthesis with the uptake of organic carbon compounds, offers interesting opportunities to optimize biomass production and specific high-value metabolites. Studying the combined effects of light and mixotrophy is a crucial step for developing more efficient cultivation strategies. The first case study of the Thesis explored the impact of different cultivation conditions on the green microalga Ettlia oleoabundans on a laboratory scale. In particular, the influences of different light qualities (white and red-enriched white) and trophic modes (photoautotrophy and mixotrophy) on growth kinetics and biochemical composition of microalgal biomass were investigated. The results showed that the combination of red-enriched white light and mixotrophic cultivation significantly promoted the accumulation of phenolic compounds and polyunsaturated fatty acids (PUFAs), as well as increasing biomass yield. The studies presented in Chapter 3 and 4, aimed to bridge the gap between laboratory-scale studies and industrial applications, investigating the impact of light spectrum on the growth and biochemical composition of E. oleoabundans in 70-liter photobioreactors, both in photoautotrophy and in mixotrophy. To overcome challenges associated with large-scale mixotrophic cultivation, a two-stage cultivation strategy was implemented. By initially cultivating E. oleoabundans photoautotrophically followed by a mixotrophic phase, contamination issues were mitigated, and biomass productivity was enhanced. The results demonstrated that specific light spectra, particularly those enriched in red light, can significantly influence biomass production and the accumulation of valuable biomolecules such as proteins, carbohydrates, and antioxidants. Mixotrophy can significantly impact the growth kinetics and biochemical composition of the microalga, leading to increased accumulation of valuable biomolecules. These findings highlight the importance of optimizing large-scale microalgal cultivation to ensure an effective transition towards industrial-scale production processes. In the study presented in Chapter 5, the cultivation of E. oleoabundans was extended to 60 days, an unusually long duration for this alga. This choice allowed for a thorough characterization of biomass evolution and identification of the best time for harvesting both biomass and conditioned culture medium, which was surprisingly enriched in extracellular vesicles. The results showed that the early stationary phase is the most promising for the isolation of extracellular vesicles and for obtaining biomass rich in proteins and phenolic compounds. To address the increasing demand for sustainable and efficient agricultural practices, microalgae-based biostimulants have emerged as a promising alternative to synthetic chemicals. In Chapter 6, the potential of microalgae-conditioned medium (MCM) derived from E. oleoabundans and Auxenochlorella protothecoides as a novel biostimulant for strawberry plants, was investigated. Field trials demonstrated that foliar application of MCM significantly enhanced plant growth, yield, and fruit quality. These findings highlight the potential of microalgae-based biostimulants to promote sustainable agriculture and improve crop productivity.