Biotechnological potential of microalgae: morpho-physiological and biochemical studies

However, after 7 days, glucose-grown cells entered the stationary phase and started to accumulate lipids. Conversely, in cells grown in presence of AWP lipid accumulation was induced after 21 days of growth. In a second moment, to promote a more rapid lipid accumulation, cells grown in AWP and in normal autotrophic medium for 7 days were transferred under nutrient depletion. Both samples suddenly reached the stationary phase. Moreover, a decrease in pigment content, alteration of the cell morphology and concomitant lipid synthesis were observed. This study confirmed that glucose can be considered a very suitable substrate for the obtainment of high-lipid enriched N. oleoabundans. However, it is considered a very-expensive substrate. Then, N. oleoabundans growth can be alternatively coupled in a bioremediation process to obtain high biomass concentrations enriched in lipids. This Thesis provides also advanced insights in the organization of the thylakoid protein complexes which characterize the photosynthetic membranes when N. oleoabundans is grown mixotrophically. Indeed, very little is known about this topic, but investigation in mechanisms which regulate photosynthetic light reactions and carbohydrate metabolisms might be useful for the scaling up of mixotrophic microalgal cultivation, for instance to plan the most fruitful type of illumination. In order to better understand the effects of mixotrophy on the organization of the thylakoid protein complexes which characterize the photosynthetic apparatus, thylakoids from cells of N. oleoabundans grown in presence of glucose were isolated to perform biochemical and biophysical analyses. On the whole, the results obtained showed dramatic changes in PSII activity and linear electron flow in mixotrophic samples with respect to autotrophic controls, with probable modifications in state-transition capability and reduced photosynthetic performance. However, further investigation are needed to provide a complete background. For this reason, this work can be considered a starting point from which further research might be developed. Finally, in a third part the effects of the expression of two exogenous phytoene synthase from Arabidopsis thaliana (AtPSY) and Oryza sativa (OsPSY1) genes in the green microalga Chlamydomonas reinhardtii have been studied, with particular regard to carotenoid (Car) accumulation, Car profile changes and photosynthetic performance. The expression of the transgenes was confirmed in only one transformant with AtPSY, which showed increased amounts of Car. However, by further experiments in which cells were grown in different light regimes, it was shown that Car accumulation was light-dependent, while initial increased amounts of zeaxanthin, antheraxanthin, violaxanthin and lutein, were always observed in transformed cells. This altered Car profile caused a different use of the light during photosynthesis. In order to optimize exogenous Car production in microalgae like C. reinhardtii, basic knowledge of the Car biosynthetic pathway and its regulation needs to be improved. However, this work allowed to gain important knowledge on the genetic engineering of microalgal cells and the methods used might as well be applied to the fatty acid metabolic pathway for the increase in lipid accumulation in a very next future.