Biomass productivity enhancement and lutein enrichment of an acidic environment microalga

Resumen

Cultivation of extremophilic microorganisms has gained interest due to their ability to accumulate and produce high value compounds, namely metabolites, enzymes, carotenoids among others. Coccomyxa onubensis is an acid-environment microalga. Coccomyxa pigments profile is rich in carotenoids, especially lutein, of which naturally accumulates around 4 to 6 g Kg1 dry weight, that is in accordance with other lutein producing species. Furthermore, C. onubensis has the practical advantage of growing well in an extremely selective culture medium at high concentrations of heavy metals, and very low pH which preserves cultures from microbial contamination. This gives the microalga attractive potential as a producer of this photosynthetic pigment This thesis was aimed at increasing biomass productivity and carotenoid content of this lutein-rich microalga, C. onubensis. Chapter II intends to show that acid-environment microalgae can be produced at similar productivities of non-extreme microalgae, with the added advantage of their highly selective culture medium. The existence of carbon concentration mechanisms and conditions that might lead to their increased activity in C onubensis was studied as a tool to increase alga productivity. C. onubensis was grown under high and low CO2 concentrations, showing external and internal carbonic anhydrase activities. Best carbon uptake capacity and growth were showed to occur at acid pH, proving acid-tolerant behavior of C. onubensis. Incubation in air followed by shift to high carbon conditions enhanced carbon use efficiency in terms of growth rate and biomass productivity, based on the activity of both carbonic anhydrases. High concnetrations of lutein was accumulated and did not depend on inorganic carbon conditions. Consequently, chapter II showed that repeated cycles of air-incubation and high CCh-incubation of C. onubensis might become a suitable tool to perform production processes of lutein-enriched biomass. In Chapter III, the influence of Cu (II) on productivity and accumulation of value carotenoids of the microalga was studied. Copper was added in range between 0.06 and 0.4 mM, and an increase in algal viability, biomass and carotenoid productivities was obtained. A copper concentration of 0.2 mM was found to be as the most appropriate one to enhance productivity and lutein accumulation and was used in semicontinuous cultures. C onubensis was found to have great potential as lutein producer when compared to known lutein accumulating microalgae. At low light intensities, most algae produce more light-harvesting pigments to improve their photosynthetic efficiency while at high light intensities, some algae produce high concentrations of "sunscreen" pigments to protect the cell from excess light. In these terms, the aim of the work described in Chapter IV was to study the effect of low to moderate light intensity shifts on lutein accumulation of Coccomyxa cultures, doubly aimed at understanding the light'dependent role of main carotenoids in acid-tolerant microalgae and at developing strategies to induce lutein accumulation with applied purposes. Coccomyxa reached its maximum growth rates and carotenoid productivities at 400 pmol photons nv2 s1 that corresponds to maximal irradiance that reaches the surface of a flat panel reactor placed vertically under summer conditions in southern Spain. Cell density was also shown to influence short-time course evolution of carotenoids, which can be used for increasing lutein content of C. onubensis in outdoor production. According to that, the aims of the Chapter V were to evaluate the feasibility of outdoor cultivation of the acid-environment alga C. onubensis and to assess its potential as alternative source of lutein. C. onubensis was cultivated in acid culture medium under springtime outdoor conditions in a pilot tubular photobio reactor in southern Spain. The results showed that C. onubensis is able to withstand high temperature levels. Maximum daily biomass productivity was obtained at maximum irradiance of 1000 {imol photons nr2 s'1, however higher irradiances were detrimental for the culture. Intracellular lutein content increased at the end of the day, probably related to light absorption functions. This thesis gives a guideline of some of the parameters that influence both biomass productivity and carotenoid enrichment of the acid-environment microalga Coccomyxa onubensis. Therefore, the obtained results should allow to design mass production strategies of a potential large-scale cultivation of acid-environment microalgae and should allow to design lutein-rich biomass production strategies of this microalga.