Phototrophic pigment production with microalgae

Abstract Microalgal pigments are regarded as natural alternatives for food colorants. To facilitate optimization of microalgae-based pigment production, this thesis aimed to obtain key insights in the pigment metabolism of phototrophic microalgae, with the main focus on secondary carotenoids. Different microalgal groups each possess their own set of primary pigments. Besides, a selected group of green algae (Chlorophytes) accumulate secondary pigments (secondary carotenoids) when exposed to oversaturating light conditions. In this thesis it was found for the first time that nutrient-depleted Isochrysis. aff. galbana T-ISO (Haptophytes) accumulates 3-hydroxyechinenone, a precursor of astaxanthin. Besides, it was found that nitrogen-depleted Chromochloris (Chlorella) zofingiensis (Chlorophyes) accumulates astaxanthin, presumably synthesised via echinenone, and ketolutein. Inhibition of production of β-carotene derivatives (e.g. echinenone and astaxanthin) did not lead to increased production of primary carotenoids (e.g. lutein) or ketolutein in this species, suggesting that the regulatory mechanisms controlling the flux towards ketolutein and primary carotenoids are not affected by the decreased levels of β-carotene derivatives. Besides, optimal yields of secondary carotenoids and triacylglycerol (TAG) on light were reached with C. zofingiensis for a range of biomass concentrations at the moment of nitrogen depletion. This indicated that the biomass-specific photon absorption rate did not affect the amounts of energy used for secondary carotenoid and TAG production, for the range of biomass concentrations tested. It was also found that nitrogen-depleted C. zofingiensis resupplied with nitrogen hardly degraded astaxanthin, whereas the other major secondary metabolites were degraded rapidly. This indicated that the overall carotenoid yield on light as well as its content may possibly be improved by applying a repeated batch instead of a series of single batch cultivations, which are traditionally applied. Finally, it was discussed that the highest increases in carotenoid yield on light can be reached by optimizing strain performance (using targeted genetic engineering and/or random mutagenesis), rather than by optimizing the cultivation conditions/operation mode or reactor design.