Phototrophic, heterotrophic and mixotrophic growth in cold-adapted marine microalgae

Abstract

Microalgae are grown in different habitats and conditions, such as high salinity, extreme temperatures, and variable pH. Cold-adapted microalgae may have the capacity of generating high biomass production, which can potentially be produced under cold and light-limited conditions in the Arctic. Most microalgae are obligated photoautotrophs; however, few species have been found living in complete darkness, using heterotrophy as a metabolic path, which allows the algae to obtain energy from organic compounds, such as glucose, glycerol, and acetate. This present thesis investigated the effect of different trophic conditions – phototrophic, heterotrophic and mixotrophic – in the growth performance and the macromolecular composition (protein, carbohydrate, and lipid content) of five different strains of cold-adapted microalgae – Nannochloropsis oceanica, Dunaliella tertiolecta, Tetraselmis suecica, Chlorella ovalis, and Chlorocystis cohnii –. The results show that only C. ovalis and C. cohnii were able to grow under heterotrophic conditions. The protein content of all microalgae had the highest production under phototrophic condition, except N. oceanica, which had it under mixotrophic condition. However, values of carbohydrates and lipids are only trustful for C. ovalis, due to complications in the selected techniques. Therefore, C. ovalis had the higher carbohydrate content under the heterotrophic condition while the lipid content was higher in the mixotrophic condition. Thus far, the strains of N. oceanica, D. tertiolecta, and T. suecica are obligated photoautotrophs in cold environments, while C. ovalis and C. cohnii are facultative heterotrophs in cold environments.