Effect of inorganic carbon limitation on the conversion of organic carbon to total fatty acids by Monodus subterraneus.

Traditional autotrophic microalgae exhibit low rates of organic carbon assimilation and conversion to useful compounds when switching to mixotrophic or heterotrophic growth. The goal of this study was to investigate the effect of inorganic carbon limitation on the efficiency of organic carbon (glycerol) assimilation and conversion to total fatty acids (TFAs) or the long-chain polyunsaturated fatty acid eicosapentaenoic acid (EPA). An oleaginous Monodus subterraneus was selected and six cultivation conditions were set, including Autotrophy-no aeration, Autotrophy-aeration, Mixotrophy-no aeration, Mixotrophy-no aeration & no Na2CO3, Mixotrophy-aeration, and Heterotrophy. The results showed M. subterraneus could utilize glycerol and grow under mixotrophic condition, while it was not occurred under heterotrophy. Superiority of mixotrophy to autotrophy on biomass productivity was more obvious under inorganic carbon limitation (no aeration or no Na2CO3) than inorganic carbon supply (aeration and existing Na2CO3 in the medium). CO2 limitation (no aeration) decreased content (of dry weight) and production (in medium) of TFAs, which was not evident in mixotrophy. CO2 limitation and inorganic carbon substrate stress largely improved the COD yield of TFAs and EPA under mixotrophic condition. TFA yield (%COD) under Mixotrophy-no aeration & no Na2CO3 was maximum (22.82%), and was almost two-fold higher than that under Mixotrophy-no aeration and nearly three-fold higher than that with Mixotrophy-aeration. EPA yield (% COD) under mixotrophy-no aeration & no Na2CO3 was maximum (6.58%). These results suggested that inorganic carbon limitation is a potentially useful method to enhance conversion of organic carbon to TFAs. Furthermore, the results suggest an application to obtain high value compounds (TFAs or EPA) combined with a high assimilation rate of waste glycerol from biodiesel and epichlorohydrin production by microalgae.