Effect of pH on metabolite excretion and cell morphology of Euglena gracilis under dark, anaerobic conditions

Abstract

Euglena gracilis is a unicellular eukaryotic microalga found in aquatic environments. It can acclimate its morphology in response to various environmental stress factors such as changes in temperature, light, and ion concentrations. E. gracilis cells excrete succinate and amino acids under dark and anaerobic conditions; however, only a few studies on the effect of these conditions on the cell morphology of E. gracilis have been conducted thus far. In the present study, we aimed to evaluate the effects of pH on succinate, glutamine and glutamate excretion in E. gracilis and the correlation between the levels of these metabolites and cell morphology under dark, anaerobic conditions. The excretion of succinate, glutamine and glutamate by E. gracilis is pH dependent. Glutamate and glutamine excretion increased under acidic conditions (pH 3–5). On the contrary, succinate production depended on the buffer salts; succinate levels were higher when GTA [3,3-dimethyl-glutaric acid, tris(hydroxymethyl)aminomethane, and 2-amino-2-methyl-1,3-propanediol] or citrate buffer were used and were lower when acetate buffer was used (pH 4 or 5). The number of spindle cells tended to increase under acidic conditions and the aspect ratio of the cells was positively correlated with glutamine and glutamate levels. Increase in cell density particularly enhanced glutamine and succinate production; in particular, the succinate titer reached 1.5 g/L. Our findings indicate that pH of the media alters metabolite production and cell morphology of E. gracilis cells under dark, anaerobic conditions. A deeper understanding of eukaryotic fermentation will benefit the biorefinery industry, mainly for the production of value-added products using anaerobic microorganisms.