Biomass and docosahexaenoic acid formation by Schizochytrium mangrovei Sk-02 at low salt concentrations

Growth and docosahexaenoic acid ( DHA) formation in Schizochytrium mangrovei Sk-02 were studied in yeast extract-glucose ( YEG) media supplemented with either artificial sea salts ( ASS) or a simplified salts medium consisting of magnesium sulfate and/or sodium chloride. With increasing concentrations of ASS, biomass increased from 9 ( no ASS) to a maximum of 21 g l(-1) ( at 2.1 M ASS or 68 g l(-1)). Addition of manganese increased these values to 17 ( no ASS) and a plateau of 24 g l(-1) ( at) 0.25 M ASS). Replacing ASS with magnesium sulfate and/or sodium chloride and manganese as an additional factor suggested that maximal biomass ( 24 g l(-1)) could only be achieved when both magnesium sulfate and manganese were added. As significant carry-over of elements in the inoculum was observed, the latter was washed in further experiments. Addition of an optimal concentration of ca. 5 mM magnesium sulfate to YEG-media then resulted in a maximum biomass level of ca. 24 g l(-1) and 5.2 g l(-1) DHA in 72 h. Additional supplementation with sodium chloride ( 8 mM) significantly stimulated biomass formation when magnesium sulfate addition was below 0.8 mM, but not at higher levels of the latter. However, cultivation time was reduced by ca. 24 h, resulting in formation of ca. 5.5 g l(-1) DHA in only 48 h. By replacing ASS with magnesium sulfate plus sodium chloride, the sum of the concentration of these four elements in media could be reduced ca. nine-fold, while achieving similar biomass- and DHA-yields. The concentration of chloride, a compound highly corrosive to steel fermenters, was reduced more than ten-fold.