## 3 Results
### Conversion of Waste Glycerol with Free and Immobilized Cells of G. oxydans ATCC 621
Conversion of Waste Glycerol with Free and Immobilized Cell Extract Obtained from G. oxydans ATCC 621  
Time profiles of glycerol conversion and DHA formation during bioconversion of 30 g-L\({}^{-1}\) of a waste glycerol fraction with the use of free or immobilized cell extract from _G. oxydans_ cells are presented in Figures 6 and 7. In the first day of the process with the use of a free cell extract as a biocatalyst, only 5 g\(\cdot\)L\({}^{-1}\) of waste glycerol was consumed (Figure 6). The substrate consumption rate increased on the next day to 14.6 g\(\cdot\)L\({}^{-1}\). After 72 and 96 h, 13 g\(\cdot\)L\({}^{-1}\) of unutilized glycerol remained in the medium (Figure 6). A significant increase in DHA concentration was recorded after 48 h, and it was about 13 g\(\cdot\)L\({}^{-1}\). On the next day, it's concentration increased by 3 g\(\cdot\)L\({}^{-1}\), but after 48 h it decreased by 0.5 g\(\cdot\)L\({}^{-1}\) (Figure 6). The degree of substrate consumption in the bioconversion process conducted with an immobilized cell extract and 3% of crude glycerol was low, as a significant portion of glycerol (ca. 20 g\(\cdot\)L\({}^{-1}\)) still remained unutilized in the reaction medium after 96 h (Figure 7). The concentration of the expected product was also insufficient. After 24 h, only 2 g\(\cdot\)L\({}^{-1}\) of DHA was produced, after 48 h its production was at around 6 g\(\cdot\)L\({}^{-1}\), while in the further reaction DHA concentration did not exceed 8 g\(\cdot\)L\({}^{-1}\) (Figure 7). Changes in glycerol and DHA concentrations during bioconversion of 50 g\(\cdot\)L\({}^{-1}\) of a waste glycerol fraction with the use of free or immobilized cell extract from _G. oxydans_ cells are presented in Figures 8 and 9. Reaction with 50 g\(\cdot\)L\({}^{-1}\) initial waste glycerol concentration and free cell extract resulted in low consumption of glycerol (only 5 g\(\cdot\)L\({}^{-1}\) and 7.5 g\(\cdot\)L\({}^{-1}\) after 24 and 48 h, respectively) (Figure 8). The final concentration of DHA successively increased throughout the reaction from about 4 g\(\cdot\)L\({}^{-1}\) (after 24 h), through 9 g\(\cdot\)L\({}^{-1}\) (after 72 h), to the maximum value of 15 g\(\cdot\)L\({}^{-1}\) (after 96 h) (Figure 8). The course of utilization of 5% of waste glycerol with the immobilized cell extract as a biocatalyst showed a low rate of substrate consumption. After 96 h of biotransformation, a substantial portion of the glycerol remained unutilized in the medium (Figure 9). The concentration of the expected product increased successively until 72 h, and decreased further from 6 to about 5 g\(\cdot\)L\({}^{-1}\) after 96 h (Figure 9). Substrate Consumption Rate and DHA Production Yield during Bioconversion of Waste Glycerol by G. oxydans ATCC 621 Cells and Their Extract  
The rates of glycerol consumption, DHA production, and DHA yield during bioconversion of a waste glycerol fraction conducted using all tested variants are presented in Tables 1 and 2. During bioconversion of a waste glycerol fraction with free cells, the glycerol consumption rate was on average 0.75 g\(\cdot\)L\({}^{-1}\)\(\cdot\)h\({}^{-1}\) after 24 h, and after the next 24 h it was on average 0.6 g\(\cdot\)L\({}^{-1}\)\(\cdot\)h\({}^{-1}\), irrespective of the initial concentration (30 or 50 g\(\cdot\)L\({}^{-1}\)) (Table 1). The further course of the process clearly showed that the glycerol consumption rate was much higher in the reaction proceeding in a solution with a lower initial substrate content (30 g\(\cdot\)L\({}^{-1}\)). In these experimental variants (free cells and 30 or 50 g\(\cdot\)L\({}^{-1}\) initial content of glycerol), the rate of DHA production was very similar at each time of measurement (Table 1). The immobilization of _G.