The effects of different culture methods on the growth and biochemical composition of Chlorella sp.

Studies on microalgae culture are gradually shifting towards heterotrophic and mixotrophic cultures in order to eliminate the limiting factors in phototrophic culture and to increase the maximum amount of dry matter that can be obtained. While heterotrophic culture occurs by the reproduction of microalgae on organic carbon sources independently of photosynthesis, in mixotrophic culture both CO2 and organic carbon source are used simultaneously. Myo-inositol, which is among the sugar alcohols, is a cyclohexane with the formula C6H12O6. In this study, possible use of myo-inositol as a carbon source in Chlorella sp. culture and its effects on the biochemical structure were investigated. Experiments were carried out in flasks and tubular photobioreactor. According to the data obtained from the results of the experiments performed in 1 l flasks, the highest cell counts were found as 52±0.71 x 106 cells/ml and 49.28±0.75 x 106 cells/ml, with 1000 mg/l and 2000 mg/l myo-inositol groups, respectively. The highest dry weight amount was determined as 1.62±0.02 g/l in the group containing 1000 mg/l myo-inositol, and the lowest dry weight amount was determined as 0.11±0.01 g/l in the control group. The highest amounts of chlorophyll a and total carotenoids were also determined in the groups containing 1000 and 2000 mg/l myo-inositol. At the end of the experiment, the highest amount of chlorophyll a per cell was determined as 0.092±0.005 pg/cell in the group containing 2000 mg/l myo-inositol. The same group included the highest amount of total carotenoids per cell, and this value was calculated as 0.031±0.001 in the analyzes were conducted at the end of the experiment. The lowest chlorophyll a and total carotenoid accumulation were determined as 0.051±0.002 pg/cell and 0.018±0.001 pg/cell, respectively, in the group containing 2000 mg/l glycerol. The experimental groups with the highest lipid content were the groups containing 1000 and 2000 mg/l myo-inositol, and were contain 10.01±0.36% and 15.19±0.75% lipid, respectively. The lowest lipid content was determined as 4.04±0.21% in the group using 100 mg/l myo-inositol. Saturated fatty acids were determined at the highest rate (44.74±2.2%) in the group containing 100 mg/l myo-inositol, while the highest monounsaturated fatty acids were determined in the group containing 2000 mg/l myo-inositol by 18.63±0.09%. Lastly, polyunsaturated fatty acids were determined as 62.15±0.23% in the group containing 500 mg/l myo-inositol at the highest rate. The photobioreactor culture trial took approximately 5 weeks. CO2 was used in the first week, both CO2 and myo-inositol in the following 2 weeks, and only myo-inositol was used as carbon source in the last 2 weeks. The average dry weight for the first week using CO2 was 3.22±0.08 g/l. The average dry weight was determined as 4.05±0.15 g/l in the period when only myo-inositol was used. Both chlorophyll a and total carotenoid amounts were decreased with the use of myo-inositol. The amount of chlorophyll a per cell decreased by approximately 22% in the period when myo-inositol was used compared to the period when CO2 was used. In addition approximately 29% decrease was determined for the total amount of total carotenoids per cell during the period of using myo-inositol as carbon source. The lipid ratio increased from 17.47±1.02% to 19.57±0.39% when myo-inositol was used as only carbon source. A decrease in the ratio of polyunsaturated fatty acids, an increase in the ratio of monounsaturated and saturated fatty acids were determined during the same period. While the ratio of saturated fatty acids was 32.77% on average in the periods when CO2 and CO2 and myo-inositol were used together, it was determined as 33.1% in the period when only myo-inositol was used. The mean ratios of monounsaturated fatty acids were calculated as 1.79% in the period of using only CO2, 1.89% in the period of using CO2 and finally in the myo-inositol period as 4.84%. Polyunsaturated fatty acids, on the other hand, were detected at an average rate of 65-66% during the periods when CO2 and CO2 and myo-inositol were used, but decreased to low as approximately 60% in the period when only myo-inositol was used. Even tough, myo-inositol is a more costly carbon source than CO2, it will be a competitive carbon source due to both the lower amount of carbon source consumed per unit biomass and lower energy cost because of reducing the dependency on the light source is considered.