Lipid production in marine microalgae under different salinity, temperature and nutrient levels

The marine microalgae Chlorella sp. (UPMC-A0013) and Chaetoceros calcitrans (UPMC-A0010) contain relatively high lipid levels (15.0 to 20.0% of dry weight)that can contribute as important components for the formulation of feed in aquaculture industry. However, lipid levels in microalgae vary according to the culture conditions. This study was carried out to determine the various environmental factors that control the growth and lipid contents of marine Chlorella sp. and C.calcitrans by subjecting the algae to different levels of stress in terms of salinity and temperature changes and nutrient depletion in their early stationary phase. The effects of salinity stress on lipid production of Chlorella sp. and C. calcitrans were performed by culturing both microalgae species in Conway medium of various salinity levels (15, 20, 25, 30, 35 and 40‰). The cultures were centrifuged and resuspended into culture medium of lower salinity levels of -5‰ (S1i) and -10‰ (S2i) from the initial salinity levels in their early stationary phase. The highest growth rate (P<0.05) of Chlorella sp. and C. calcitrans were observed at 25‰ (μ=0.37 day-1) and 30‰ (μ=0.28 day-1), respectively. Salinity changes from S30 to S130 and S230 increased P<0.05) the production of total lipid in Chlorella sp. to 10.0% and 19.0% of d.w., respectively. Total lipid of C. calcitrans increased significantly (P<0.05) to 13.2% of d.w. when stressed to S230. Prior to the temperature stress, marine Chlorella sp. and C. calcitrans were cultivated at 20, 25, 30 and 35oC (Ti). Then, the microalgae were shifted to higher temperature of +5oC (TSi) from the initial temperature levels in their early stationary phase. Before stress, Chlorella sp. showed the highest growth rate (μ= 0.35 day-1, P<0.05) at 25oC (T25). Total lipid of Chlorella sp. significantly increase (P<0.05) from 31.0% of d.w. (T25) to 41.0% of d.w (TS25). Diatom, C. calcitrans showed the highest growth rate at T30 (μ= 0.26 day-1) and lipid content increased significantly (P<0.05) from 31.0% to 39.0% after stress (TS30). The effects of nutrient stress on lipid production was determined by culturing the algae into Nitrogen and Phosphorus deprived Conway medium (25.0% reduction = N25 and P25; 50.0% reduction = N50 and P50; 75.0% reduction = N75 and P75) in their early stationary phase. Lipid concentrations significantly increased (P<0.05) by 9.0% in both cultures subjected to 75% nitrogen and phosphorus reduction (N75 and P75). This study illustrated that various growth conditions such as salinity and temperature changes, and reduction in nutrient concentration enhanced lipid content of Chlorella sp. and C. calcitrans.

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