Screening, Cultivation And Cells Harvesting Of Oleaginous Microalgae From Tropical Ocean

Microalgae biodiesel is one of the most potential candidates for biodiesel production. And obtaining the outstanding microalgal strains with high lipid content is the first and most important issue for producing biodiesel from microalgae in large-scale. To exploit the indigenous species of microalgae with high lipid content in Hainan, microalgal strains were isolated from several fresh and marine waters in Hainan, and screening and identification were performed for oleaginous microalgal strains. Whereafter, the effects of culture conditions on growth and lipid accumulation were investigated, as well as cells harvesting with flocculants was preliminarily studied in this research.Fifty-four water samples were collected from Hainan. One hundred and one microalgal strains, twenty four isolates from marine waters and the others from freshwaters, were separated by repeated plating and streaking. Eight isolates were preliminarily screened with Nile Red stain and identified by morphological and biomolecular method. These eight cultureswere identified as Tetranephris brasiliensis DL12, Ankistrodesmus gracilis DL25, Desmodesmus subspicatus WC01, Desmodesmus sp. WC08, Ankistrodesmus sp. CJ02, Ankistrodesmus gracilis CJ09, Chlorella vu/garis CJ15and Chlorella sorokiniana XS04, respectively.By further cultivation, characters of growth and lipid accumulation were analyzed. The results showed C. vulgaris CJ15had the highest special growth rate (0.64d-1), and the higher lipid content were obtained by A. gracilis CJ09(47.90%) and C. vulgaris CJ15(47.39%), respectively. Besides, the highest biomass (2.32g/L) concentration was gained by microalgae Desmodesmus sp.WC08. And C. vulgaris CJ15and Desmodesmus sp.WC08had higher lipid yield in all eight isolates in terms of biomass concentration and lipid content. The fatty acid composition of all tetested microalgae strains were mainly the fatty acids with carbon atoms among16-18by GC-MS. And in compared with the other isolates, Desmodesmus sp.WC08was more appropriate as a candidate for biodiesel production.The effects of different culture media, carbon source, nitrogen source and phosphorus source on growth and lipid accumulation of marine microalgae Desmodesmus sp.WC08were investigated by bath cultivation. The results indicated that brine BG11was the optimal culture media for growth and lipid accumulation of Desmodesmus sp.WC08. Sodium carbonate and sodium bicarbonate had no significant difference on growth and lipid accumulation of Desmodesmus sp. WC08based on brine BG11, but there was significant effect for sodium carbonate concentrations. Without any addition of sodium carbonate, the growth, biomass and lipid yield of Desmodesmus sp. WC08were worse, and it was on the contrary for higher concentration. For test of nitrogen sources, ammonium chloride was unfit as nitrogen source for the cultivation of Desmodesmus sp. WC08, because the pH was liable to decrease with the assimilation of NH4+in culture media. There were no significant effects on the biomass and lipid content for the sodium nitrate, sodium nitrite and urea, but it was opposite for lipid yield. When urea was as the nitrogen source, biomass concentration and lipid yield of Desmodesmus sp.WC08were highest among four nitrogen sources. Besides, nitrogen concentration and phosphorus concentration both had significant effect on the growth and lipid accumulation of Desmodesmus sp.WC08. When the concentration was low for either nitrogen or phosphorus in culture media, higher lipid content was obtained, and the lipid content was highest when the nitrogen concentration and phosphorus concentration were5and0.1mmoL/L, respectively. But it was adverse on the growth and lipid accumulation both for high concentration. In one word, for nitrogen source and phosphorus source, the optimal concentration on growth and lipid accumulation of Desmodesmus sp.WC08were10and0.5mmoL/L, respectively.Eight cells harvesting methods including seven flcculants and pH regulation were studied for harvesting cells of Desmodesmus sp.WC08from culture broth. The better flocculation efficiency was obtained by aluminum sulfate, ferric chloride, chitosan and aluminum chloride, and chitosan was the optimal flocculant, due to less dosage, non-toxic and environment friendly. When the pH of culture broth was lower (5or6), the flocculation efficiency was better and the flocculation time was less by using chitosan. But it was on the contrary for the higher pH (7). There was no significant effect on the flocculation efficiency for using acetic acid and hydrogen chloride as the dissolved solution of chitosan. In addition, growth phase of Desmodesmus sp.WC08had some effects on flocculation efficiency of chitosan, the dosage increased with the extension of growth phase.Oleagininous microalgal strain Desmodesmus sp. WC08was isolated and screened from the tropical ocean, based on its better growth, higher biomass, lipid yield and more suitable fatty acid profile. So it can be possible candidate specie for biodiesel production. And higher carbon concentration or lower nitrogen and phsorus concentration were the optimal culture conditions for growth and lipid accumulation of Desmodesmus sp.WC08based on borine BG11culture media. In addition, when the pH of the culture broth was set to6and the dissoveled, the flocculation efficiency of Desmodesmus sp.WC08is approximately98%by using0.03g/L chitosan dissolved in hydrochloric acid solution at the end of cultivation. And beyond1g of microalgal biomass can be obtained by per10mg chitosan.