LIPID PRODUCTION, PHOTOSYNTHESIS ADJUSTMENT AND CARBON PARTITIONING OF MICROALGA CHLORELLA SOROKINIANA CULTURED UNDER MIXOTROPHIC AND HETEROTROPHIC CONDITIONS
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Microalgae have attracted growing attention for renewable biofuel production. In this work, microalga Chlorella sorokiniana was explored for its potential for lipid production. The results showed that C. sorokiniana could tolerate high temperature (up to 42 oC) and had high productivity. Under batch fermentation condition, 37.6 g L‒1 of biomass could be achieved with 80 g L‒1 of initial glucose supplementation. Rapid growth rate and high lipid content were obtained through mixotrophic cultivation under the experimental conditions. The specific growth rate (3.40 d─1) and the biomass yield (0.82 g g─1) were 1.8- and 2.4-folds of those in heterotrophic culture. Most significantly, lipid content in mixotrophic cells reached 45%, whereas it was only 13% in the control heterotrophically cultured cells. Linear electron flow and CO2 refixation were demonstrated to be critical factors for boosting the cell growth and lipid accumulation in mixotrophic culture. To gain further insights into the mixotrophic growth, regulation of photosynthesis was examined. Photosynthetic activities and fluorescence parameters were measured and characterized, followed by analysis of 77K fluorescence spectra, enzyme activities and enzyme expression levels. In the exponential phase, the mixotrophic cells exhibited enhanced photosynthetic activity; in the stationary phase, down-regulation of photosynthesis was observed, which was proved to be closely correlated with nitrogen limitation. Starch is another major carbon and energy storage compound in many microalgal cells. This study revealed the pattern of starch and lipid accumulation in C. sorokiniana controlled by nitrogen levels: when nitrogen was replete, starch was the predominant form of carbon storage; after nitrogen was depleted, large amounts of lipid accumulation occurred accompanied by starch degradation. Finally, lipid became the only carbon sink in the biomass partially contributed from starch degradation and the turnover of some primary metabolites. These results provide insight for using microalga C. sorokiniana for biofuel production.