Photosynthetic Mechanism And Directed Optimization Of Metabolic Productions In Heterotrophic And Anaerobic Chlorella

With the development of microalgae biotechnology, microalgae have been used to produce many valuable products, such as food, feeds, pharmaceuticals, biodiesel and hydrogen. However, the traditional photoautotrophic growth and open cultivation system bring more cost for the slow growth rates and the need of aeration and light. With the need to reduce cost on production, heterotrophic and anaerobic cultivation of microalgae is paid more and more attention.Firstly, a microalga strain was isolated from a wastewater treatment pond and identified as Chlorella sorokiniana GXNN01 in terms of morphology, physiology, and phylogeny. The strain grows rapidly in heterotrophic or mixotrophic conditions with addition of various carbon sources, and even in anaerobic conditions. The protein content of the microalgae was 75.32% in cell dry weight. The strain was shown to be capable of (1) utilizing D,L-malate only with light with the maximum growth rate reached 0.28 d-1, (2) inhibiting photosynthesis in mixotrophic growth, and (3) growing in anaerobic conditions with regular photosynthesis and producing oxygen internally.Secondly, the effects of acetate and microaerobic conditions on the growth, photosynthesis, and respiration of the strain were examined after acetate addition to autotrophic cultures. As the acetate concentration increased cells needed a longer lag phase to grow, and 1% acetate completely inhibited growth. Acetate addition induced an immediate response in photosynthesis and respiration. The activity of both PSII and PSI was impaired and then recovered compared with autotrophic cells. Carbonic anhydrase and Rubisco activities were also inhibited at the start, and respiration was confounded. A transition from state I to state II was revealed by 77-K chlorophyll fluorescence. We hypothesize that ATP consumption for acetate assimilation resulted in surplus NADPH, which was dehydrogenated by NAD(P)H dehydrogenase -dependent cyclic electron flow. This resulted in a higher transthylakoid proton gradient that induced non-photochemical quenching of PSII and an over-reduced PQ pool, which redistributed energy between PSI and PSII during state transitions. These observations point to an adaptation process after acetate addition that involves interaction between photosynthesis and respiration across ATP and reducing power.Lastly, effect of carbon sources on growth and lipid accumulation of the strain was studied in order to understand the relationship between heterotrophy and lipid production. With the addition of carbon sources, the alga exhibited fast growth rate, which reached stationary phase after 48 h at 30℃. Glucose and NaAc had a significant effect on the lipid accumulation in the early-stationary phase. With glucose as carbon source, the lipid content were 0.237 g·g-1 cell dry weight and 0.272 g·L-1 cultures, whereas the lipid content could reach 0.287 g·g-1 cell dry weight and 0.288 g·L-1 cultures with NaAc as carbon source. Neutral lipid content was found firstly to decrease and then to increase over time in the growth phase. The optimum concentration of glucose was 20 mM for the maximal lipid yield and the optimum harvest time was the early-stationary phase.