Trans-omics Study Of The Cellular Metabolism Of Chlorella Sorokiniana Cultivation

A trans-omics approach was employed to study the change of the levels of oilproduction related metabolites, phospholipids and proteins in Chlorella sorokinianaunder different inoculum sizes to reveal the most critical regulatory factors whichaffect microalgal oil production and provide more comprehensive information for thestudy of metabolism and oil production mechanism of microalgae.The metabolome of C. sorokiniana under different initial cell densities wasanalyzed to reveal the metabolic relevance between lipid production and inoculumsize by GC-TOF-MS. Furthermore, a tight correlation between metabolism and lipidaccumulation in C. sorokiniana was revealed by partial least-squares to latentstructures （PLS） analysis. It was interesting that most of the key metabolites selectedby PLS were involved in the fixed carbon metabolism in photosynthesis and werenegatively correlated to the daily productivity and cetane number（CN） of FAME.Moreover, most of the levels of these metabolites were remarkably reduced in thehighest inoculum size culture. One possible explanation for this interestingobservation is the accessibility of light to microalgae cells in culture. We thereforeasked whether high cell density lessened the light permeability into the culture and ledto low photosynthetic capacity of cells, which then reduced the carbon-relatedmetabolic levels.Proteomic was integrated with the metabolomic research to study the change ofthe proteins which caused the metabolic differences. The results showed thatphotosynthesis related proteins were decreased with the improvement of inoculationdensity, and meanwhile78.4%of the other proteins also showed the similar trend inthe stationary phase. It’s clear that the inoculum size had a great effect on the level ofphotosynthetic protein in C. sorokiniana, which directly decided the energymetabolism and carbon fixation, and then changed the levels of other metabolism inthe cell. This might be the crucial reason for the decline of overall level ofcarbon-related metabolism. The phospholipidome studies showed that a cleardiscrimination from the culutre with the inoculum size of1×106cells·mL-1to theothers was found and the key phospholipids responsible for the discrimination werePhosphatidylethanolamine（PE） and phosphatidylglycerol（PG）, which means the degree of membrane density and the fluidity of thylakoid membrane might play animportant role in inoculum-associated mircoalgal growth and metabolism.All of the above omics resultes pointed that the lack of photosynthetic capacitymight be a potential limiting factor in high inoculum size culture. To verify thishypothesis, we compared the metabolism level and FAME product in the culture withthe highest inoculums size with different light intensity(40-160μmol·m^-2·s^-1). Theresults showed that the highest daily productivity and CN of FAME were achievedwith the light intensity of110μmol·m^-2·s^-1. Further analysis revealed that amino acidmetabolism was negatively correlated to the production of FAME while sugarmetabolism and free fatty acid metabolism were positively correlated to the lipidaccumulation. Based on this study, we comfirmed that light intensity did improve themetabolic status and lipid production of C. sorokiniana under high inoculum sizeculture and the light regulation was the key metabolic regulation mechanism ininoculum-depended cultivation of C. sorokiniana.