Effects of Total Inorganic Carbon and Nitrogen Concentrations On Lipid Formation In Chlorella Vulgaris

Effects of total inorganic carbon and nitrogen concentrations on lipid formation in Chlorella vulgaris

Jinsoo Kim¹, Joo-Youp Lee¹, Kaniz F. Siddiqui²

¹ Chemical Engineering Program, University of Cincinnati, OH 45221-0012

² The Metropolitan Sewer District of Greater Cincinnati, Cincinnati, OH 45204-2002

Microalgae have recently received great attention as a promising alternative bioenergy source because they can produce a substantial amount of triacylglycerols (e.g. 20-50 % dry cell weight) and can be readily converted into biodiesel.  Chlorella vulgaris has been selected for this growth kinetic study since it is one of the fastest growing microalgae and contains a reasonable amount of triacylglycerols (e.g. 14-22 % dry cell weight).  Dissolved carbon dioxide (CO₂(aq)) and bicarbonate ion (HCO₃^-) are essential inorganic carbon forms required for autotrophic algal growth, but few studies have been systematically conducted on the impact of inorganic carbon on algal growth.  In this respect, the growth kinetics of Chlorella vulgaris was previously investigated dependent on the total inorganic carbon concentration (i.e., CO₂(aq) and HCO₃^-)  As a result, it was observed that the growth kinetics was increased when the total inorganic carbon concentration was increased in the culture medium.

Under many nutrient limiting conditions, lipids are accumulated as a means of storage when energy (i.e. light) and carbon (i.e. CO₂ for autotrophic growth) sources are available.  However, many of these nutrients are essential for photosynthesis responsible for metabolic flux generation into lipids formation.  These nutrient-deficient conditions decrease biomass growth rate, and offset the benefit of lipid accumulation.  The growth of algal biomass such as Chlorella vulgaris is dependent on bicarbonate ion concentrations available in the culture medium.  Bicarbonate ion also plays a critical role as a catalyst in the first step of fatty acids biosynthesis for the synthesis of malonyl-CoA.  In this study, a change in total lipids content has been investigated using the Nile red stain method when the total inorganic carbon concentration in culture medium is increased.  This is expected to provide fundamental understanding of the utilization of bicarbonate ions for the design and operation of bioreactors.

Key words:  Chlorella vulgaris, total inorganic carbon, nitrogen deficiency, lipid content.