Evaluation of nitorgen and carbon supplementation strategies for optimizing biomass generation during cultivation of Chlorella sorokiniana, strain SLA-04

Abstract

Algal cultivation requires significant nitrogen and carbon inputs, which are expensive and can offset benefits associated with biofuel production. This research investigates growth of an alkali-tolerant Chlorella sorokiniana, strain SLA-04, using different nitrogen and carbon regimes to improve physiological knowledge of this novel organism, and improve biomass production and resource demand. Nitrate, ammonium, and urea were used efficiently by SLA-04, however pH changes during utilization of nitrate and ammonium impacted inorganic carbon availability (species and concentration). Generation of OH- during use of nitrate increased pH, increasing mass transfer of CO 2 into solution and increasing the ratio of HCO 3-/CO 2. Ammonium utilization resulted in proton generation, lowering pH and inhibiting growth. When bicarbonate, rather than CO 2, was provided, productivity improved for the urea and mixed nitrogen conditions. This likely resulted from upregulation of genes related to nitrogen and carbon assimilation in the presence of bicarbonate, however Na + cotransport with urea and nitrate is required in some organisms. It is possible that Na + was insufficient when CO 2 was provided, but not in conditions with bicarbonate since it was added as NaHCO 3-. The impact of Na +, as well as other ions, on nitrogen and carbon utilization is not well understood, but it may alter gene regulation. Bicarbonate and CO 2 both promoted increased growth relative to cultures without inorganic carbon supplementation. The highest productivities were observed when carbon supplementation, either as continued CO 2 augmentation to the air sparge or as a 50mM bicarbonate amendment, was provided during nitrogen deplete growth. Glucose availability improved productivity for conditions without CO 2 supplementation. The use of urea or a combination of nitrogen sources with bicarbonate, instead of CO 2, was promising due to (a) the low cost of urea, relative to the other nitrogen sources; (b) the potential for using wastewater containing a mix of nitrogen sources; and (c) the low cost and easy transport of bicarbonate. Future research should evaluate changes in SLA-04 gene expression resulting from the supply of different nutrients, including nitrogen and carbon sources, as well as other ions essential for growth.