The effect of diverse nitrogen sources in the nutrient medium on the growth of the green microalgae Chromochloris zofingiensis in the batch culture

Abstract

The effect of three nitrogen (N) sources in the nutrient medium – sodium nitrate (NaNO3), urea (CO(NH2)2), and ammonium chloride (NH4Cl) – on the morphological and physiological characteristics of the green microalga Chromochloris (Chlorella) zofingiensis, a potential commercial producer of lipids and a ketocarotenoid astaxanthin, was studied. The alga was batch-cultivated in glass conical flasks from starting cell density (n) around 2.3·106 per mL and dry weight (DW) content of 0.06 g·L−1 in all variants at 120 μmol·m−2·s−1 PAR, +20…+21 °C, and air bubbling at a rate of 0.3 L·min−1·L−1. The concentration of nitrogen sources (as elemental N) in the modified BBM nutrient medium was 8.83 mmol·L−1, the cultivation duration was 17 days. The dynamics of n and cell volumes, DW content, chlorophylls a and b (Chla and Chlb), total carotenoids (Car), and lipids (Lip) in the cultures, concentration of N sources in the nutrient medium, and its pH were recorded. It was shown that the growth rate, size distribution of the cell populations, and the biomass chemical composition depended significantly on the nitrogen source in the nutrient medium. Using NH4Cl as N source caused on the second day growth inhibition, cell swelling, aggregation, and discoloration; by the seventh day, it caused culture crash. C. zofingiensis cells took up NaNO3 and CO(NH2)2 from the medium at a similar rate (0.626 and 0.631 mmol N·L−1·day−1, respectively), but the growth of the culture fed with CO(NH2)2 lagged; its cell volume and Chla, Chlb, and total Car contents declined profoundly. The average dry matter productivity (PDW) in the culture grown on CO(NH2)2 [(0.086 ± 0.004) g·L−1·day−1] was 32.6 % lower than in the culture grown on NaNO3 [(0.114 ± 0.005) g·L−1·day−1]. At the same time, lipid productivity (PLip) of the urea-fed culture was comparable with that of the nitrate-fed culture (PLip of 28 and 26 mg·L−1·day−1, respectively). The lipid DW percentage of the former exceeded significantly that of the nitrate-fed culture (31.6 % vs 23.1 %, respectively). From the standpoint of profitability, the lag in biomass accumulation recorded in the urea-fed culture on PDW is not critical since it is compensated by lowering the cost of nitrogen source for the nutrient medium (approximately by 230 %) and a higher biomass lipid content. C. zofingiensis grown in media with urea as the only N source deserves further investigation.