The effect of light on the cellular stoichiometry of Chlorella sp. in different growth phases: implications of nutrient drawdown in batch experiments

Identification of nutrient status is essential to Chlorella sp. cultivation for biodiesel production. Critical cellular elemental ratio is a commonly used parameter to assess nutrient status, but the definition of critical cellular elemental ratio is inconsistent, and the influence of light and growth phase on this ratio is still unclear. In this study, Chlorella sp. was batch cultured under different conditions of light and nutrient supply ratio. Nitrate and phosphate consumption in the medium and particulate cellular carbon (C), nitrogen (N), and phosphorus (P) contents were measured. We first obtained the critical nutrient supply ratio to assess nutrient limitation and then utilized this ratio to deduce the critical cellular elemental ratio. The critical nutrient supply ratio was significantly affected by light intensity, and was approximately 45, 15, and between 30 to 45 under light intensities of 50, 100, and 200 μmol photons m⁻² s⁻¹, respectively. Critical cellular C/N/P ratios of Chlorella sp. at three light intensities of 50, 100, and 200 μmol photons m⁻² s⁻¹ in exponential growth phase were C_{159.91–237.30}N_{15.90–22.52}P, C_{104.02–167.39}N_{11.71–17.35}P, and C_{144.30–243.66}N_{12.84–19.84}P, and in stationary growth phase the ratios were C_{201.47–406.14}N_{16.55–32.67}P, C_{125.05–298.44}N_{10.12–24.24}P, and C_{168.65–342.42}N_{14.47–29.18}P, respectively. These results supported the hypothesis that the critical cellular elemental ratio, which is used to predict nutrient status, varies and depends on light intensity and growth phase.