The effect of nitrogen limitation on carbohydrates and β-glucan accumulation in Nannochloropsis oculata

Abstract

Nitrogen (N) is the second most abundant element in microalgae biomass, following carbon. Microalgae have a high carbon and nitrogen metabolism system, making nitrogen limitation (N-limitation) a crucial factor that modifies their biochemical composition. In Nannochloropsis genus cultures, N-limitation can cause lipid or carbohydrate accumulation. This research aims to evaluate the effect of N-limitation on carbohydrate and β-glucan accumulation in Nannochloropsis oculata during maximum biomass production (i.e., finished logarithmic phase and early start stationary phase in batch culture) in the two-stage process context. To achieve this goal, we tested the effect of five levels of N availability (treatments 100, 75, 50, 25, and 0 % of N, concerning F medium) in batch culture using sodium nitrate (NaNO3) as the N source. The initial availability of N in each treatment was 1.7, 1.3, 0.8, 0.4, and 0 mmol∙l−1. The results showed that the highest concentration of N. oculata biomass, carbohydrate, and β-glucans content was produced during the logarithmic phase at 0.8 and 1.32 mmol∙l−1 N initial batch culture concentration (50 and 75 % N treatments), without a significant difference (P > 0.05) from the F medium used. Total carbohydrates obtained were 15.2 %, 19.7 %, 17.2 %, 12.7 %, and 14.2 % (dry weight) for the 100 %, 75 %, 50 %, 25 %, and 0 % N treatments, respectively. Similarly, the β-glucans in microalgal biomass were 5.0 %, 7.4 %, 6.7 %, 4.4 %, and 1.7 % (dry weight) for treatments 100 %, 75 %, 50 %, 25 %, and 0 % N, respectively. The 25 % and 0 % N treatments had a negative effect (P < 0.05) on cell population growth, carbohydrates, and β-glucans accumulation. Based on these results, we recommended conducting studies on the effect of N-limitation in N. oculata culture primarily at 50 % and 75 % N treatments, during the stationary phase and/or second stage of the two-stage process context, such as a chronic or and long-term effect, along with a partial supply of N or testing other inductors might also be explored. This knowledge can help to optimize the industrial production of microalgae β-glucans.