Identification of carbohydrates as the major carbon sink of the marine microalga Isochrysis zhangjiangensis (Haptophyta) and optimization of its productivity by nitrogen manipulation

•Carbohydrates were the major carbon sink in Isochrysis zhangjiangensis.•Carbohydrate production was optimized by nitrogen manipulation.•The parameter Fv/Fm can serve as an indicator of carbohydrate accumulation.

Microalgae represent a potential feedstock for biofuel production. During cultivation under nitrogen-depleted conditions, carbohydrates, rather than neutral lipids, were the major carbon sink of the marine microalga Isochrysis zhangjiangensis (Haptophyta). Carbohydrates reached maximum levels of 21.2 pg cell−1 on day 5, which was an increase of more than 7-fold from day 1, while neutral lipids simultaneously increased 1.9-fold from 4.0 to 7.6 pg cell−1 during the ten-day nitrogen-depleted cultivation. The carbohydrate productivity of I. zhangjiangensis was improved by optimization of the nitrate supply mode. The maximum carbohydrate concentration was 0.95 g L−1 under batch cultivation, with an initial nitrogen concentration of 31.0 mg L−1, which was 2.4-fold greater than that achieved under nitrogen-depleted conditions. High performance liquid chromatography (HPLC) analysis showed that the accumulated carbohydrate in I. zhangjiangensis was composed of glucose. These results show that I. zhangjiangensis represents an ideal carbohydrate-enriched bioresource for biofuel production.