Effects of CO2 restriction on growth of Nitzschia closterium: Evidence from stable isotopes and fatty acids

Algae blooms frequently occur in the coastal areas of China, and pose numbers of adverse effects to marine environment. Nitzschia closterium is one of the algal species associated with algae blooms and was selected as the target algae of this work. This study was the first attempt to investigate the effect of carbon source restriction on the growth of N. closterium during marine phytoplankton blooms. Experiments were implemented in the CO2-restricted (include three nutritional conditions) and CO2-unrestricted systems, respectively. The stable isotope ratios (δ13C and δ15N) and fatty acids (FAs) profiles were measured. Here we hypothesized that the deficient carbon source could lead to the changes of stable isotope ratios and FAs profiles in the N. closterium. The results showed that the δ13C of N. closterium enriched under the CO2 restriction during the culture time (the isotopic difference greater than 27.8‰), whereas depleted under CO2-unrestricted system. Furthermore, within the CO2 restriction, δ15N showed the enrichment trend with increasing culture time under the nitrogen deficiency conditions, where leveled off under the nitrogen non-deficiency conditions. A total of 12 FAs in N. closterium were detected. Within CO2 restriction, FA 20:5n-3c was the predominant congener in all the three conditions. In addition, FA 16:1n-7c was highest in nitrogen deficient, while FA 16:3 was highest in phosphorus deficient condition. Consequently, the deficient carbon sources leaded to inhibit the FA (14:0, 16:0, 16:1n-7c, 16:3, 18:0, 18:3n-3c) synthesis. Overall, the present study provided the new approach to investigate the forming mechanisms of marine phytoplankton blooms by using stable carbon and nitrogen isotope compositions as well as FAs profiles.