Stable carbon isotope fractionation (δ13C) of microalgae on CO2 assimilation

A study on the stable carbon isotopic fractionation (δ13C) in the process of CO2 assimilation of microalgae is very significant to investigate the role of microalgae in estimate of potential carbon dioxide (CO2) sink. However, till now it is yet unknown how to measure the fractionation on CO2 assimilation of microalgae in vivo. Bidirectional tracers, which two types of sodium bicarbonate have different δ13C-PDB values, are used as the carbon isotope labeling to establish the method of calculation of the fractionation value in this study. Furthermore, the different proportions of metabolic pathways of inorganic carbon utilization are quantified by the fractionation value in microalgae. The carbon isotope fractionation (δ13C) value of three types of algae, Chlamydomonas reinhardtii (CR), Chlorella pyrenoidosa(CP), and compound algae collected from surface water of Hongfeng Lake, are 15.3‰, 14.8‰, and 21.7‰, respectively in this paper. The difference of the fractionation values are closely related to cell size of these microalgaes. That means, the bigger cell size of microalgae, the greater the resistance produces and the longer the reaction time of process in the CO2 assimilation. This leads to greater fractional values. The percent share of bicarbonate ions calculated by the fractionation values are 100%, 81.1% and 97.8% respectively in Chlamydomonas reinhardtii, Chlorella pyrenoidosa and compound algae. The results demonstrate that the main pathway of inorganic carbon utilization of microalgae is the bicarbonate ions in karst lakes.