Examining the effects of elevated CO2 on the growth kinetics of two microalgae, Skeletonema dohrnii (Bacillariophyceae) and Heterosigma akashiwo (Raphidophyceae)

Carbon dioxide (CO 2 ) serves as the primary substrate for the photosynthesis of phytoplankton, forming the foundation of marine food webs and mediating the biogeochemical cycling of C and N. We studied the effects of CO 2 variation on the Michaelis-Menten equations and elemental composition of Skeletonema dohrnii and Heterosigma akashiwo . CO 2 functional response curves were conducted from 100 to 2000 ppm. The growth of both phytoplankton was significantly affected by CO 2 , but in different trends. The growth rate of S. dohrnii increased as CO 2 levels rose up to 400 ppm before reaching saturation. In contrast to S. dohrnii , the growth rate of H. akashiwo increased with CO 2 increasing up to 1000 ppm, and then CO 2 saturated. In addition, H. akashiwo showed a slower growth rate than S. dohrnii for all CO 2 concentrations, aside from 1000 ppm, and the Michaelis-Menten equations revealed that the half-saturation constant of H. akashiwo was higher than S. dohrnii . An increase in CO 2 concentration was seen to significantly affected the POC: Chl- a of both S. dohrnii and H. akashiwo , however, the effects on their elemental composition were minimal. Overall, our findings indicate that H. akashiwo had a more positive reaction to elevated CO 2 than S. dohrnii , and with higher nutrient utilization efficiency, while S. dohrnii exhibited higher carbon fixation efficiency, which is in line with their respective carbon concentrating mechanisms. Consequently, elevated CO 2 , either alone or in combination with other limiting factors, may significantly alter the relative relationships between these two harmful algal blooms (HAB) species over the next century.