Comparative study of the physiological responses of Skeletonema costatum and Thalassiosira weissflogii to initial pCO₂ in batch cultures, with special reference to bloom dynamics

Extensive studies have documented the responses of diatoms to environmental drivers in the context of climate change. However, bloom dynamics are usually ignored in most studies. Here, we investigated the effects of the initial pCO₂ on the bloom characteristics of two cosmopolitan diatoms, Skeletonema costatum and Thalassiosira weissflogii. Batch cultures with two initial pCO₂ conditions (LC: 400 μatm; HC: 1000 μatm) were used to investigate bloom dynamics under current and ocean acidification scenarios. The simulated S. costatum bloom was characterized by fast accumulation, a rapid decline in biomass, and a shorter stationary phase. The T. weissflogii bloom had a longer stationary phase, and cell density remained at high levels after culturing for 19 days. The physiological performances of the two diatoms varied significantly in the different bloom phases. We found that the initial pCO₂ has modulating effects on biomass accumulation and bloom dynamics for these two diatoms. The higher initial pCO₂ enhanced the specific growth rate of T. weissflogii by 6% in the exponential phase, leading to higher cell densities, while 86% higher decay rates were observed in the HC cultures of S. costatum. Overall, ocean acidification may alter the dynamics of diatom blooms and may have profound impacts on the biological carbon pump.