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.

Combined effects of seawater acidification and benzo(a)pyrene on the physiological performance of the marine bloom-forming diatom Skeletonema costatum.

The combined effects of polycyclic aromatic hydrocarbons and seawater acidification are poorly understood. Hence, we exposed the bloom-forming diatom Skeletonema costatum to four concentrations (0, 0.1, 1 and 10 µg L-1) of benzo(a)pyrene and two pCO2 levels (400 and 1000 µatm) to investigate its physiological performance. The growth and photosynthesis of S. costatum were tolerant to low and moderate benzo(a)pyrene concentrations regardless of the pCO2 level. However, the highest benzo(a)pyrene concentration had remarkably adverse effects on most parameters, decreasing the growth rate by 69%. Seawater acidification increased the sensitivity to high light stress, as shown by the lower maximum relative electron transport rate and light saturation point at the highest benzo(a)pyrene concentration. Our results suggested that benzo(a)pyrene could be detrimental to diatoms at a habitat-relevant level, and seawater acidification might further decrease its light tolerance, which would have important ramifications for the community structure and primary production in coastal waters.