Stoichiometric and sterol responses of dinoflagellates to changes in temperature, nutrient supply and growth phase

Climate changes as well as the inherent properties of phytoplankton have important and complex effects on the ecology and biogeochemical cycles of the oceans. Dinoflagellates are often dominant groups in harmful algae blooms (HABs) and red tides, while the impact of environmental changes on chemical composition in typical dinoflagellates is currently unclear, which limits our understanding of the role of dinoflagellates in planktonic trophodynamics in the changing ocean. Here, we investigated the responses of elemental stoichiometry and sterol contents in three dinoflagellate species (Prorocentrum donghaiense, Prorocentrum minimum and Karenia mikimotoi) to changes in temperatures (15, 20 and 25 °C), N:P supply ratios (molar ratios 10:1, 24:1 and 63:1) and growth phases (exponential and stationary growth phases) in batch culture experiments. Both dinosterol and brassicasterol were produced by all the three species, but the cellular contents of dinosterol were up to 10 times higher than brassicasterol. Overall, growth phase changes showed the most pronounced effects, while warming and nutrient deficiency had moderate effects on the ratios of particulate organic nitrogen and carbon (PON:POC) and sterol contents. Our observations imply that under future ocean scenarios (warming and imbalanced N:P supply ratios), PON:POC may decrease but carbon-normalized sterol contents may increase in the three species. Such parallel changes in PON:POC and sterol contents may influence the dinoflagellate-grazer relationship and have significant consequences for ecological functions of the future ocean.