Responses of a harmful algal bloom-causing dinoflagellate Prorocentrum obtusidens to elevated temperature and urea

Elevated temperature and nitrogen (N) availability affect dinoflagellates differently; however, their interactive effects remain largely unexplored. This study investigated the physiological and transcriptomic responses of a harmful algal bloom-causing dinoflagellate, Prorocentrum obtusidens, to elevated temperature (22 °C vs. 26 °C) under three N conditions (16 μM nitrate, 8 and 16 μM urea) after 32 days of exposure. Elevated temperature enhanced cell growth across all N conditions, with a more pronounced increase in urea-grown cells, regardless of the insignificant interaction between temperature and N. Physiological responses to elevated temperature varied under N conditions. The nitrate-grown cells had higher particulate organic carbon (POC) content, C:N ratio, and activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO), phosphoenolpyruvate carboxykinase, and urease, but lower particulate organic N (PON) content. The low urea-grown cells showed higher C:N ratio, total carbohydrate content, and activities of carbonic anhydrase (CA), glutamine synthetase (GS), and urease, but lower phosphoenolpyruvate carboxylase activity, total protein, and total antioxidant capacity. The high urea-grown cells exhibited higher maximum photosynthetic efficiency, POC content, C:N ratio, and activities of CA, RubisCO, GS, and urease. Transcriptomic analysis revealed that elevated temperature increased the expression of genes associated with photosynthesis under all N conditions. The nitrate-grown cells produced more energy to mitigate thermal stress, whereas the urea-grown cells decreased energy production. These findings suggest that P. obtusidens is more resilient to future ocean warming when grown with urea, and predictions of dinoflagellate responses to warming oceans should consider N conditions in the environment.