Dynamic photo-physiological responses of the dinoflagellate Karenia brevis to short-term changes in temperature and nitrogen substrates

On the West Florida Shelf, annual blooms of the neurotoxin-producing harmful dinoflagellate,
Karenia brevis (and other Karenia species) cause negative effects on the environment, the
economy, and on human health. Blooms primarily occur in the fall months and may overwinter
into winter and spring, but they rarely persist throughout the hot summer months when water
temperatures are above 30°C. During 2020 and 2021, an unusually prolonged (~12 months)
K. brevis bloom occurred. During the early phase of this bloom (January, 2021) when K.
brevis were present in low-moderate abundances in southwest Florida, experiments on rates
of photosynthesis were conducted on surface samples from two offshore stations within the
bloom. Photosynthesis-irradiance responses were examined using PAM fluorometry to assess
responses to varying nitrogen conditions and temperatures. Treatments included controls and
exposures to temperatures of 15, 20, 25, 30°C with and without 10 μM additions of three
nitrogen forms (nitrate, ammonium, urea). Although there were variations between stations, in
general decreasing trends of fluorescence parameters with increasing temperature from 15°C
to 30°C were observed, suggesting that 30°C might be physiologically stressful for Karenia.
However, when cells were treated with urea, this reduction was substantially less. These results
suggest K. brevis dependence on nitrogen form may change with temperature, and availability
of chemically-reduced organic forms of nitrogen may be favored at the highest temperatures
that Karenia may experience.