The impact of fluctuating light on the dinoflagellate Prorocentrum micans depends on NO3(-) and CO2 availability

Transcriptome analysis of two bloom-forming Prorocentrum species reveals physiological changes related to light and temperature

Temperature and light substantially influence red tide succession. However, it remains unclear whether the molecular mechanisms differ among species. In this study, we measured the variation in the physiological parameters of growth and pigments and transcriptional levels of two bloom-forming dinoflagellates, namely Prorocentrum micans and P. cordatum. This was undertaken in four treatments that represented two factorial temperature combinations (LT: 20 °C, HT: 28 °C) and light conditions (LL: 50 µmol photons m^-2 s^-1, HL: 400 µmol photons m^-2 s^-1) for 7-day batch culture. Growth under high temperature and high light (HTHL) was the fastest, while growth under high temperature and low light (HTLL) was the slowest. The pigments (chlorophyll a and carotenoids) decreased significantly in all high light (HL) treatments, but not in high temperature (HT) treatments. HL alleviated the low light-caused photolimitation and enhanced the growth of both species at low temperatures. However, HT inhibited the growth of both species by inducing oxidative stress under low light conditions. HL mitigated the HT-induced stress on growth in both species by upregulating photosynthesis, antioxidase activity, protein folding, and degradation. The cells of P. micans were more sensitive to HT and HL than those of P. cordatum. This study deepens our understanding of the species-specific mechanism of dinoflagellates at the transcriptomic level, adapting to the future ocean changes including higher solar radiation and higher temperatures in the upper mixed layer.

Ocean acidification interacts with variable light to decrease growth but increase particulate organic nitrogen production in a diatom

Phytoplankton in the upper oceans are exposed to changing light levels due to mixing, diurnal solar cycles and weather conditions. Consequently, effects of ocean acidification are superimposed upon responses to variable light levels. We therefore grew a model diatom Thalassiosira pseudonana under either constant or variable light but at the same daily photon dose, with current low (400 μatm, LC) and future high CO₂ (1000 μatm, HC) treatments. Variable light, compared with the constant light regime, decreased the growth rate, Chl a, Chl c, and carotenoid contents under both LC and HC conditions. Cells grown under variable light appeared more tolerant of high light as indicated by higher maximum relative electron transport rate and saturation light. Light variation interacted with high CO₂/lowered pH to decrease the carbon fixation rate, but increased particulate organic carbon (POC) and particularly nitrogen (PON) per cell, which drove a decrease in C/N ratio, reflecting changes in the efficiency of energy transfer from photo-chemistry to net biomass production. Our results imply that elevated pCO₂ under varying light conditions can lead to less primary productivity but more PON per biomass of the diatom, which might improve the food quality of diatoms and thereby influence biogeochemical nitrogen cycles.