Photosynth Hetic Performance In Response To Solar Ultraviolet Radiation And Nitrogen Limitation In Diatoms

Global warming led to the increase of temperature in the surface seawater, i.e. ocean warming. Such warming will potentially decrease the depth of the ocean upper mixing layer, accelerate the vertical mixing rate and lead to more nutrient limitation in the surface seawater. At the same time, the depletion of ozone layer resulted in the enhancement of solar ultraviolet radiation reached on the earth, therefore, marine phytoplankton will be affected by a variety of environmental stresses. It is important to investigate the impacts of these environmental variations on the photosynthesis of phytoplankton. In this study, we incubated three diatom species, Thalassiosira pseudonana, Thalassiosira weissflogii and Phaeodactylum tricornutum, to investigate the photosynthetic responses of these diatoms to multiple environmental variations, such as enhanced ultraviolet radiation, nitrogen limitation and temperature rise. The main results are as the follows:T. pseudonana was exposed to full spectrum solar radiation(PAR + UV-A + UV-B, PAB, 280-700nm) and photosynthetically active radiation(PAR, 400-700nm) and incubated at two mixing rates(2 or 48 cycles) for 4 hours, it showed that the PSII photochemical efficiency of cells(Fv/Fm, ΦPSII) were decreased with increasing irradiance level under both PAR and PAB treatments, the presence of UV radiation accelerated the inhibitory effects. At fast cycling speed, the photochemical responses were significantly different for PAR and PAB treatments, and the UV-induced inhibition reached to 33% at 600μmolm-2s-1 of PAR level. In addition, photosynthetic efficiency(α) and relative maximum electron transport rate(r ETRmax)were not significantly changed with increasing irradiance level under PAR treatment, but notably decreased under PAB treatment, no significant change was found in the light saturation point(Ek) under both PAR and PAB treatments. We also found that the absorption flux per RC(ABS/RC) and the dissipation flux per RC(DIo/RC) were not significantly changed with increasing irradiance level under PAR treatment, but increased under PAB treatment. The two radiation treatments did not bring about any changes in the electron transport flux per RC(ETo/RC). Non-photochemical quenching(NPQ) were induced under both PAR and PAB treatments, particularly in cells incubated at slow cycling speed. These results showed that fast cycling could reduce the photoinhibitory effect on T. pseudonana caused by high PAR, however, the presence of UVR(PAB treatment) intensified the inhibitory effect.When cells of T. weissflogii and T. pseudonana were grown under nigrogenlimited conditions, their Fv/Fm was progressively decreased with the prolonged incubation time. It showed a rapid recovery when nitrogen nutrient was resupplied. The D1 protein and Chla content were changed similarly. NPQ was induced in these two diatom species. The analyses of the polyphasic rise of fluorescence transients demonstrated that nitrogen limitation inhibited the electron transfer from QA- to QB. High light significantly inhibited the PSII photochemical activity of the two diatoms, and nitrogen limitation accelerated the photoinhibition. In addition, T. pseudonana showed higher σi under nitrogen-repleted conditions, indicating that they are more sensitive to high light. In contrast, T. weissflogii possessed higher value of σi under nitrogen-limited conditions, its resistance to highlight was significantly higher than that in T. pseudonana. In the context of ocean warming, nutrient limitation and solar radiation increase may reduce the photosynthetic capacity of diatom cells, and then changed their contributions to the primary production.Nitrogen limitation inhibited the growth of P. tricornutum cells both at 18℃ and 24℃. Fv/Fm in cells grown at 18℃was decreased to a great extent as compared with that grown at 24℃. r ETRmax and α decreased significantly under nitrogen limited conditions, the rise of temperature(24℃) did not bring about any changes on it. Ek maintained stable regardless of these environmental changes. It was also shown that cells incubated under nitrogen-limited conditions showed higher ABS/RC, DIo/RC and ETo/RC, while temperature rise had little effect on these power flow distributions. In addition, nitrogen limitation inhibited electron transfer from QA- to QB, it was less inhibited in cells grown at 24℃ than that grown at 18℃. Highlight led to serious photoinhibition of cells grown at nitrogen-limited conditions regardless of the temperature changes. These results showed that temperature rise could alleviate the influence of nitrogen limitationon the PSII functionof P. tricornutum.