Studies On The Photosynthetic Responses Of Diatom To Environmental Changes Associated With Ocean Warming

The report of ocean warming resulting from global climate change has been received great concern. Ocean warming will potentially enhance water column stratification, thus lead to more nutrient-depleted conditions and increased average light availability to circulating cells in the surface mixed layer. It is important to investigate the impacts of these environmental variations on the photosynthesis and growth of phytoplankton, if their contributions to the primary production would be changed is an important question needs to be explored. Diatoms(Bacillariophyceae) is a key taxon of the unicellular photosynthetic eukaryotes, contributing to about 40% of the aquatic primary production. However, we still know little about the sole or interactive effect of ocean environmental variations associated with global warming, such as enhanced temperature, decreased nitrogen supply and increased solar radiation, on the photosynthetic characteristics of diatoms and the underlying mechanisms. In this study, we incubated three representative diatom species, Phaeodactylum tricornutum, Thalassiosira pseudonana and Thalassiosira weissflogii at different temperature or under nitrogen depleted/repleted conditions, to systematically investigate the photosynthetic responses of these diatoms to multiple environmental factors by carrying out a simulated in situ experiment with the natural solar radiation. The main results are as follows:The PSII function and repair capacity in P. tricornutum cells were affected by temperature rise at different cycling speeds. Cells incubated at high temperature showed higher ΦPSII, r ETRmax and α, indicating that light was efficiently absorbed and transmitted. Higher Ek at slow clycing speed suggested that the light saturation for photosynthesis was shifted to a higher irradiance, so that excess energy absorbed can be used for photochemistry, minimising the occurence of photodamage. Analyses with the data from the upward shift and recovery experiment showed that cells grown at high temperature posses smaller value of effective target size for photoinactivation of PSII(σi), particularly for cells with fast cycling speed, indicating that they are more resistant to PSII photoinactivation, which is consistant with their lower PSII repair rate.The PSII function was also significantly affected under nitrogen depleted conditions in Thalassiosira pseudonana and Thalassiosira weissflogii. Our study showed that nitrogen depletion led to the inhibition of electron tranfer from QA- to QB, and increased the energy flux per active reaction center. Nitrogen depletion and temperature rise synergistically reduced the photosynthetic activity of the smaller cells of T. pseudonana, and caused serious photoinhibition. In contrast, no significant changes were shown in the larger cells of T. weissflogii.In addition, solar UV radiation inhibited the PSII photochemistry of T. pseudonana, and this inhibitory effect could be alleviated at fast cycling speed. The absorption flux per RC(ABS/RC) and the dissipation flux per RC(DI0/RC) were increased under both PAR and PAB treatments with increasing irradiance level, while the electron transport flux per RC decreased, particularly for cells treated with PAB. It was also shown that at low cycling speed, PAB treatment significantly reduce the efficiency with which a trapped excition can move an electron into the electron transport chain further than QA-(Ψ0), and the quantum yield of electron transprot beyond QA(ΦE0).In conclusion, the enhanced temperature, decreased nitrogen supply and increased solar radiation associated with ocean warming significantly affected the photosynthesis of diatom. Temperature rise changed the responses of diatom species to high level of irradiance at different cycling speeds, causing serious photoinhibition in smaller diatom cells, and the inhibitory effect of UV radiation was alleviated at fast cycling speed. Responses of different phytoplankton functional groups to ocean warming and the underlying molecular mechanisms need to be further investigated.