Physiological Responses And Potential Acclimation Mechanisms Of Marine Diatom Thalassiosira Pseudonana To Reduced Dissolved Oxygen

Since 1960s,under the influence of ocean eutrophication and global warming,the frequency and extent of marine hypoxia has increased,and seasonal or permanent hypoxic areas have formed in some marine areas.Previous studies have shown that low O2/hypoxia impairs the growth of oxygen-consuming organisms such as fish and invertebrates,reduces their ability to reproduce,and even leads to death.However,there have been few reports on how hypoxia affects phytoplankton,which are the main oxygen-producing organisms in the ocean.For phytoplankton supporting the entire marine ecosystem,O2 is both photosynthate and respiratory substrate,and declines in dissolved O2(DO)in the water will certainly affect their physiological metabolism.Field investigations revealed that the distribution depth of phytoplankton in the low O2/hypoxic zone largely overlaps with the depth of the hypoxic water layer,i.e.,phytoplankton will also be affected by hypoxia in the in-situ environment.Therefore,present study used the representative marine phytoplankton Thalassiosira pseudonana as an example to investigate the effect of low O2/hypoxia on the physiological metabolism and biochemical components of the diatom under different light and dark periods and light intensity conditions,and the physiological mechanism of the response.The main research results can be summarized as follows:1)Under conditions of optimal temperature and light intensity,low O2 increased the growth rate(μ)of T.pseudonana by 34%at short photoperiod(L:D 8:1 6),as compared with ambient O2(0.49±0.05 d-1),while hypoxia reduced it by 22%;at long photoperiod(L:D 16:8),μ reduced with the decrease of DO.Prolonged photoperiod reduced Chl a content but increased protein content.In addition,the maximum photochemical efficiency of PhotosystemⅡ(FV/FM),photosynthetic rate(Pn),and respiration rate(Rd)decreased under low O2/hypoxia,accompanied by increased malondialdehyde(MDA)content and superoxide dismutase(SOD)activity.The results of transcriptome analysis were consistent with the above physiological changes.Most differentially expressed genes(DEGs)related to photosynthesis,respiration,and carbon metabolism were significantly downregulated under low O2/hypoxia conditions,but the degree of downregulation was different at short or long photoperiod.2)Low O2 promotes the μ of T.pseudonana at lower light intensity(30-120 μmol photons m-2 s-1)but inhibits it at higher light intensity(220-400 μmol photons m-2 s-1).However,hypoxia inhibits the μ at all light intensity conditions,accompanied by the highest inhibition occurring at 300 μmol photons m-2 s-1.Accordingly,low O2 increased cellular carbon(C)content at lower light conditions but decreased it at higher light conditions.In addition,the nitrogen(N)assimilation rate of T.pseudonana decreased with decreasing DO,and the altered cellular distribution of assimilated N resulted in decreased cellular N and protein content and increased C:N ratio,as such the maximum reduce in protein content under low O2 and hypoxia was 46%and 58%,respectively.The increased SOD activity caused by low O2/hypoxia failed to remove the damage,resulting in an increase in MDA content that further increased with increasing light intensity.Lipid analysis showed that decreased O2 reduced the content of fatty acids,specifically 34 of 37 detected fatty acids significantly reduced at 300 μmol photons m-2 s-1.Proteomic analysis showed that more than 70%of the differentially expressed proteins(DEPs)related to photosynthesis,protein synthesis,and dark respiration were downregulated under low O2/hypoxia;the significantly downregulated DEGs related to protein synthesis and the significantly upregulated DEGs that related to protein degradation from transcriptome analysis further confirmed the above changes.3)Comparison of the effect of low O2 and hypoxia at light and dark conditions(L:D 12:12)revealed that the μ of T.pseudonan increased by 7%under low O2 compared with ambient O2(1.17±0.04 d-1)but decreased by 192%in the dark,with total μ showing no significant change.Hypoxia showed no significant effect on μ at light condition,but significantly decreased total μ and dark μ by 11%and 249%,respectively.In addition,cell sink rate increased with decreasing DO at light condition,but decreased by 10%and increased by 12%under low O2 and hypoxia after 84 h of dark treatment,respectively.Low O2 resulted in better adaptation of cells to the dark condition and increased carbon and nitrogen fluxes by 88%and 59%at 84 h,respectively,while nitrogen flux of hypoxia was significantly reduced by 48%at 48 h.In conclusion,low O2/hypoxia promoted and/or inhibited diatom growth and decreased cellular C,N,pigment,protein,and other content,as well as antioxidant activity.Moreover,the effect of low O2/hypoxia was closely related to light intensity and dark period.The results of this study not only deepen the understanding of how the decrease of marine DO affects phytoplankton,but also provide certain data that support the understanding of the effects of low O2/hypoxia on the marine ecosystem from the viewpoint of how the change of the components of marine primary producers affects primary consumers and even secondary consumers.