The Biological Uptake Of Dissolved Inorganic Carbon And Dissolved Iron By Marine Phytoplankton And The Influencing Factors

Iron(Fe)is an essential nutrient of organisms.As a cofactor for many cellular enzymes,Fe is involved in a series of cellular physiological processes,such as cellular respiratory electronic chains,nitriogen fixation,nitrate/nitrite reduction and detoxification of reactive oxygen species.Therefore,Fe uptake by marine phytoplankton plays an important role on the cellular growth,metabolism,and primary productivity.In this study,the eastern tip of the Antarctic Peninsula and the Amundsen Sea which both located in the high latitudes and the Daya Bay which located in the subtropical zone were selected as research areas.The rates of carbon fixation(CFR)and Fe uptake(FeUR)by phytoplankton were measured with radioisotopes 14C and 55Fe as tracers,to analyze the spatiotemporal virations and their influencing factors.Using the three-endmember model with oxygen isotopic composition(?18O)and salinity of seawater,the contributions of meteoric water and sea ice meltwater were quantified to discuss the influence and mechnism of different freshwater fractions on the CFR and FeUR in the Southern Ocean.Meanwhile,the seasonal virations of CFR and FeUR in Daya Bay and their relationships with environmental factors were studied to reveal the spatiotemporal viration pattern of CFR and FeUR in the near-shore bay which is affected by human activities heavily and has high dissolved Fe(DFe)content.In addition,we selected two typical coastal algae species with different cell sizes(Skeletonema costaum(S.costatum)and Chlorella vulgari(C.vulgaris))to study the varitions of CFR and FeUR to changes in DFe and light levels,and explore the role of Fe in the coastal ecosystem.During the austral summer of 2016-2017 in the eastern tip of the Antarctic Peninsula,the CFR and FeUR in the surface layer were between 0.15 and 1.79 mmol C m-3 d-1,5.16 and 16.31 pmol Fe L-1 d-1,respectively.In the chlorophyll maximum layer,the CFR and FeUR were between 0.08 and 1.11 mmol C m-3 d-1,0.35 and 10.86 pmol Fe L-1 d-1,respectively.The freshwater input would affect the CFR and FeUR via the DFe supply and water stratification.Compared to the sea ice meltwater,meteoric water(mainly glacial water)has a more significant influence on CFR and FeUR.The contribution of size-fraotionated FeUR demonstrates smaller phytoplankton have a larger specific surface area and a competitive advantage in Fe uptake than larger phytoplankton.The difference in the Fe uptake kinetic by different size-fractionated phytoplankton would be one of the driving factors for the miniaturization of the phytoplankton population structure.During the late summer and early autumn in the Amundsen Sea,the CFR in the surface and chlorophyll maximum layer ranged from 0.01 to 0.15 mmol C m-3 d-1 and 0.02 to 0.11 mmol C m-3 d-1,respectivity,which are lower than those in the eastern tip of the Antarctic Peninsula.The FeUR in the surface and chlorophyll maximum layer ranged from 1.66 to 38.19 pmol Fe L-1 d-1 and 4.58 to 20.29 pmol Fe L-1 d-1,respectivity.Considering the effects of freshwater components on the CFR and FeUR in the Amundsen Sea,the sea ice meltwater has a more obvious effect on the CFR and FeUR than the meteoric water,and the extent of this effect is largely regulated by the phytoplankton biomass.Although the meteoric water has little influence on the CFR and FeUR,the increase of meteoric water fraction promotes the growth of large algal cells which are susceptible to Fe deficiency.The sea ice formation in the late summer and early autumn inhibites the carbon fixation in the Amundsen Sea,but has little effect on the Fe uptake,which resulted in the Fe:C ratio of phytoplankton is much higher than that in other areas of the Southern Ocean.In Daya Bay,the seasonal virations of CFR and FeUR showed CFR(summer)>CFR(autumn)>>CFR(winter),FeUR(summer)? FeUR(autumn)>>FeUR(winter),and the CFR and FeUR were much higher than that in the eastern tip of the Antarctic Peninsula and the Amundsen Sea.This study demonstrates the phytoplankton biomass and growth rate dominate the CFR variation:in Daya Bay.The increase in temperature and dissolved inorganic nitrigen(DIN)in the seawater promote the carbon fixation by phytoplankton.Larger phytoplankton are the main contributor to CFR nowadays,but the contribution of smaller phytoplankton would increase with the increase of seawater temperature and DIN in the future.The FeUR in Daya Bay is mainly controlled by the Fe species.The decrease of pH and DO would elevate the bioavailable Fe concentration and promote the FeUR of phytoplankton.Due to the difference of illumination intensity in surface and bottom layer,the CFR in the surface layer was much higher than that in the bottom layer.But light-induced Fe reduction is not the only way for phytoplankton to obtain Fe.Thus,low light or no light would not inhibit the FeUR in the bottom layer.The uptake of dissolved inorganic carbon(DIC)and DFe by phytoplankton differ in cell size.Larger algal cells have higher photosynthetic efficiency,while smaller algal cells have larger specific surface area and higher FeUR,resulting in the Fe:C ratio of phytoplankton with smaller cell size to be higher than that with larger cell size.The results of laboratory simulation experiment using S.costatum and C.vulgaris as two typical coastal algae shows that cell growth and carbon fixation of these two algae are almost restricted compeletely uner dark.While Fe uptake are affected rarely.The Fe uptake were comparable or even higher under dark than that under light condition,revealling the light is one of the most important factor to regulate the Fe:C ratio.The DFe concentration is another factor to affect the CFR and FeUR.Higher DFe could promote the carbon fixation and Fe uptake by S.costatum and C.vulgaris.Comparision of these two algae species,the higher FeUR and lower Fe demand for S.costatum are a competitive advantage in Fe-deficiency environments than C.vulgaris.Based on the results of the above studies,we can get the following conclusions:(1)the bioavailable Fe content is one of the most important factors to influence the FeUR and CFR.In the eastern tip of the Antarctic Peninsula in summer and the Amundsen Sea in the late summer and early autumn,the source of DFe and its determined DFe content regulate the spatial change of FeUR and CFR.In Daya Bay with high DFe content in seawater,the Fe species which affected by pH and DO,reflecting the Fe bioavailability,would influence the spatial change of FeUR.In the laboratory experiment,the increase of DFe could also significantly promote the CFR and FeUR by S.costatum and C.vulgaris.(2)The difference in the response to changes in light levels affects the spatial change of CFR and FeUR and the Fe:C ratio of phytoplankton.CFR is restricted obviously under low light or dark conditions,while FeUR is not.This results in the CFR in the actual sea being significantly reduced at lightconstrained stations,while FeUR is still high,which improves the Fe:C ratio of phytoplankton.(3)The CFR and FeUR by phytoplankton cells differ in cell size.Larger cells have higher photosynthetic efficiency,while smaller cells have higher FeUR because of the larger specific surface area.The difference in the uptake kinetic of DIC and DFe by different size-fractionated phytoplankton would affect the phytoplankton composition.