| Marine nitrogen-fixing organisms convert atmospheric nitrogen(N2)into bioavailable ammonia(NH3),provide a nitrogen source for non-nitrogen-fixers in oligotrophic sea,regulate net sequestration of carbon dioxide(CO2).Trichodesmium is one of the main nitrogen fixing organisms in tropical and subtropical gyres,and its nitrogen fixation amount accounts for 50%of the total marine nitrogen fixation amount.In addition,recent studies have found that the nit:rogen fixed by unicelluar cyanobacteria equals to that of Trichodesmium.One third of the CO2 emitted by human activities is absorbed by the ocean,which leads to the increase of pCO2 in the seawater and the decrease of pH,i.e.ocean acidification.In recent years,quite more studies have focused on how ocean acidification affects biological nitrogen fixation,and most of the research objects are Trichodesmium and Crocosphaera.In addition,CO2 emissions also cause global warming,while the research on nitrogen fixation under the coupling effect of pH and temperature is almost blank.As a result,the research in this paper has cerrtain scientific significance.Nitrogen fixation is accompanied by isotope fractionation.Previous studies reports on the ?15N value of nitrogen fixation(Molybdenum-Iron nitrogenase type)were all about-1‰.Recent studies have found that if low-efficiency vanadium or iron,nitrogenase is used,the isotope fractionation of nitrogen fixation shown by 8i5N will be lower,ranging from-6‰ to-8‰.Previous studies have found that lower pH decreases the efficiency of nitrogenase,and according to the Thorneley-Lowe mechanism,changes in pH and temperature may directly or indirectly affect fractionation.When assessing the contribution of nitrogen fixation to the the Nitrogen pool,if the nitrogen fixation ?15N value drops by 2‰,the contribution of nitrogen fixation to the particulate nitrogen storage would decrease by 22-30%.Therefore,it is of theoretical and practical significance to study the effects of pCO2(pH)and temperature on nitrogen fixation isotope fractionation in order to correctly understand the marine nitrogen cycle under climate change.In this paper,the dominant nitrogen-fixing cyanobacteria,Trichodesmium,and unicelluar nitrogen-fixing cyanobacteria,Crocosphaera,were selected as the research objects.The effects of pCO2(pH)and temperature chaneges on their growth,nitrogen fixation,element composition and nitrogen-fixing isotope fractionation were studied The mechanism is preliminarily explored by combining the efficiency of nitrogenase and the ratio of two kinds of nitrogen-fixing rate(R ratio).The main conclusions of this paper are as follows:(1)Compared with the current concentration of pCO2 in seawater(400 ppm),the increase of pCO2 to 1500 ppm or the decrease to 180 ppm both inhibited the growth rate and nitrogen fixation rate of Trichodesmium and Crocosphaera.However,the change of pCO2 did not affect the C/N ratio of the two diazotrophs.Although POC and PON decreased simultaneously under some conditions,their ratios remained unchanged When pCO2 increased to 1500 ppm or decreased to 180 ppm,the value of ?15N decreased from-1.1‰ to to-2.1‰ and-3.1‰ respectively for Trichodesmium.The value of ?15N decreased from-0.9‰ to-2.2‰ and-3.6‰ respectively for Crocosphaera.Two parameters characterizing the efficiency of nitrogenase,Nitrogenase efficiency(nitrogen fixing rate/NifH protein abundance)and R ratio(defined as R=acetylene reduction rate/N2 fixation rate by nitrogenase)decreased when pCO2 increased or decreased.The possible reason is that the change of environmental pH decreases the nitrogenase efficiency of the nitrogen-fixing organism,thereby reducing the growth,nitrogen fixation rate and nitrogen isotopic fractionation 815N value.(2)The increase of temperature inhibited the growth rate and nitrogen fixation rate of Trichodesmium.The growth rate decreased by 3%-12%and the nitrogen fixation rate decreased by 7%-19%when the temperature increased from 27? to 3 1 ?.Temperature increase had no significant effect on the growth of Crocosphaera.When the temperature rose from 27? to 31?,the nitrogen fixation rate of Crocosphaera was inhibited by 5%-20%.The increase of temperature did not change the C/N ratio in the cells of the two diazotrophs.The increase of temperature increased the value of ?15N of the two diazotrophs.Especially when pCO2 was 1000 ppm and the temperature rose by 4?,the value of ? 5N in Trichodesmium increased from-2.8‰ to-1.7‰.When pCO2 was 1500 ppm and the temperature rose by 4?,the value of 8 5N in Crocosphaera increased from-3.6‰ to-2.2‰.The temperature rise had no significant effect on the Nitrrogenase efficiency and R ratio of the two diazotrophs.The effect of temperature on the growth,nitrogen fixation and isotope fractionation of the two diazotrophs remains to be elucidated.(3)The coupling effect of pCO2 change(increase or decrease)and temperature increase showed greater inhibition on nitrogen fixation rate of Trichodesmium and Crocosphaera.When pCO2 increased to 1500 ppm or decreased to 180 ppm and the temperature increased by 4?,the nitrogen fixation rate of Trichodesmium decreased by 47%or 39%respectively.When pCO2 increased to 1500 ppm or decreased to 180 ppm and the temperature increased by 4?,the nitrogen fixation rate of Crocosphaera decreased by 46%or 37%respectively.Understanding the effects of two environmental factors(pH and temperature)on the growth,nitrogen fixation and element composition of two typical nitrogen-fixing cyanobacteria is of great significance to the study the response of marine nitrogen-fixing organisms in the face of environmental changes in different periods.The changes of the ?15N value of isotope fractionation of nitrogen fixation affected by two environmental factors(pH and temperature)have certain reference significance in evaluating the contribution of marine nitrogen fixation in the nitrogen cycle in the future or the depict of paleomarine habitat. |
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