| The latest research shows that the loss of oxygen in the ocean has been about 2%in the last 50 years,which has led to the continuous expansion of the Oxygen Minimum Zones(OMZs)in the ocean,and the number of low-oxygen zones found in coastal waters has increased.The study found that there is a serious imbalance in the input and output of nitrogen in the ocean,and a large part of the fixed nitrogen is reduced to N2 or N2O and is lost from the ocean.Although the OMZs in the ocean is considered to be the most important area for nitrogen loss,there is still a unclear understanding of the nitrogen transformation process and influencing factors in the OMZs.In order to explore the biogeochemical cycle of nitrogen in the hypoxic environment,laboratory simulation cultivation was used in this study to explore the nitrogen migration process in the hypoxic water.This study measured dissolved inorganic nitrogen(DIN)and other related parameters in overlying water at different dissolved oxygen(DO)concentrations,calculated and analyzed the transformation characteristics,main reactions and proportions of DIN in different stages of hypoxic water,and further estimated the total annual loss of nitrogen in the OMZs.Meanwhile,a complete method for extracting organic matter was established in this study.The ladderane core lipids and n-alkanes in surface sediments in different DO concentrations were detected,the indicator effect of the ladderane core lipids on anammox and the changes of them in different stages were analyzed.This study also explored the change characteristics of n-alkanes in different DO environments and the relationship with nitrogen conversion.The main findings are as follows:1.In this culture system,there was a closed correlation between the variation of nitrogen in water and the DO concentration in it.The anammox was a key process affecting nitrogen transformation in hypoxic environment,but aerobic denitrification was a key process affecting DIN conversion in an environment with high DO concentration.The change of nitrogen in hypoxic water could be divided into three stages:nitrate dissimilatory nitrate reduction to ammonium(DNRA)and denitrification,anammox and denitrification,and the anammox.During the first stage in the hypoxic environment(days 1?10),the concentration of NH4+ in water continued to rise while the concentrations of NO2-and NO3-kept decreasing.Since the concentration of DO in the hypoxic environment could inhibit denitrification and anammox,and there was a little concentration of H2S in the water that can promote DNRA,it could be determined that the DNRA contribute to the rise of NH4+and the decline of NO2-and NO3-.The final data showed that the ratio of DNRA and denitrification in this stage should be less than 40:60,and the formation of nitrogen-containing gas in the system should be higher than 11.63 ?mol/L.In the second stage(days 11?36),the main reactions were anammox and denitrification,and the rate of them should be more than 40:60.During the third stage of the reaction(days 37?64),the main reaction was anaerobic ammonia oxidation.In the early period of this stage,the concentration of NH4+ increased rapidly,and the time of starting to rise coincided with the time when nitrite reached the minimum value.It shows that when the NO2-in water was exhausted,the nitrite inputted from the sediment mineralization was not enough to maintain the original anammox rate and weaken it.In the later period,the concentration of NH4+ remained relatively stable,and there was also no accumulation of NO2-/NO3-concentration in the water.Based on the DIN after stabilization in this stage,the total nitrogen loss in global OMZs was estimated to be 240?260 Tg/a,which was higher than previous estimates.The change of nitrogen in near oxygen-saturated or oxygen-saturated environment could be divided into two stages:aerobic denitrification,heterotrophic nitrification-aerobic denitrification.The changes of nitrogen in high DO saturation environment indicated that the related reactions were basically the same,and the microbial activities in the system are similar.The analysis results of various factors showed that there was a co-respiration/co-metabolism process of oxygen and nitrate in water during first stage(days 1-18),and its reaction rate was higher than the nitrification rate.Meanwhile,the formation of micro-reducing environment on the surface of the sediment will also inhibited the nitrification and promoted denitrification.In the second stage(days 19?64),the main reaction was heterotrophic nitrification-aerobic denitrification.In these two systems with high DO concentration,there was a rapid decline and re-stable change of NH4+ concentration in this stage.This was because when NO2-and NO3-were consumed,the microbial community in the system would change with the increase of the cultivation time.And when the consumption rate of NH4+increased to exceed the maximum generation rate of it provided by mineralization,the concentration of NH4+ can be reduced.After comprehensive analysis of the actual effect factors in the cultivation experiment,it was believed that the heterotrophic nitrification and aerobic denitrification in the system could be performed simultaneously,so that NH4+ generated by mineralization was directly transformed the nitrogen-containing gas to escape from the culture system.And the DIN in the overlying water has also reached the equilibrium state.2.As a biomarker of anammox,the content of the ladderane core lipids in the surface sediments of the hypoxic system was increasing,which was mainly generated from the anammox of the upper water.The change of n-alkane was closely related to the strength of anammox in water.The ladderane core lipids mainly came from anammox in upper water,and its content increased with the increase of the culture time,which was consistent with the changes of DIN in overlying water.The results show that with the increase of time,the composition of the basic components of ladderane core lipids(C18-[5]-ladderane FAME(?),C18-[3]-ladderane FAME(?),C20-[5]-ladderane FAME(?)and C20-[3]-ladderane(?))did not change.The content of the ladderane core lipids in the hypoxic environment was speculated it mainly came from the anammox in the overlying water.When the culture system was in high DO concentration,the content of the ladderane core lipids maintained stable.The conversion coefficient between the anammox rate in the stabilized hypoxic water and the formation rate of the ladderane core lipid in the surface sediment should not be less than 314.61?416.77(nmol ·ng)/(g·L),and the ladderane core lipid deposition rate should not be more than 0.12?0.17 ng/(g·m2·d).These conversion coefficient and sedimentation rate are applicable to hypoxic water where the stratification of upper and lower water is not obvious.The degradation of n-alkanes in the three culture systems could be divided into two stages,and it was also found that in the medium/late period of hypoxic environment,there was a closed correlation between the changes in n-alkanes and the degree of anammox.In the early stage of the reaction(days 1?30),due to the high resistance to degradation of high carbon n-alkanes,the L/H had a small change range,and the advantage of high carbon n-alkanes was relatively stable.But in the later stage(days 31?120),due to the simultaneous effects of multiple degradation reactions and without external input,the content of high carbon n-alkanes was significantly reduced,and the L/H increased significantly.Because of the degradation of algae residues and the selective degradation of fatty acids by bacteria to generate short-chain n-alkanes,the content of C15 in the sediments rapidly increased in the early stage and became the peak center,and then the synergistic effect of biological factors and evaporation factors made its content decrease slowly.In the environment with high DO concentration,the rate and degree of change about n-alkanes were significantly higher than those in the hypoxic environment,which indicated that the aerobic respiration of organisms played a significant role in the degradation.Comparing the changes in the content of n-alkanes and high carbon n-alkanes(C21?C33)in the hypoxic environment with the changes of DIN in the overlying water,it was found that there was a closed correlation between the reaction rate of n-alkanes in the medium/later period and the intensity of anammox in water.It was speculated that this may be due to the formation of ladderane lipids which could promote the fumarate degradation of n-alkanes.And in the environment with high DO concentration,the changes of the total n-alkanes content and the high carbon n-alkanes content further proved the existence of the biological oxidative degradation of aerobic microorganisms. |
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