| Nitrogen,as an essential element of the growth of the marine organisms in the ocean,limits marine biological productivity and plays an important role in marine biogeochemical cycle.Nitrification refers to the process that ammonia(NH4+)was oxidized to nitrite(NO2-)by marine ammonia oxidizing bacteria and archaea,then NO2-was further oxidized by nitrite oxidizing bacteria eventually became nitrate(NO3-).The process changes the distribution of inorganic nitrogen that marine phytoplankton rely on,and links the source and sink of fixed nitrogen.Nitrification is central transformation pathway in the marine nitrogen cycle.All the time,Ammonia oxidation is regarded as the limiting step of nitrification,becoming the focus of the nitrification research.Nitrite oxidation has not received corresponding attention,although which is the primary source of NO3-.So far,we know little about nitrite oxidation rate of photic zone and the whole water column.What's more,NO2-accumulated at some specific layer such as primary nitrite maximum(PNM)at the base of euphotic layer.This is contrary to the notion that NH4+ oxidation is the limiting step of nitrification but still lacks unified cognition,so the study of nitrite oxidation process has important scientific significance.This study is set in oligotrophic South China Sea(SCS),using artificial isotope labeling technology and molecular biology techniques.This study investigated the vertical distribution characteristics of nitrite oxidation rate and functional microbes in summer of the SCS,and further analyzed the kinetics characteristics of ammonia oxidation and nitrite oxidation.The formation and maintaining mechanism of PNM and the relationship between nitrate oxidation and new productivity are discussed in our work.We guess that the relative relationship between ammonia oxidation and nitrite oxidation rate under different stages determines the vertical distribution characteristics of nitrite concentration.Effective removal of the added 15N-NO2-is an important basis for accurately testing the rate of nitrite oxidation.We got stable growth rate of 15NO3-in the time series incubation after NO2-were removed by using sulfamic acid,indicating that the removal efficiency of NO2-was stable and the cultivated way was reasonable.The effect of community structure changes or dilution by ammonia oxidation during incubation were insignificant for the calculation of the rate.The results of nitrite oxidation rate are reliable.The nitrite oxidation rate is over 4 orders of magnitude in the SCS,from 0.001 nmol L-1 d-11 to 26.50 nmol L-1 d-1.In addition,the rate maximum of nitrite oxidation(NOM)of the water column at the seat station in 2016 was 13.73 nmol L-1 d-1.The NOM that was measured at the SEATS station in 2017 was close to that of 2016,which was 8.04 nmol L-1 d-1.The rates of NO2-oxidation showed distinct subsurface maxima in summer of the SCS.The surface velocity is usually lower or lower than the detection limit,which was possibly caused by light inhibition and the competition of phytoplankton for substrate NO2-.The rate increases rapidly from the bottom of the mixing layer to the base of the euphotic layer.However,with the supply of substrate NO2-gradually decreasing,the nitrite oxidation rate gradually decreases in the middle and deep ocean.The abundance of Nitrospina and Nitrospira on the distribution characteristics of nitrite-oxidizing bacteria(NOB)also had a peak at the depth of 5-200 m,but some stations had a"bimodal" distribution pattern.The abundance of Nitrospina is generally higher than Nitrospira,indicating that the dominant group of nitrite oxidation bacteria(NOB)is Nitrospina in SCS basin area in summer.The depth of the maximum abundance of Nitrospina and Nitrospira is not coupled with the layer of the peak of ammonia oxidation rate and nitrite oxidation rate,suggesting a complex regulation relationship between microbial abundance and activity distribution.When investigated the influence of substrate concentration on nitrite oxidation in the SCS,we found that the relationship between NO2-concentration and nitrite oxidation rate followed the Michaelis-Menten relationship.The mean value of Km was 0.21 ± 0.11?mol L-1 in our research.This is closed to Km values(0.25±0.16?mol L-1)measured by Sun et al.(2017)in the Eastern Tropical North Pacific,while was 1-3 orders of magnitude lower than the Km of pure strains cultivated NOB(Nitrobacter,Nitrotoga and Nitrospira;Ks=6-544 ?mol L-1)(Blackburne et al.,2007;Nowka et al.,2015;Ushiki et al.,2017).The nitrite oxidation bacteria(NOB)(probably mainly Nitrospina)showed a strong affinity for NO2-in the SCS,implying that it can adapt to the marine environment of low NO2-.In addition,the Km value constantly decrease with increasing of depth.We guess that is because of the different community structure or activity type of NOB.There was not significant linear correlation relationship between the nitrite oxidation rate and NO2-concentration in euphotic layer(1%Par).In contrast,the nitrite oxidation rate and NO2-concentration have a significant linear relationship(R2 =0.908,p<0.05),suggesting that light is an important impact factor for nitrite oxidation.Moreover,the nitrite oxidation rate under dark culture condition was higher than under the cultivated condition of in situ light at SS1 station light and dark incubation experiment,directly and fully proved that light has inhibitory effect on nitrite oxidation process.Light is the key environmental factor to regulate nitrite oxidation in photic zone.In the dark ocean,substrate concentration is the main factor to control nitrite oxidation.The vertical distributions and 100 m water column integral results of ammonia oxidation rate and nitrite oxidation rate show the fact that ammonia oxidation rate is greater than nitrite oxidation rate,which means that the ammonia oxidation process is not limiting step of nitrification in the photic zone.NOB may be more sensitive to light,and perhaps because the degree of light sensitive for AO A?AOB and NOB is different,leading to the accumulation of NO2-in the euphotic zone.Ammonia oxidation rate was considered as the parameters of the new productivity calibration in current study,but NO3-produced by nitrite oxidation process in fact.And according to our research,there was a significant difference between ammonia oxidation rate and nitrite oxidation rate.Therefore,nitrite oxidation rate rather than ammonia oxidation rate should become the revised basis of new productivity.This study helps to fill the blank on the research of nitrite oxidation in aerobic area,and explore the formation and maintaining mechanisms of PNM in oligotrophic sea.At the same time,it gives theoretical support to accurate quantify the biological pump export efficiency and productivity. |
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