Nitrogen Fixation In The Aphotic Zone Of The Oligotrophic South China Sea And The Eutrophic Jiulong River Estuary

Marine nitrogen fixation is the process that marine nitrogen-fixing organisms?mainly cyanobacteria?convert N2 into bio-available N.This process mainly occurs in the euphotic zone of oligotrophic waters at low latitudes,which supporting nearly half of new productivity.It is generally believed that nitrogen-fixing autotrophic cyanobacteria?Photosynthetic nitrogen fixation?are controlled by light and nutrient.However,for some non-cyanobacteria nitrogen-fixers,they are not limited by light and nutrient?Heterotrophic nitrogen fixation?so that they can widely distribute in some dark and/or nutrient-rich ecosystems,such as coastal up-welling zones and oceanic aphotic zones?aphotic nitrogen fixation?.These regions transcend the environmental boundaries where nitrogen fixation has traditionally occurred,which may affect current estimates of global ocean nitrogen input.As a result,researchers are increasingly focusing on waters that were previously thought to be unimportant for nitrogen fixation,with the aim of better understanding and exploring non-cyanobacteria nitrogen fixers and attempting to address global ocean nitrogen budget.This paper investigated the nitrogen fixation rate?NFR?and primary productivity of the euphotic zone in the oligotrophic South China Sea?SCS?and primarily explored the nitrogen fixation process in the aphotic zone of SCS and Jiulong River Estuary?JLR?.The main conclusions are as follows:?1?In SCS?spring?,maxima of nitrogen fixation rate and carbon fixation rate in the euphotic layer occurred in the subsurface layer about 25 m.SEATS station has higher abundance of Trichodesmium and UCYN-A,however its NFR is lower than A2 station.There are not Trichodesmium at A2 station.Instead,the dominant nitrogen fixers are Het-2.Combined with the hydrological characteristics and nutrient parameters,it can be seen that A2 station is agitated by strong wind in spring and has stronger water mixing capacity.A2 station shows the characteristics of high biogenic silicon concentration,high phosphate concentration and low silicate concentration.So it is speculated that the high nitrogen fixation rate of A2 station is mainly supported by DDAs.?2?The measured dark NFR in the aphotic zone of SCS ranges from 0.01-0.14 nmol N L-1 d-1,which is one order of magnitude lower than the euphotic zone?0.03-2.5 nmol N L1-d-1?,and equivalent to that measured in the existing literature.After 48 h and 96 h dark incubation,it is concluded that the nitrogen fixation at 200 m in the two stations are relatively significant.Although the dark NFR is low,the nitrogen fixation flux can be found to be equivalent to that in the euphotic layer when integrating the water column roughly.It suggests that the aphotic zone has certain significance for the global marine nitrogen balance.?3?In JLR,nitrogen fixation process is active and the rates range from 0.01-0.14 nmol N L^-1 h^-1,among which the station?S=5.8?has the highest light nitrogen fixation rate.At the same time,a certain NFR is detected after 12 h dark incubation.For inshore low-salinity waters,the daytime NFR was higher than the dark NFR,suggesting that the nitrogen fixation communities may be dominated by photosynthetic nitrogen fixers,and some heterotrophic nitrogen fixers also played a role in the absence of light High salinity areas shows that the daytime NFR was the same as the dark NFR.the nitrogen fixation rate in JLR(0.07-1.3 nmol L^-1 h^-1)was higher than that in the coastal zones of the Atlantic ocean and even equivalent to that in the SCS surface layer where affected by kuroshio?0.09-0.71 nmol L-1h-1?.?4?By using the high throughput sequencing method,we can find that The nitrogen-fixers communities at low salinity area are obviously different from that at high salinity areas.However,the nitrogen fixation communities of the two stations with low salinity or high salinity are relatively similar.In the nutrient-rich estuary environment of jiulong river,the nitrogen-fixing rmicroorganism community is mainly dominated by proteobacterna.?-proteobacteria and Y-Proteobacteria may contribute the light nitrogen fixation mainly while ?-proteobacteria may be the most important contributors to the dark nitrogen fixation.