The Release Of Fixed Nitrogen To Dissolved Pool Of Trichodesmium And Crocosphaera Under Laboratory Culture

The nitrogen element is one of the essential elements of all living things and is an important element of the organism.In the ocean,biological productivity is mainly limited by nitrogen.Therefore,nitrogen cycle plays an important role in the ocean.Nitrogen fixation refers to theprocess of reduction of N2 gasunavailable for most organisms to reduced forms.of N which iscatalyzed by the enzyme nitrogenase.Theproduct of the reaction is ammonia(NH3)which is biologically available tomany plants and microorganisms for amino acid synthesis.Nitrogen fixation will also release some nitrogen in the nitrogen fixation process,which was not calculated in previous studies,leading to an underestimate of nitrogen fixation rates.The dissolved nitrogen released at the same time may result in the growth of the pico-phytoplankton,,bacteria and diatoms,and change the ecological composition of the Marine algae.Through the burial of diatoms,the transfer of fixed nitrogen to diatoms can also lead to indirect export of that.Therefore,the release of nitrogen fixation is of greatscientific significance.In the experiment,we cultivate unicellular nitrogen-fixing cyanobacteria Crocosphaerain exponential phase for ten days under laboratory conditions.We collect the particulate nitrogen,dissolved nitrogen and intracellular dissolved nitrogen before and after the incubation of 24h.With the addition of 13C and 15N tracer,we can calculate the carbon and nitrogen fixation rate of particle,the release rate of fixed nitrogen to dissolved nitrogen pool and nitrogen fixation rate of intracellular dissolved pool by measuring the concentrations and isotopic values of the collected samples.In order to further understand the factors that influence the dissolved nitrogen release,another experiment was designed.In this experiment,we did the incubation experiment in different stage(exponent phase,stationary phase or decline phase)of the growth cycle and under different environmental stress(light-limited,Fe-limited or P-limited).The release rate of fixed nitrogen to dissolved pool was calculated to investigate whether the physiological changes of nitrogen-fixing cyanobacteria will cause the increase of nitrogen release.Through the first experiment,we found that:(1)Unicellular cyanobacteria Crocosphaerahas a very small release rate of nitrogen fixation in the culture environment,accounting for less than 1%of the total nitrogen fixation rate.This shows that in the laboratory culture,the unicellular cyanobacteria Crocosphaera releases little dissolved nitrogen into the environment.(2)The ratio of cellular carbon and nitrogen amount of Crocosphaera is close to the Redfield ratio.But the ratio of carbon and nitrogen fixation rate is far more than the Redfield ratio,showing that nitrogen-fixing cyanobacteria fixes carbon and nitrogen not strictly following the Redfield ratio.This may be due to nitrogen-fixing cyanobacteria have different characteristics from the general phytoplankton.(3)In this experiment,the turnover time of the intracellular nitrogen pool of Crocosphaera was less than 1 day,so the release of nitrogen fixation to the dissolved nitrogen pool was not underestimated.Through the second experiment,we found that:(1)In the different stages of growth cycle and under the stress of different environmental conditions,the release of fixed nitrogen to dissolved nitrogen pool accounts for 0.5%to 1.5%of total fixed nitrogen.(2)There is no relationship between the ratio of release to total fixed nitrogen and the Nitrogen-fixing cyanobacteria species.For both Trichodesmium and Crocosphaera,the ratio is similar to each other.(3)The ratio of carbon fixation to nitrogen fixation of Trichodesmium and Crocosphaera was much higher than Redfield value,regardless of the stage of the cell growth cycle and under the conditions of environmental stress.But with the growth of cyanobacteria,the rate increases,which may indicate a shift in the metabolism of nitrogen,carbon or both assimilation processes.(4)In this experiment,the turnover time of the intracellular nitrogen pool was larger than one day,indicating that the nitrogen fixation rate may be underestimated.Especially in the later stage of the growth cycle,the long turnover time can lead to severe N isotope imbalance between inside and outside the cell,which in turn can lead to serious nitrogen fixation rate underestimation.(5)Rapid growth of bacteria in the later stages of the culture cycle may accelerate the decomposition of particulate nitrogen and dissolved organic nitrogen.At the same time,the bacteria also contain nitrogen,and its contribution to extracellular dissolved nitrogen cannot be ignored.