Study On Iron-Nitrogen Coupling Cycle And Its Influencing Factors In Sediments Of Taihu Lake

Photosynthesis and respiration of cyanobacteria can cause diurnal variation of dissolved oxygen in the waterbody.Such anaerobic/aerobic alternation is considered as a hot spot for nitrogen removal by iron-nitrogen coupled cycle.Thus,we speculate that in addition to the traditional denitrification and Anammox,there are more nitrogen removal pathways in lakes during algal bloom outbreaks and decay,such as Anaerobic ferric ammonia oxidation(Feammox)and nitrate-dependent ferric oxide(NDFO).Therefore,the sediments of different periods and regions in Lake Taihu were collected to explore the temporal and spatial distribution characteristics of iron,nitrogen and related bacterial community structure in the sediments..In addition,the nitrogen removal potential of Feammox and NDFO and the effect of substrate concentration on the two processes were investigated by isotope tracer,high-throughput sequencing and X-ray photoelectron spectroscopy.Furthermore,the effects of cyanobacteria degradation on the nitrogen removal potential of Feammox in the sediments of Lake Taihu were investigated by adding cyanobacteria dry powder.The main results are as follows:1.Field investigation showed that that the active Fe(Ⅱ)and active Fe(Ⅲ)contents were relatively high in the sediments from different regions of Lake Taihu.The active Fe(Ⅱ)content and active Fe(Ⅲ)in the sediments of Lake Taihu in winter were lower than that in summer(P<0.01),the contents of different form of iron in the sediments collected from Meiliang Bay were slightly higher than that in the open area,but the difference was not significant(P>0.05).Moreover,the high-throughput sequencing results show that iron and nitrogen cycling related bacteria such as Geobacter,Thiobacillus,Anaeromyxobacter,Thauera,Aromatoleum and Halomonas are also abundant in the sediments of Lake Taihu,indicating that the sediments in Lake Taihu possess the substrate and microbial conditions for the Fe-N coupling cycle.2.After anaerobic culture,the changes of the surface color and morphology,the concentration of NH4+-N,Fe(II),NO3--N and the increase of the relative abundance of Fe-reducing bacteria in the sediment of the treatments with the addition of Fe(Ⅲ)and NH4+-N proved the occurrence of the Feammox reaction.In the process of Feammox,the increase of Fe(Ⅲ)concentration would promote the reaction of Feammox,while the high concentration of NH4+-N may have an inhibitory effect.The results of isotope incubation experiments showed that the nitrogen removal rate of Feammox in the sediments of Lake Taihu could reach 16%-23%of the denitrification rate,indicating that Feammox may play an important role in the nitrogen self-purification process of Lake Taihu.3.After the additions of Fe(Ⅱ)and NO3-and anaerobic culture,the NO3--N concentrations reduced significantly,while the results of XPS photoelectron spectroscopy showed that a large amount of Fe(Ⅱ)was transformed into Fe(Ⅲ).Moreover,a large amount of brown flocs were produced on the surface of the sediment,and the relative abundance of NDFO-related bacteria increased to a great extent.All these phenomena indicated the occurrence of NDFO reaction in the system during anaerobic cultivation process.In NDFO process,the addition of Fe(Ⅱ)can promote the reduction of NO3--N and lead to the accumulation of NO2--N,while the addition NO3--N exhibited a relatively small impact on the oxidation of Fe(Ⅱ).The isotopic results showed that the nitrogen removal rate of the sediments in Lake Taihu was 367-434 mg N/(m2·d)after the addition of Fe(Ⅱ),indicating that the addition of Fe(Ⅱ)could promote the removal of NO3--N from Lake Taihu.4.The relative abundance of Fe-cycling-related bacteria in the sediments increased after 180 days of dark incubation with the addition of dry cyanobacterial powder,indicating that cyanobacteria may promote the proliferation of Fe-cycling-related bacteria.The results of isotope culture showed that the production rates of 30N2 and Fe(Ⅱ)in the treetments with the addition of cyanobacteria were higher than that in control,sugestting that cyanobacterial bloom may promote the Feammox in Lake Taihu.In summary,the research on the coupling process of iron and nitrogen in Lake Taihu can not only enrich and improve the theoretical knowledge on the process and mechanism of microbial-mediated iron and nitrogen Biogeochemistry cycle in eutrophic lakes.At the same time,it can also provide innovative methods for eutrophication control and nitrogen reduction.