| Nitrite-dependent anaerobic ammonium oxidation(ANAMMOX)and anaerobic methane oxidation(N-DAMO),two recently discovered processes in microbial nitrogen and carbon cycles,which are significant to accomplish the global nitrogen and carbon cycles,are unique in linking the two important biogeochemical cycles.Mangrove wetlands located in the land/ocean interface are periodically flooded by seawater,which can create an anaerobic environment in the sediment,and the anoxic conditions of the sediments can provide suitable habitats for ANAMMOX and N-DAMO bacteria.And mangrove wetland can accept all kinds of pollutants from sea and land,so the material resources and biological resources are very rich.At present,the research of the ANAMMOX bacteria mainly focuses on marine environment,but the study about N-DAMO process mainly concentrates on freshwater habitats.However,the interaction between the two different microorganisms in mangrove sediments are rarely reported and the niche and activity of ANAMMOX and N-DAMO bacteria in the mangrove ecosystem have not been confirmed.Here,we report the community structures,diversity,abundance,activities and ecological benefits of ANAMMOX and N-DAMO bacteria in different layers of mangrove sediments.The major results are as follows:1.The community structures and diversity of ANAMMOX and N-DAMO bacteria in mangrove sedimentsDifferent genera of ANAMMOX bacteria were detected in mangrove sediments in summer and winter,indicating a high diversity of ANAMMOX bacteria,which affiliated to Candidatus Brocadia,Candidatus Kuenenia and Candidatus Scalindua based on ANAMMOX bacterial 16S rRNA gene.Candidatus Kuenenia and Candidatus Scalindua were the dominant ANAMMOX bacteria in mangrove sediments.In the meantime,the genera collected from mangrove sediments of hzsB gene in winter affiliated to Candidatus Kuenenia and Candidatus Scalindua.It was found that the community structures of ANAMMOX bacteria in upper layers(0-20 cm),middle layers(20-40 cm and 40-60 cm)and deeper layers(60-80 cm and 80-100 cm)of Aegiceras corniculatum(AC),Rhizophora candel(RC)and Avicennia marina(AM)habitats were extremely similar,respectively.At the same time,the community structures of ANAMMOX bacteria in mangrove sediments were most similar with that in bay environment and it had some similarity with that in rhizosphere soil and Taihu sediments.Correlation analysis showed that the sediment total organic carbon concentration,water content,the redox potential and nitrate concentration were the most significant factors affecting the community structures of ANAMMOX bacteria in mangrove sediments.Different clusters of N-DAMO bacteria were found in mangrove sediments in summer and winter,suggesting a high diversity of N-DAMO bacteria,which belonged to group B,D,E in summer and group A,B,D in winter based on N-DAMO bacterial 16S rRNA gene.Group B was the dominant N-DAMO bacteria in mangrove sediments.Meanwhile,the genera obtained from mangrove sediments of pmoA gene in summer and winter belonged to M.oxyfera and M.sinica.The research discovered that the community structures of N-DAMO bacteria in upper layers(0-20 cm),middle layers(20-40 cm and 40-60 cm)and deeper layers(60-80 cm and 80-100 cm)of Aegiceras corniculatum(AC),Rhizophora candel(RC)and Avicennia marina(AM)habitats were extremely similar,respectively.In the meanwhile,the community structures of N-DAMO bacteria in mangrove sediments were most similar with that in marine and Mai Po wetland and it had some similarity with that in natural freshwater wetland soil.Correlation analysis indicated that the sediment total organic carbon concentration,water content,nitrite concentration,methane concentration and the redox potential were the most significant factors influencing the community structures of N-DAMO bacteria in mangrove sediments.2.The abundance of ANAMMOX and N-DAMO bacteria in mangrove sedimentsLarge numbers of ANAMMOX bacteria were detected in mangrove sediments.The abundance of ANAMMOX bacterial 16S rRNA gene ranged from 0.41×107 to 4.07×107 and 1.39×107 to 9.74×107 copies per gram of dry soil in the examined sediment cores in summer and winter.And the abundance of hzsB gene ranged from 0.42×106 to 6.44×106 copies per gram of dry soil in the examined sediment cores in winter.Nevertheless,the abundance of ANAMMOX bacteria in the surface layer of mangrove sediments was obviously higher than that in the deep layer,showing that ANAMMOX bacteria minaly distributed in the surface layer of mangrove sediments.Large numbers of N-DAMO bacteria were detected in mangrove sediments.The abundance of N-DAMO bacterial 16S rRNA gene varied from 0.24×107 to 2.09×107 and 0.16×104 to 5.20×104 copies per gram of dry soil in the examined sediment cores in summer and winter.And the abundance of pmoA gene varied from 0.21×107 to 3.38×107 and 0.59×106 to 2.72×106 copies per gram of dry soil in the examined sediment cores in summer and winter.However,the abundance of N-DAMO bacteria in the surface layer of mangrove sediments was obviously higher than that in the deep layer,indicating that N-DAMO bacteria minaly distributed in the surface layer of mangrove sediments.3.The activities of ANAMMOX and N-DAMO bacteria in mangrove sedimentsExtensive ANAMMOX activities were observed in mangrove sediments,with the potential rates varying from 4.83 to 277.36 nnmolN2·g-1 soil·d-1.The potential ANAMMOX rates in Aegiceras corniculatum habitat decreased first and then it rised with the increase of sediment depth,but the potential ANAMMOX rates in Rhizophora candel and Avicennia marina habitats rised gradually with the increase of sediment depth.Moreover,the potential ANAMMOX rates in Aegiceras corniculatum habitat were obviously lower than those in Rhizophora candel and Avicennia marina habitats.Hence,we may reasonably conclude that the deep layer of mangrove sediments was the reaction hotspots for ANAMMOX bacteria.Extensive N-DAMO activities were observed in mangrove sediments,with the potential rates varying from 24.19 to 1077.44 nmolCO2·g-1 soil·d-1.The potential N-DAMO rates in Aegiceras corniculatum habitat rised first and then it decreased with the increase of sediment depth,but the potential N-DAMO rates in Rhizophora candel and Avicennia marina habitats decreased gradually with the increase of sediment depth.Furthermore,the potential N-DAMO rates in Aegiceras corniculatum habitat were obviously higher than those in Rhizophora candel and Avicennia marina habitats.Therefore,we may reasonably conclude that the surface layer of mangrove sediments was the reaction hotspots for N-DAMO bacteria.4.The relationship between physicochemical characteristics of mangrove sediments and ANAMMOX and N-DAMO bacteria.Pearson correlation analysis indicated that the sediment redox potential,nitrite and total organic carbon concentration had positive impact on the diversity of ANAMMOX bacteria in mangrove sediments(P<0.05 or P<0.01);the sediment pH and temperature had negative impact on the diversity of ANAMMOX bacteria in mangrove sediments(P<0.05 or P<0.01).And it was found that the sediment water content,nitrite and total organic carbon concentration had positively significant correlated to the abundance of ANAMMOX bacteria in mangrove sediments(P<0.05 or P<0.01);the sediment pH and temperature had negatively significant correlated to the abundance of ANAMMOX bacteria in mangrove sediments(P<0.05 or P<0.01).In addition,the sediment total nitrogen and total organic carbon concentration had the negatively important impact on the activities of ANAMMOX bacteria in mangrove sediments(P<0.05).Pearson correlation analysis revealed that the sediment redox potential,water content and methane concentration had positive influnce on the diversity of N-DAMO bacteria in mangrove sediments(P<0.05 or P<0.01);the sediment pH,temperature and ammonium concentration had negative influence on the diversity of N-DAMO bacteria in mangrove sediments(P<0.05 or P<0.01).And it was found that the sediment redox potential,temperature and ammonium concentration had positively important correlated to the abundance of N-DAMO bacteria in mangrove sediments(P<0.05 or P<0.01);the sediment salinity and methane concentration had negatively important correlated to the abundance of N-DAMO bacteria in mangrove sediments(P<0.05 or P<0.01).In addition,the sediment water content,nitrite and total organic carbon concentration had the positively significant influence on the activities of N-DAMO bacteria in mangrove sediments(P<0.05 or P<0.01);the sediment pH had the negatively significant influence on the activities of N-DAMO bacteria in mangrove sediments(P<0.01).5.The ecological benefits of ANAMMOX and N-DAMO bacteria in mangrove sedimentsThis study confirmed that ANAMMOX and N-DAMO microorganisms can coexist in the mangrove wetland ecosystem.Furthermore,the results of stable isotope tracer experiment showed that the reaction hotspots of ANAMMOX bacteria was in the deep layer,but the reaction hotspots of N-DAMO bacteria was in the surface layer.As a result,we may reasonably come to a conclusion that the niche of ANAMMOX and N-DAMO bacteria segregated in mangrove sediments.According to stable isotope tracer experiment and flux calculation,the ANAMMOX bacteria could remove 5.4%of nitrogen loading in mangrove sediments.This suggests that the ANAMMOX process is a significant nitrogen sink in wetlands,which can alleviate eutrophication of rivers,lakes and marine to a large extent.It is estimated that the N-DAMO bacteria could remove 36.2%of nitrogen loading in mangrove sediments,which plays a vital role in alleviating water eutrophication.In addition,it is considered that about 2%-6%methane gas produced by wetlands ecosystem could be oxidized via the N-DAMO process in mangrove sediments,indicating that the N-DAMO bacteria may be a previously overlooked metane sink in mangrove sediments. |
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