Community Structure And Biogeographic Distribution Of Diazotrophs In The Indo-west Pacific Region

The Indo-West Pacific region mainly includes the Eastern Indian Ocean,the Western Pacific Ocean and their confluence areas.The Indo-West Pacific region has a complex geographical environment and is a hot research area in the field of marine biodiversity.Marine diazotrophs,as a functional group of nitrogen-fixing,play an important role in the nitrogen cycle and primary productivity in the ecosystem.Understanding the diversity of marine diazotrophs,the temporal and spatial patterns of communities and their influencing factors is of great significance for assessing the ecological functions and biogeochemical effects of diazotrophs.Surveys were carried out in the Eastern Indian Ocean,Western Pacific and South China Sea from 2016 to 2019.Using the analysis method of the nitrogen-fixing functional gene nif H high-throughput sequencing and microscopy,the composition,structure and biogeographical distribution of diazotrophs communities were studied at large scale,and the differences in the structure and distribution of diazotrophs communities were compared among different regions.The results of high-throughput sequencing showed that Proteobacteria is the dominant group,followed by Cyanobacteria;Sagitula castanea belonged to α-Proteobacteria and Crocosphaera watsonii are dominant species.Trichodesmium contributed to a small proportion in the community.Microscopic results showed Trichodesmium thiebaultii was the main species of planktonic nitrogen-fixing cyanobacteria.In the Easten Indian Ocean,α-proteobacteria,γ-proteobacteria and cyanobacteria were the main groups.The α-proteobacteria have the highest species richness,the relative abundance of which located in the equatorial region.The relative abundance ofγ-proteobacteria in the north of the equator was higher than that in the south of the equator,and reaches the highest value in the 75 m water layer of the equator.The high relative abundance of cyanobacteria was located in the 50 m to 100 m water column near Sri Lanka,and the highest value appeared in the 100 m layer.The microscopic cell abundance of Trichodesmium showed an inter-annual decline.In deep water column from 500 m to 2000 m,Trichodesmium(OTU343)occupied a certain proportion in the deep-sea,which may be related to the output of particulate carbon to the deep sea.The results of ANOVA analysis showed that diazotrophs communities had no significant differences between different stations and water depths.Besides α-proteobacteria in the diazotrophs communities in the western Pacific,cyanobacteria and γ-Proteobacteria accounted for a relatively high proportion,and the contribution of cyanobacteria to the community was significantly higher than that in the Eastern Indian Ocean(p<0.05).In the 25 m water layer near the Philippine Sea,the proportion of Crocosphaera watsonii in the community was up to 96.53%.The proportion of Trichodesmium is only 0.16% averagely.Combined with the results of microscopy,Trichodesmium mainly distributed between 75 m and 100 m.The main diazotrophs groups had no significant differences between water layers,while the δ-proteobacteria,β-proteobacteria and cyanobacteria showed significant differences between different stations(p<0.01).Compared with the Eastern Indian Ocean,the diazotrophs communities in the Western Pacific had lower diversity.In the South China Sea,the dominant group α-proteobacteria varied significantly with water depth,its high relative abundance presented in the central area of the South China Sea.Another main group γ-proteobacteria mainly distributed in the western South China Sea.Crocosphaera watsonii was the unique annotated cyanobacterial speices,which accounted for 2.07% in the community.In the region deeper than 500 m,the structure of diazotrophs community in the South China Sea were different from that of the East Indian Ocean.The proportion of β-proteobacteria and cyanobacteria in the South China Sea tended to increase with the increase of water depth;in the deep sea deeper than 1000 m,the contribution of δ-proteobacteria and β-proteobacteria to the community was higher than their contribution in the East Indian Ocean.The superiority of Proteobacteria suggested its key role in the ecosystem and biogeochemical cycle.The significant variation of the cyanobacteria in different habitats indicated that cyanobacteria may be more sensitive to changes in the marine environment under global changes.The biogeographical characteristics of the communities in the Indo-West Pacific region showed significant distance attenuation rather than latitude diversity.No significant relationship showed between nutrients and diazotrophs in the Indo-West Pacific region.Temperature and salinity have significant effects on diazotrophs in the Eastern Indian Ocean and the Western Pacific.It was speculated that the hydrological conditions may impact the geographic characteristics of diazotrophs community.This research provides insights into the succession of global marine diazotrophs community,provides a scientific basis for understanding the ecological response mechanism of diazotrophs community under global changing.