Distribution And Diversity Of Magnetotactic Bacteria In Sedimens Of Continent Shelf Of The Yellow Sea And East China Sea

Prokaryotes that exhibit magnetotaxis,collectively known as magnetotactic bacteria(MTB),have directional motility that is influenced by the Earth’s geomagnetic field and by externally applied magnetic fields.They are cosmopolitan in sediments of freshwater and marine habitats.They form variously shaped intracellular biomineral magnetic crystals,either magnetite(Fe3O4)or greigite(Fe3S4),termed magnetosomes,which cause the MTB to passively align along magnetic field lines as they swim.MTB have been classified into the phyla Proteobacteria,Nitrospirae and so forth.Different morphological types of MTB were found in marine,limnetic and pedologic environments since MTB were reported in Science by Richard P.Blakemore.Most studies of marine MTB have concerned intertidal zones,and there have been few reports of MTB from seafloor sediments.There is no systematic research focusing on MTB in sediments of continent shelf.In this study,a combination of ecological investigation and experimental technique,such as light microscopic observation,transmission electron microscopic observation,and high-throughput sequencing,were utilized to study the ecology of marine MTB inhabiting in sediments of the continent shelf of the Yellow Sea and East China Sea,including horizontal distribution,vertical distribution,seasonal variation,diversity and the relationship between MTB and environmental factors.MTB were ubiquitous in sediments of continent shelf of Yellow Sea,from nearshore to far off-shore.The percentage of stations where MTB occurred in summer,winter and spring was 27.27%,30.43% and 60.00% and the average abundance in these three seasons was 260 ind./dm3,130 ind./dm3 and 470 ind./dm3.The highest abundance of MTB in these three seasons was 2400 ind./dm3,820 ind./dm3 and 5000 ind./dm3 at station H17(H19)(121.33oE,35.00oN).At most stations except H17(H19)the dominant morphology of MTB was coccoid whereas at H17(H19)rodshaped MTB dominated in three seasons.The abundance of MTB varied vertically,with the highest abundance in top layer(0-2 cm,22 ind./dm3;2-4 cm,19 ind./dm3).With depth increasing,the abundance of MTB decreased(4-6 cm,13 ind./dm3;6-8 cm and 8-10 cm,12 ind./dm3)and correlated to redox potential.The horizontal distribution of MTB in sediments of the Yellow Sea continent shelf was influenced by the grain size of sediments,with the abundance being positive correlated with the median sediment grain size.A trend of increasing MTB abundance with decreasing percentages of TC,TN,TS,and TOC,and a decreasing concentration of Fe,was also observed.The sediments at station H17(H19)were significantly coarser than at any other station.Percentages or concentration of some chemical elements at station H17(H19)were almost the lowest of all stations.Sediment grain size can influence the MTB motility which is indispensable for the metabolism and survival of MTB.Relative low concentrations of nutrients appear more favorable for MTB and Cu,Co and Zn inhibit the motility of MTB.These comprehensive factors can make a contribution to the negative correlation between MTB abundance and median sediment grain size.The hypervariable region V3–V5 of bacterial 16 S r RNA genes was amplified by PCR using the bar-coded universal primers 338 F and 806 R and then library construction and high-throughput sequencing were performed.High-throughput sequencing showed MTB of the Yellow Sea in summer and winter can be divided into 72 OTUs(26 OTUs belonged to Alphaproteobacteria;7 Deltaproteobacteria;38 Nitrospirae).There were 19-46 OTUs at each station.The number of reads was 2447 and 3271 in summer and winter respectively.There were 71-439 reads at each station.The proportion of reads of MTB affiliated to Nitrospirae was approximately 75%.The dominant group of MTB in sediments of the Yellow Sea continent was Nitrospirae.In sediments of continent shelf of East China Sea,MTB distributed in both nearshore and far off-shore stations.The average abundance of MTB was 290 ind./dm3 and 590 ind./dm3 in 2014 and 2016.The highest abundance in 2014 was 11,000 ind./dm3 at station 6-5 and the highest abundance in 2016 was 6,500 ind./dm3 at station DH4-4.These two stations were adjacent.The dominant morphology of MTB at near-shore stations was coccoid while that at far off-shore stations was coccoid and rod-shaped.The types of sediments distribute were paralleled with coastline and sediment in near-shore region is clay which composed with smaller particles while that in far off-shore region is sand which is composed with larger particles.MTB abundance was influenced by sediment type and sediment grain size and thus MTB abundance was higher at near-shore region and lower at far off-shore region.This is the first systematic report of spatial and temporal distribution and diversity of MTB in sediments of a continental shelf and propose that sediment grain size is the most important factor which influences MTB abundance and explain the mechanism.Knowledge of the distribution of MTB on the continental shelf of the Yellow Sea may serve as a general guideline to investigate the locations of MTB in seafloor habitats.