Distribution Patterns And Biological Characteristics Of Magnetotactic Bacteria In Different Marine Habitats

Magnetotactic bacteria（MTB）are unique in their ability to synthesize intracellular nano-sized minerals of magnetite and/or greigite magnetosomes for aligning and swimming along geomagnetic field lines.Thus,they provide an excellent model for investigating the mechanisms of biomineralization,paleomagnetism,phylogenetics,microbial metabolism and microbial ecology.For a long time,studies of MTB focused on their biological and phylogenetic characteristics,as well as the biomineralization and application of magnetosomes and physiological metabolism of the cultivated strains.There is few study on the long-term ecological investigation of the uncultivated MTB.Most of the known MTB are found from sediments of freshwater or coastal environments.There is few report on the MTB of the deep ocean.To understand the diverse distribution patterns of MTB in different ecosystems,we carried on the research of seasonal changes in vertical distribution of multicellular magnetotactic prokaryotes（MMPs）and the multistation distribution of seamount MTB.In addition,we identified some environmental factors related to the distribution patterns of these organisms and try to gain insight into the physiological mechanism.These findings extend our knowledge of the distribution patterns and biological characteristics of MTB.MMPs are gram-negative cell aggregates that are unique because of the synchronized cell division and the coordination in complex movements.There are two genetically distinct morphological types of multicellular magnetotactic prokaryotes（MMPs）in intertidal zone of Lake Yuehu（China）:ellipsoidal MMPs（eMMPs）and spherical MMPs（sMMPs）.We studied the vertical distribution of both types of MMPs in the sediment of Lake Yuehu during one year.Both types of MMPs were observed at sediment depths ranging from 1 to 34 cm,depending on the seasons.Analysis by depth and season indicated that the maximum abundance of eMMPs was 148 individuals per cm³（ind/cm³）and the maximum abundance of sMMPs was 182 ind/cm³.There were seasonal variations in the vertical distribution of both eMMPs and sMMPs.The eMMPs distributed at depths of 2-34 cm during spring,1-11 cm during summer,2-21 cm during autumn,and 9-32 cm during winter.During spring,sMMPs were present at depths of 3to 27 cm;during summer,1 to 18 cm;during autumn,1 to 21 cm;and during winter,most sMMPs were at depths of 10 to 30 cm.The eMMP species Candidatus Magnetananas rongchenensis with magnetite magnetosomes dominated at all distribution depths.These results suggested that Cand.M.rongchenensis migrated vertically during four seasons.The sMMP population varied during four seasons.The same dominant species occurred in different sediment depths in four seasons.It suggested that sMMP population also migrated vertically and there were also seasonal replacement in s MMP population.The vertical distribution of oxidation-reduction potential（ORP）in Lake Yuehu changed seasonally.These changes coincided with the seasonal distribution of MMPs,suggesting that ORP affected the vertical distribution of MMPs.Both types of MMPs preferred low concentration of inorganic nutrients.In addition,high concentrations of ammonium and silicate might interfere with the vertical migration of MMPs.Therefore,the seasonal vertical distribution of MMP was closely related to the seasonal variation of environmental factors,which reflacted the unique adaption of MMPs among MTB.Here we report the first systematic detection of live MTB in deep sea seamount habitat.MTB distributed in sediments at all 14 stations ranging in depth from 238 to2,023 m in a seamount（Mariana M2）of the Mariana volcanic arc,in the western Pacific Ocean.Cocci of approximately 2.3μm diameter were the dominant MTB present,although rod-shaped and spiral-shaped MTB were occasionally observed.The abundance of MTB in the sediments rangeed from 7 to 226 ind/cm³.High abundance of MTB were found at sampling sites having a low dissolved oxygen concentration（approximately 2 mg/L）in the overlying water.MTB here had high diversity and high species-specificity.Phylogenetic analysis based of 16S rRNA gene sequences identified 20 MTB OTUs affiliated to the Alphaproteobacteria.These appeared to represent 14 novel genus and 16 novel species and three known species.Transmission electron microscopy（TEM）revealed cubo-octahedral and prismatic magnetosomes in magnetotactic cocci,bullet-shaped magnetosomes in magnetotactic rod and irregular magnetosomes in magnetotactic vibriod cells.All of the magnetosomes are magnetite particles with Fe₃O₄ in nature.A novel flagellar apparatus,comprising one or two bundles of 19 flagella arranged in a 3:4:5:4:3 array,was observed in the magnetotactic cocci,and was the most complex yet reported.This architecture might have evolved as an adaptation to the unique seamount habitat.This study extended the known occurrence of magnetotactic bacteria to seamount ecosystems.Taken together,we studied the distribution and biological characteristics of MTB in two different Marine habitats:intertidal coastal environment and deep-sea seamount environment.Our data indicated the seasonal migration of eMMPs and sMMPs.Additionally,for the first time,a systematic analysis of the diversity and distribution of MTB in a seamout ecosystem was reported.These studies provided a novel perspective for understanding the biological characteristics of MMPs,and also paved the way for the research on the biological characteristics of seamount MTB.