Microbial Effects On Magnetite Formation And Modification And Its Geological And Environmental Significance

Iron（Fe）is a major element on Earth and plays important roles in the global material cycle and life activities.Nature Fe is often present in the form of iron-oxides（hydroxides）minerals.Among them,magnetite[Fe³⁺(Fe²⁺Fe³⁺)O₄]is one of the most common iron oxide minerals,which may contain significant information about paleoenvironment and microbial activities due to their unique crystal structure,variable redox state,and excellent magnetic properties.The coevolution between microbe and magnetite is a common phenomenon in earth surface.Firstly,microbes may participate in the formation of magnetite.Compared with inorganic magnetite,microbial magnetite contains more paleoenvironmental information.However,accurately distinguishing between the inorganic and microbial origin of magnetite is an urgent issue for paleoenvironmental reconstruction utilizing magnetite.Secondly,microbes can play important roles in magnetite modification,which could rewrite the original environmental signature of magnetite.The above processes often be neglected in paleoenvironment research.Finally,magnetite may influence microbial metabolism.Unfortunately,there is still unclarified on the role of this process in modern and early Earth.The above problems greatly limit the application of magnetite in paleoenvironmental research,as well as the understanding of“microbe-magnetite interactions”in the evolution of earth environment.In this study,a series of anaerobic experiments are conducted to investigate the roles of bacteria on the formation and redox modification of magnetite,aiming to providing multiple identification indicators of microbial effects in natural samples.Meanwhile,the influence of magnetite on microbial nitrogen（N）metabolic pathways is explored and its environmental significance is also discussed.The main insights are obtained as follows:Ⅰ.Microbial magnetite is characterized by poorer crystallization,smaller particle size,and narrower particle size range,which could be regarded as its identification markers in sedimentary systems.Ferrihydrite and green rust are selected as the precursors in magnetite synthesis via inorganic and microbial routes,respectively,under 10～50℃.The inorganic reduction products of ferrihydrite are goethite and magnetite[including superparamagnetic（SP）,single domain（SD）,pseudo single domain（PSD）and multi domain（MD）particles],while the inorganic oxidation products of green rust are mainly SD and SP magnetite.In contrast,microbial products exclusively consist of magnetite SP particles with poorer crystallization,smaller size,and more uniform particle size.In addition,high temperature favors the increase in magnetite yield and median particle size of SP particles.Ⅱ.Magnetite can be anaerobic modified by various bacteria effects（reduction,oxidation,and redox cycling）,which suggests that its original paleoenvironmental signatures may be potentially rewritten during diagenesis.In microbial reduction,a dissimilatory iron-reducing bacterium Shewanella oneidensis MR-1 is utilized for structural iron reduction of magnetite.All magnetites can act as electron acceptors for the bacteria.The magnetite with smaller particle size,poorer crystallinity,and lower surface oxidation is observed with higher bioavailability.After bio-reduced,the Fe（II）proportion,lattice parameters,magnetic susceptibility,and remanent magnetic loss in the Verwey interval of the magnetite increase significantly,but the remanent magnetic recording capacity and coercivity decrease（i.e.,the magnetism become“softer”）.In microbial oxidation,magnetites are modified in incomplete and normal denitrification systems using a denitrifying bacterium Pseudogulbenkiania ferrooxidans2002.Bio-oxidation of magnetites are found to be occurred in both systems,as evidenced by the increases in Fe（II）proportion and lattice parameters,as well as the“hardening”of magnetic properties,i.e.,appearance the shift to the“magnetite-magnetite solid solution”sequences.In microbial redox cycling,a mixture of S.oneidensis MR-1 and P.ferrooxidans2002 is used to anaerobic modified of magnetite.We found magnetite can act as a“mineral battery”for bacteria,being cyclically reduced and oxidized,with its magnetic susceptibility and dissolved Fe cycling fluctuations.Bacterial organic matter may impede the electron transfer between magnetite and bacteria,decreasing or even stalling the redox rate.Ⅲ.Magnetite can influence microbial nitrogen metabolism,which could not only increase nitrous oxide（N₂O）yield in denitrification,but also transform microbial nitrate reduction from denitrification to dissimilatory nitrate reduction to ammonium（DNRA）.These findings are significant for the nitrogen cycle in iron rich water bodies.In the incomplete denitrification system with P.ferrooxidans 2002-mediated and metallo coenzymes-limited,the absence of metallo coenzymes in magnetite free group stall the denitrification mainly in the nitrite（NO₂^-）phase,with minute N₂O production.However,the N₂O yields are remarkably enhanced with the amendment of magnetite.In addition,magnetite with a higher surface oxidation may alleviate the limitation of metallo coenzymes,leading to the increase of NO₂^-consumption and N₂O yield.In the normal denitrification system mediated by P.ferrooxidans 2002,nitrogenous gas（N₂O/N₂）is the major product of microbial nitrate reduction in the absence of magnetite,and minute amounts of ammonium（NH₄⁺,～6%）can be detected.However,the NH₄⁺yields are remarkably enhanced with the amendment of magnetite（up to 36.6%）,which especially true for the bio-systems with smaller magnetite crystal sizes and lower acetate concentrations.