Factors Influencing The Formation Of Iron Plaques On The Root Of Typical Coastal Wetland Plants And Their Ecological And Environmental Implications

Iron（Fe）plaque is a typical phenomenon on wetland plant root surfaces.Oxygen（O₂）and other oxidizing substances permeate from the roots,oxidizing divalent iron(Fe²⁺)to ferric iron(Fe³⁺)and precipitating Fe（hydr-）oxide on the wetland plant roots.The presence of Fe²⁺and sufficient O₂ concentration in the rhizosphere are both required to produce Fe plaques.Periodic flooding alters the redox potential（Eh）,p H,and relative concentration of Fe redox cycling bacteria in the sediment of different tidal flats,causing the valence states of Fe to shift between trivalence and bivalence.Many variables influence the formation of Fe plaque,which may be separated into abiotic and biological causes.So far,there are still few researches on the influence of comprehensive factors on the formation of Fe plaque.Besides,because of the discharge of industrial and domestic sewage in estuarine and coastal areas,a large amount of nitrogen（N）,phosphorus（P）,and heavy metal pollutants accumulate in coastal wetlands with tidal dynamics,having a significant impact on the health and stability of the coastal wetland ecosystem.The Fe plaques help the survival of plants in polluted environment by influencing the translocation of above elements.This paper investigated the effects of abiotic factors such as artificial sewage,flooding,and Fe²⁺concentration,as well as biological factors such as iron redox cycling bacteria,on the formation of Fe plaque on the surface of typical coastal wetland plants Spartina alterniflora（S.alterniflora）and Kandelia obovate（K.obovate）root,using indoor control experiments and field investigations.The impacts of Fe plaque development on the root surface of wetland plants on the absorption and transport of heavy metals（Cu,Zn,Pb,Cr）,N,P,and other nutrients in sewage,as well as the stress adaptation of wetland plants,were also investigated.The main outcomes are as follows:（1）Iron plaque aids in the immobilization of wastewater-borne pollutants,although both the development of Fe plaque and the immobilization of pollutants are regulated by flooding conditions.The concentration of Fe plaque on the roots of S.alterniflora increased as the synthetic wastewater level increased but declined dramatically as the flooding time increased.Even though synthetic wastewater treatments aid in the production of Fe plaques,the levels of major heavy metals（Cu,Pb,Cr）in Fe plaques have not shown a significant variation;the root system is the predominant pool for heavy metals.Iron plaque in the roots of S.alterniflora did absorb heavy metals such as Cu,Zn,and Cr,according to SEM-EDX examination.The buildup of heavy metals in Fe plaque varies with flooding time and is related to Fe plaque levels in roots.The increased use of synthetic wastewater causes an increase in P accumulation in the aerial portions of S.alterniflora.（2）The production of iron plaques on the root surface of S.alterniflora is facilitated by exogenous Fe²⁺input,which also increases plants’resistance to wastewater containing heavy metals.The concentration of Fe plaque in the roots of S.alterniflora increased when Fe²⁺levels increased.When the Fe²⁺concentration is low,amorphous Fe（hydr-）oxide dominates the Fe plaque,but crystalline Fe（hydr-）oxide dominates as the Fe²⁺concentration increases.Sewage stress reduced plant weight and chlorophyll content while increasing malonaldehyde（MDA）level.Plant weight and chlorophyll increased considerably to control levels as Fe plaque concentration increased,whereas peroxidation damage to S.alterniflora was reduced.Exogenous Fe²⁺supplementation decreased Cu,Zn,Pb,and Cr levels in S.alterniflora leaves and protected the plants against sewage stress.（3）The amount of Fe plaque formed in the roots of S.alterniflora and Phragmites australis（P.australis）decreased significantly with increasing soil Eh in the tidal flat,and the amount of Fe plaque formed in the roots of the two plants corresponded to the relative abundance of Fe redox cycling bacteria in the two plants’rhizospheres.Iron plaque concentrations（both amorphous and crystalline Fe（hydr-）oxides）in S.alterniflora and P.australis roots reduced dramatically as soil Eh increased.Regardless of tidal levels,the Fe plaque content of P.australis roots was much greater than that of S.alterniflora.The production of crystalline Fe plaque was aided by high soil Eh,and the fraction of crystalline Fe（hydr-）oxides increased.P.australis had considerably higher rhizosphere microbial community diversity than S.alterniflora with similar soil Eh.The relative abundance of the main Fe oxidizing bacteria（Fe OB）and Fe reducing bacteria（Fe RB）in the rhizosphere of P.australis,such as Desulfuromonas,Geothermonbacter,Pseudomonas,Paracoccus,Geobacter,Amaricoccus,Pelobacter,and Gallionella,was generally greater than that of S.alterniflora in the low tidal flat.However,the relative abundance of Fe RB like Desulfuromonas and Fe OB like Pseudomonas in the rhizosphere of two plants increased as soil Eh and total Fe increased,indicating that Fe redox cycling bacteria enrichment in the rhizosphere of S.alterniflora and P.australis is not the primary determinant of Fe plaque formation.（4）Inoculation of a Fe RB strain,Pseudomonas Sp.SCSWA09（P.Sp.SCSWA09）,significantly promoted the decomposition of Fe plaque on the root of K.obovata,increased the activity of antioxidant protective enzyme（POD）in K.obovata leaves,and increased chromium（Cr）translocation to the aboveground part of K.obovata.P.Sp.SCSWA09,a kind of Fe redox cycling bacterium,was isolated from the rhizosphere of K.obovata in this study.In the context of periodic floods and heavy metals,the bacterial strain demonstrated exceptional Fe reduction capabilities.Inoculation with P.Sp.SCSWA09 decreased the amount of Fe plaque on the root of K.obovata,particularly the amount of amorphous Fe plaque.Periodic flooding aids in the change of Fe oxide in the Fe plaque from amorphous to crystalline form,whereas P.Sp.SCSWA09 speeds this process.P.Sp.SCSWA09 exhibits plant growth promoting bacterial characteristics in the rhizosphere of K.obovata,which can considerably increase the activity of antioxidant enzymes（POD）in K.obovata leaves,however the presence of heavy metals weakens this effect.Inoculation with P.Sp.SCSWA09promoted the translocation of Cr to the aboveground part（leaves）of K.obovata seedlings.In conclusion,the production of Fe plaque is regulated by a combination of abiotic variables such as flooding conditions,Fe²⁺,and biotic factors such as Fe redox cycling bacteria.In addition,the Fe plaque generated on the root of wetland plants can improve plant stress tolerance to pollutants by immobilizing heavy metals（Cu,Zn,Pb,Cr）,N,and P.The findings have crucial implications for studying the biogeochemical processes of common heavy metals and nutrient components in estuarine and coastal intertidal zones,as well as wetland plants for heavy metal and other pollution ecological restoration.