Microabial Methanogenesis In Response To The Ferrihydrite Biomineralization In The Presence Of Phosphate

Iron oxides are the important minerals in soils and sediments,and they are diversity in types,such as poorly crystalline Fe(OH)3 and ferrihydrite,or highly crystalline goethite,hematite and magnetite.Previous studies have demonstrated that different iron oxides show differnt effects on microbial methanogenesis under anaerobic conditions.Poorly crystalline Fe(?)oxides,such as ferrihydrite,has inhibitory effect on microbial methanogenesis,whereas conductive iron oxides of hematite and magnetite were reported to accelerate the conversion of organic matter to methane.With time,the transformation of iron oxides under natural environments always follows the direction from thermodynamically unstable to thermodynamically stable.This implies that the changes in iron mineralogy caused by biomineralization may consequently affect the correlation between iron oxides and methanogenesis.To date,the effects of changes in iron mineralogy on methanogenesis have not been characterized.Using paddy soil in South China,this study first investigated whether a change in iron mineralogy influences the methanogenesis pathway,and the different ferrihydrite biomineralization pathways are regulated by the modulation of phosphate levels.Dynamic of methane production and microbial community structure were analyzed to explore the effect of different ferrihydrite biomineralization pathways on methanogensis and methanogens.Using the typcial Fe(?)-reducing bacteria and methanogens,the co-cultures of Geobacter sulfurreducens/Methanosarcina barkeri and Geobacter metallireducens/Methanosarcina barkeri grown in a medium undergoing different ferrihydrite biomineralization were developed to investigate the response of methanogenic activity to the transformation of iron oxides.These results would provide direct evidence for the correlation between secondary iron minerals and methanogenesis.The main conclusions are listed as follows:1.The level of phosphate significantly affected ferrihydrite mineralization pathway and the production of secondary iron minerals.In the absence of phosphate,the dissimilatory Fe(?)reduction resulted in the transformation of ferrihydrite into magnetite.The formation of magnetite accelerated the rates of methanogenesis from acetate and propionate by 27.4 % and 25.8 %,respectively,compared with a control lacking ferrihydrite.The facilitated effect was proposed that magnetite,serving as electron conduit,stimulated the direct interspecies electron transfer(DIET)between syntrophic acetate-oxidizing bacteria(Clostridiaceae)/syntrophic propionate-oxidizing bacteria(Anaerolineaceae)and Methanosarcinaceae.Due to the much higher effectiveness of DIET than that of traditional hydrogen/formate interspecies electron transfer,DIET-mediated syntrophic methanogenesis was greatly accelerated.However,in the presence of phosphate,ferrihydrite was transformed into the non-conductive vivianite which has insignificant impact on methanogenesis from both acetate and propionate.2.The findings in the anaerobic paddy soil incubation was consistently observed in the co-cultures experiments.For both G.sulfurreducens-cultures(acetate)and G.metallireducens-cultures(ethanol),Fe(?)reduction resulted in the conversion of ferrihdyrite to magnetite in the absence of phosphate,whereas vivianite was the main secondary minerals of the ferrihydrite reduction in the presence of phosphate.Results showed that conductive magnetite accelerated methanogenesis from acetate in the co-cultures of Geobacter sulfurreducens/Methanosarcina barkeri and methanogenesis from ethanol in the co-cultures of Geobacter metallireducens/Methanosarcina barkeri.We proposed that the nanoparticles magnetite formed by ferrihydrite biomineralization can compensate for the electron transfer function of OmcS in promoting electrical contacts with pili,thus facilitated DIET-mediated methanogenesis.However,the non-conductive vivianite,the secondary minerals of ferrihydrite biomineralization in the presence of phosphate,has insignificant effect on syntrophic methanogenesis in the co-cultures.The correlation between magnetite formation and facilitated methanogenesis was further evidenced by the acceleration of methanogenesis from acetate and ethanol by the addition of magnetite in the defined co-cultures.The results observed here have implications for our understanding of the biogeochemical cycling of iron and carbon,particularly in iron-rich natural and agricultural wetland soils.This would be helpful for the practical applications of iron oxides in greenhouse gases mitigation and bioenergy processes.