| Pseudomonas aeruginosa is widely found in nature and frequently found in industrial water and seawater.P.aeruginosa has been proved to corrode commonly used steel materials like carbon steel,stainless steel,and even duplex stainless steel.Meanwhile,P.aeruginosa is considered a pioneer in biofilm formation,which consumes oxygen to provide an anaerobic environment for other anaerobic corrosive bacteria,such as sulfate reducing bacteria.More and more evidences show that P.aeruginosa is an important bacterium causing marine corrosion and leading to economic and environmental losses.However,the corrosion mechanism of P.aeruginosa is still unclear.It is of great theoretical value and practical significance to study the microbiogically influenced corrosion(MIC)of P.aeruginosa for enabling the satefy of industrial production.The MIC behavior of P.aeruginosa against X80 pipeline steel and 304 stainless steel was investigated by means of gene knockout technique from the perspective of the electron transfer between the metal surface and bacteria.In this study,we try to find the electron transfer mediators which may participate the MIC process of P.aeruginosa and determine the related genes controlling the synthesis of these electron transfer mediators.On this basis,we gamely make an attempt to sum up the specific electron transfer mechanism hiddened in the process of MIC process of P.aeruginosa.The main results are as follows:The addition of exogenous PYO into sterile medium does not change the surface state of X80 steel and 304 stainless steel.PYO itself could not accelerate the corrosion of two typical metal materials when the bacteria were absent.The pitting morphology observation and weight loss results suggested an accelerated corrosion of X80 steel coupons when exogenous PYO was added to the culture medium containing P.aeruginosa.Electrochemical results showed that P.aeruginosa biofilm promoted the anodic dissolution of X80 steel with the addition of PYO.The pitting corrosion of 304 stainless steel samples was aggravated with excess PYO.The electrochemical results also showed a negative shift of the breakdown potential Eb of 304 stainless steel sample under the action of external PYO.The P.aeruginosa biofilm made a fierce offensive against passive film with the help of exogenous PYO,which led to more serious pitting corrosion.In this study,the knockout of phzM and phzS genes had little effect on the growth of the corresponding mutant strains,which maintained a normal growth similar to the wild type strain.By knocking out phzM and phzS genes,the mutant strain was no longer capable of producing sufficient PYO.Deletion of these two genes lowered the corrosion rate of P.aeruginosa against X80 steel and 304 stainless steel as manifested by corrosion weight loss,pit statistics and electrochemical measurements.When exogenous PYO was added,the mutant strain recovered most of the ability to perform the extracellular electron transfer(EET)process,which confirming the crucial crucial role of phzM and phzS genes in regulating the secretion of PYO and further regulating the EET in MIC of X80 steel and 304 stainless steel.The phzM and phzS genes are only responsible for the regulation of the synthesis of PYO,but did not affect the ability of P.aeruginosa to use electron transfer mediators like exogenous PYO.The synthesis of phenazines in P.aeruginosa is in a dynamic procedure.By knocking out phzM and phzS genes,the mutant strain was no longer capable of producing sufficient PYO.However,the secretion of the other two phenazines phenazine-1-carboxylic acid(PCA)and phenazine-l-carboxylate(PCN)increased.When the synthesis pathway of PYO is inhibited,P.aeruginosa can up-regulate the expression of other phenozines such as PCN and participate in EET in MIC of P.aeruginosa,resulting in corrosion.In this study,we combined carbon starvation experiment and gene knockout technique and proved that P.aeruginosa could harvest electrons for intracellular respiration from the Fe matrix of X80 steel,which in turn accelerate the corrosion of X80 steel.Based on this,we proposed the EET mechanism in MIC of p.aeruginosa against steels in active state.In this mechanism,the active steel surface,acting as the electron donor,releases electrons and increases the amount of the reduced PYO.Electrons carried by the reduced PYO are transferred to the interior of cells through the electron transport chain and are accepted by the final electron acceptors intracellularly.In this inward EET process,P.aeruginosa bacteria served as a "biological cathode",and the whole cathode reaction takes place inside the cell.In this study,we also investigated the use of Fe oxides as electron acceptor with succinate as an electron donor and probed corrosion-enhancing mechanism of P.aeruginosa against 304 stainless steel under anaerobic condition.With enough NO3-in the culture medium,the presence of P.aeruginosa only caused mild pitting corrosion.Under reduced NO3-concentration,P.aeruginosa could use an outward EET pathway to transfer electrons to Fe oxides in the stainless steel extracellularly,which in turn facilitate the bioreductive dissolution of protective passive film of 304 stainless steel.Based on this,we proposed the EET mechanism in MIC of p.aeruginosa against staliness steel in passive state.In this mechanism,electrons donated by the oxidation of carbon source are transported through intracellular electron transport chain to reach the oxidized PYO,which convert PYO to reduced state.In the exterior of cells,electrons carried by the reduced PYO are transferred to the iron oxides in the passive film,which is the electron acceptor.Ferric ions were reduced to ferrous ions,and PYO returned to the oxidized state.In this outward EET process,P.aeruginosa bacteria served as a "biological anode" where microbial oxidation reaction takes place.P.aeruginosa have evolved an adaptive strategy,either by microbial oxidation or microbial reduction to corrode the surface they attach to. |
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