Effects Of D-amino Acids And AI-2 On Biofilm And Corrosion Behavior Of Marine Microorganisms

As a very important form of corrosion,microbiologically influenced corrosion（MIC）plays an important role in the corrosion failure of marine metal engineering facilities.Biofilm is a key feature of MIC,and regulating the state of it can affect the MIC.The formation,development and disassembly of biofilms are strictly controlled by microorganisms,and these regulations are often achieved through the response of signal molecules.Therefore,studying the effect of signal molecules on biofilms will provide a basis for in-depth analysis of MIC mechanism.In this paper,D-amino acids and quorum sensing signal molecules are selected as typical signal molecules.By means of biofilm quantification and microstructure observation,the effects of D-amino acids and quorum sensing signal molecules on the biofilm formation of two microorganisms Pseudoalteromonas sp.SC2014（SC2014）and Desulfovibrio sp.（D.sp.）which are widely found in the corrosion product layer of the marine environment were studied.By studying the surface hydrophobicity of the bacteria,and the structure and compositional changes of biofilms,the influence mechanism of signal molecules on biofilms is revealed.In combination with the corrosion weight loss method,the effect of signal molecules on the corrosion of metals caused by bacteria was explored,and the correlation between signal molecules,biofilms and corrosion of the metals was established.Main results are shown as follows:（1）The specificity and effect of D-amino acids on biofilms of SC2014 and D.sp.were revealed.Among six D-amino acids（D-Leu,D-Met,D-Pro,D-Trp,D-Tyr and D-Phe）,only D-Phe could inhibit the formation of biofilms without affecting the growth of bacteria,but its influences on the development of biofilms formed by these two bacteria were different.For SC2014,D-Phe had no significant effect on the initial adhesion of bacteria to the surface of 316L stainless steel,and had no significant effect on the monolayer biofilms left after the disassembly of the mature biofilms,and did not affect the interaction between the bacteria and the metal.However,D-Phe could inhibit the development of biofilms from a monolayer structure into a mature multilayer structure,and disintegrate the mature biofilms.D-Phe could inhibit and break the interaction between bacteria.For D.sp.,D-Phe not only inhibited the initial adhesion of bacteria to the surface of 316L stainless steel,but also prevented the further development of biofilms.D-Phe not only hindered the interaction between bacteria and metals,but also affected the interaction between bacteria.（2）The effect mechanism of D-Phe on SC2014 and D.sp.biofilms were investigated.Effects of D-Phe on the surface properties of bacteria and the composition and structure of biofilms were discussed.D-Phe could change the surface hydrophobicity of SC2014 from 27.44±1.35oto 34.21±2.79o,by changing the type and quantity of the surface composition of bacteria,and inhibiting the aggregation of extracellular polymetic substances（EPS）on the surface of the metal to influence the interaction between bacteria.Thereby,bacteria could only form a monolayer biofilm on the surface of 316L stainless steel,and could not form a mature multilayer biofilm.For D.sp.,on the one hand,D-Phe altered the shape of the bacteria from rod to inhomogeneous fibrous or spherical,and affected the type or amount of bacterial surface components.On the other hand,D-Phe inhibited the aggregation of bacteria and EPS on the surface of the metal significantly.Consequently,the structure of biofilms formed on the surface of Q235 carbon steel by D.sp.changed from mushroom shapes with uneven distribution to network structure with relative uniform distribution,and the relative layering of the distribution of biofilm components（proteins on the most outside,and bacteria,lipids and polysaccharides on the inside）disappears.（3）The correlation between corrosion of metals and the state of biofilms was revealed.For SC2014,although D-Phe could inhibit the formation of biofilms on the surface of Q235 carbon steel,it had no significant effect on the weight loss of the metal caused by bacteria.For D.sp.,D-Phe could reduce the weight loss of Q235carbon steel from 0.844±0.076 mg/cm² to 0.552±0.048 mg/cm²,and made corrosions pits with maximum depth of 5μm,width of 41.5μm and density of 251±118 mm^-2 decrease.D-Phe inhibited the weight loss of the metal caused by D.sp.by inhibiting the formation of biofilms on the surface of Q235 carbon steel,and the uniformity of the structure and distribution of biofilms made the corrosion of the metal more homogeneous.（4）The quorum sensing systems of SC2014 and D.sp.were determined,and the correlation between the quorum sensing system,biofilm and corrosion of metals was analyzed.SC2014 had LuxR/Lux I quorum sensing system,and did not have LuxS/AI-2 quorum sensing system.Signals C6-AHL and AI-2 had no effect on its biofilm formation and growth.D.sp.did not have LuxR/Lux I quorum sensing system which could be detected by the reporter strain,but had LuxS/AI-2 quorum sensing system.AI-2 inhibited the formation of biofilms on the surface of 316L stainless steel by inhibiting the aggregation of bacteria,polysaccharides and lipids,and mainly inhibited the later development of biofilms.However,AI-2 had no effect on the weight loss of Q235 carbon steel caused by D.sp.,and biofilms formed on the surface of it by the bacteria.