Why Are Some Microbes Corrosive And Some Not?

摘要

Biocorrosion is also known as microbial corrosion and microbiologically influenced (or induced) corrosion (MIC). Biofilms are responsible for MIC. At least three different types of MIC can be defined. Type Ⅰ MIC involves microbes such as sulfate reducing bacteria (SRB), nitrateitrite reducing bacteria (NRB) and methanogens, which are collectively called "XRB," in which "X" stands for sulfate, nitrate, nitrite, CO_2 or another non-oxygen oxidant and "B" for bugs that include prokaryotes, archaea and eucaryotes. These corrosive microbes respire on an oxidant to oxidize an organic carbon (or sometimes H_2) for energy in their normal metabolism. The organic carbon (or sometimes H_2) is an electron donor in their energy production. Some XRB biofilms will switch to elemental iron (Fe°) as an electron donor (or fuel) when there is local shortage of organic carbon underneath a biofilm. This type of attack is driven by the need for energy. Type Ⅰ MIC requires electrogenic microbes or microbes that are capable of utilizing electron carriers such as H_2 and formate, etc., because only these microbes can transport extracellular electrons released by iron oxidation to the cytoplasm inside the cells where reduction of an oxidant such as sulfate occurs under biocatalysis. Most microbes lack this ability and thus they cannot cause Type Ⅰ MIC directly. Type Ⅱ MIC typically involves fermentative microbes such as acid producing bacteria (APB) and the corrosion process itself caused by the secreted corrosive metabolites such as organic acids does not require biocatalysis. Type Ⅲ MIC involves microbes that secrete enzymes to degrade polymers (e.g., polyurethanes) and plasticizers in the polymers, and utilize the products as organic carbon and energy sources. This type of corrosion is better known as biodegradation. This work explained: (a) Why some biofilms are corrosive and some are not, (b) why some "non-corrosive" biofilms can suddenly become aggressive, (c) what roles non-corrosive sessile cells play in a syntrophic biofilm community that is corrosive, and (d) whether it is possible and practical to employ a "protective biofilm" to prevent MIC. Experimental work was carried out to verify that electron mediators flavin adenine dinucleotide (FAD) and riboflavin promoted MIC by Desulfovibrio vulgaris considerably because they enhanced electron transport without promoting cell growth.