PEDRO J. ALVAREZ: In our previous lecture, we learned that the metabolism of pollutants often involves energy yielding redox reaction. So it is important that we learn more about electron donors an electron acceptors. Let us start by reviewing how bacteria function, which is very similar to what combustion engine in regards to redox processes. In the sense that bacteria needs fuel, which is an electron donor-- and here, we're using the common groundwater pollutant benzene, in this example-- and they also need an electron acceptor, such as oxygen, which is present in aerobic environments. So electrons are transferred from the electron donor to the electron acceptor and eventuality mineralization can occur, which refracts to the oxidation of the target pollutant all the way to carbon dioxide and water. And the electrons are transferred to oxygen through a series of electron carriers located in the cell membrane. During this electron transfer process, protons are extruded to create the proton motive force that is eventually used to produce chemical energy stored in the form of ATP. And this produced energy can be used for cell growth and to do work, such as to move the flagella. This process is not 100% efficient. And about 30% to 40% of the available energy is often lost as heat. Sometimes, the target pollutant can be present in a relatively oxidized state, as is the case of trichloroethylene in this example. And instead of serving as electron donor, it can serve as electron acceptor in this redox process. So it is important to remember, as we learned in the last class, that contaminants can serve as electron donors, as is the case of benzene and hydrogen, which have a very negative reduction potentials, or they can serve as electron acceptors, as is the case of perchloroethylene or nitrate, which have positive reduction potentials. And this will dictate the thermodynamic feasibility of electron transfer. Here, we can see that the energetics of perchloroethylene reduction to trichloroethylene using hydrogen as an electron donor is more favorable than its oxidation by oxygen, as indicated by the larger red and blue lines, which are essentially the differences in the redox potential between the electron donor and the electron acceptor. Now, hydrogen is an excellent electron donor for reductive dechlorination. And it is produced by the fermentation of a wide variety of organic compounds in anaerobic environments. And other fermentation product, such as acetate, can also serve as electrons donor. But in both cases, there can be competition for hydrogen or for acetate by methanogenic bacteria and by other microorganisms. And if they divert those electrons donors to other nonproductive pathways, that would hinder dechlorination. Note that chlorinated solvents tend to be relatively favorable electron acceptor. Certainly, there are more favorable electron acceptors than sulfate. However, other electron acceptors, such as ferric iron oxides and nitrate that might be present at the contaminated site might out-compete the chlorinated solvents for these electrons and, therefore, hinder the reductive dechlorination process. Switching now to hydrocarbon-impacted aquifers, here the electron acceptors are often used in sequence in the order of their oxidation potential. So first, oxygen is used up until it is depleted. And then the organisms that are capable of using the next best electron acceptor with the highest oxidation potential, which in this case is nitrate, they will have a competitive advantage and proliferate and prevail until nitrate is depleted. And then in turn, manganese reducers may take over, and in sequence, that will be after maybe the iron reducers, and the sulfate reducers, until the only electron acceptor left is carbon dioxide, whose reduction generates methane. So contamination by gasoline and other fuel spills often result in these geochemical transitions shown here, where you may find strong anaerobic methanogenic conditions nearer the source zone, where the biochemical oxygen demand and the electron acceptor demand exerted by the pollutants is very high. And the preferred electron acceptors are depleted. And then you will transition eventually to aerobic processes near the fringes of the plume, where the contaminants are present in much lower concentrations. So the hydrocarbon of greatest concern in fuel spills is benzene, because of its carcinogenic potential. And benzene could be degraded through most of these redox zones. Aerobic degradation near the fringes of the plume will certainly be the most favorable going all the way to methanogenic conditions near the source zone, where the thermodynamic feasibility and rates of degradation would be much, much slower. So the key points of this lecture are that thermodynamic considerations will dictate whether microorganisms will utilize the target pollutant as electron donor or electron acceptor. Furthermore, hydrocarbons are excellent electron donors. And they degrade very well under aerobic conditions generally. And finally, chlorinated compounds can be favorable electron acceptors and degrade very well under anaerobic conditions provided that suitable electron donors, such as hydrogen that is produced by fermentation reactions, is present to promote reductive dechlorination.
