Single Substrates Several Organisms

One of the organisms fulfills the need for a growth requirement by the other, for example, vitamin requirements of one organism that is provided by the other. Examples are provided by biotin in cocultures of Methylocystis sp. and Xanthobacter sp. (Lidstrom-O Connor et al. 1983), and thiamin in cocultures of Pseudomonas aeruginosa and an undefined Pseudomonas sp. that degraded the phosphonate herbicide glyphosate (Moore et al. 1983). 

H0-(CH2-CH20) -CH2-CH2-0-CH2-CH20H H0-(CH2-CH20) -CH2-CH2-0 -CH2-CO2H H0-(CH2-CH20) .CH2-CH2-0H + HO-CH2-CO2H 

FIGURE 4.1 Degradation of parathion by a mixed culture of two pseudomonads. 

Stable metabolic associations generally between pairs of anaerobic bacteria have been termed syntrophs, and these are effective in degrading a number of aliphatic carboxylic acids or benzoate under anaerobic conditions. These reactions have been discussed in reviews (Schink 1991, 1997 Lowe et al. 1993) that provide lucid accounts of the role of syntrophs in the degradation of complex organic matter. Two examples are given here to illustrate the experimental intricacy of the problems besetting the study of syntrophic metabolism under anaerobic conditions  

Pnre cnltnres of organisms that can oxidize propionate either in the presence of a methanogen or nsing snlfate as electron acceptor have been obtained. These include both Syntrophobacter wolinii and Syntrophobacter pfenigii (Wallrabenstein et al. 1995). The interaction of two organisms, therefore, is clearly not obligatory for the ability to degrade these carboxylic acids under anaerobic conditions. 

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Both a proton and a metal ion can introduce a positive charge into a substrate molecule and effect the electronic changes listed above. A metal ion, however, is superior to a proton on several grounds. A metal ion can introduce a multiple positive charge into an organic molecule, whereas a proton can introduce only a single positive charge. Furthermore, a metal ion can operate in neutral solution, whereas a proton cannot. Finally, a metal ion can coordinate several donor atoms, whereas a proton can coordinate with only one donor atom. 

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