hydrogenation of double bonds in the side chain (in parentheses). For example, UQ-10 stands for a ubiquinone with 10 isoprene units in its side chain and MK-8(H2) represents menaquinone with 8 isoprenoid units, of which one double bond is hydrogenated with 2 atoms of hydrogen. Most microorganisms contain only one type of quinone as their major quinone, which remains unchanged by physiological conditions. The half-lives of those quinones released by dead soil microorganisms are very short (in the range of several days). The diversity of quinone molecules can be interpreted directly as an indicator of microbial diversity. About 15-25 types of quinones are detected in most soils. The distribution of the most important marker quinones within several groups of organisms is given in Table 3.6. Many bacteria contain Q-8, Q-9, and Q-10. bacteria containing Q-8 are mostly classified in the (3 subgroup of the proteobacteria and some in the y subgroup. G~ bacteria in the Cytophaga-Flavobacterium complex contain MK-6 and MK-7. G+ bacteria contain only menaquinones. The high mole ratios of mena-quinones to ubiquinones in soils support earlier results using culturing and isolation techniques that showed that G+ bacteria dominate in soil. As G+ bacteria have a wide variety of menaquinones, the sensitivity of quinone profiles to changes in the microbial community — especially with respect to G+ bacteria in soil — is higher than the sensitivity of PLFA profiles, which detect G+ bacteria based on the presence of only one signature compound (branched ester-linked fatty acids).

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