their complement is brief. As complexity increases, the time it takes for complementary strands to reanneal increases. Experimentally, this is referred to as a Cot curve, where Co is the initial molar concentration of nucleotides in single-stranded DNA and t is time. This measure reflects both the total amount of information in the system (richness or number of unique genomes) and the distribution of that information (evenness or the relative abundance of each unique genome) (Liu and Stahl, 2002), thus making it among the more robust methods for estimating extant diversity in a given sample. Yet, it provides no information on identity or function of any member of the microbial community.
The genetic complexity or genome size of several soil microbial communities was assessed using reassociation kinetics by Torsvik et at. (1990; 1998). Using this procedure, they estimated that the community genome size in undisturbed organic soils was equivalent to 6000-10,000 E. coti genomes. In comparison, a heavy-metal-polluted soil contained 350-1500 genome equivalents. Culturing produced fewer than 40 genome equivalents. Such experiments, and those employing direct counts using epifluorescence microscopy, are what substantiate the lack of abundance and diversity commonly observed using culturing methods. The data from the above study were reanalyzed using improved analytical methods, which yielded an estimate of the extant diversity contained in the undisturbed soil sample of 8.3 million distinct genomes in 30 g of soil, two orders of magnitude greater than originally derived (Gans et at., 2005). In contrast, the heavy-metal-polluted soil was estimated to contain only 7900 genome equivalents, 99.9% fewer than in the undisturbed soil.
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