Bacteria in Soil III

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     The diversity of species in bacterial communities is often studied by phenotypic characterization. A problem with this method is that phenotypic methods can be used only on bacteria which can be isolated and cultured, and most soil bacteria that have been observed by fluorescence microscope cannot be isolated and cultured.
     DNA can be isolated from bacteria in soil to obtain genetic information about the nonculturable bacteria therein. The heterogeneity of this DNA is a measure of the total number of genetically different bacteria, or the number of species. DNA heterogeneity can be determined by thermal denaturation and reassociation. In general, renaturation of homologous single-stranded DNA follows second-order reaction kinetics. In other words, the fraction of DNA that has renatured within a given time period is proportional to the genome size or the complexity of DNA, defined as the number of nucleotides in the DNA of a haploid cell, without repetitive DNA. The genetic diversity of a bacterial community can be inferred in a similar manner.
     Vigdis Torsvik, Jostein Goksøyr, and Frida Lise Daae used this process to analyze soil samples taken from the soil from a beech forest north of Bergen, Norway. The reassociation curves for the main DNA fraction did not follow ideal second-order reaction kinetics, so the half-life values gave only approximate, underestimated values for the number of genomes present. Nevertheless, the soil bacterium DNA was very heterogeneous; the diversity corresponded to about 4,000 distinct genomes of a size typical of standard soil bacteria. This diversity was about 200 times as many species as could have been isolated and cultured.
     Various procedures for isolating DNA from river sediments and seawater are known. This opens up the possibility of applying the thermal denaturation method to systems other than soil. The results of the Norway study indicated that the genetic diversity of the total bacterial community in a deciduous-forest soil is so high that heterogeneity can be determined only approximately. In environments with pollution or extreme conditions, the genetic diversity might be easier to determine precisely.                

With which of the following statements would the author of the passage most likely agree?

Review: Bacteria in Soil III


Explanation

This question asks us to make an inference without giving us any detail or subtopic to latch onto, so we can proceed by keeping the author's main idea in mind. The correct answer may be a general statement or a specific one, but it will be definitely supported and maybe even required by the passage. But then, looking at the answer choices, we can see that they are all about "isolating and culturing specific bacteria." That's the method that is not thermal denaturation and reassociation. What does the author think about this method? He definitely thinks that it's not as good at counting bacterial species in soil; that's the point of the passage. We can start there. On those grounds, choice (C) is quickly out. Choice (B) is too strong in the other direction; the author hasn't discussed isolating and culturing bacteria extensively enough to discredit it so thoroughly. So (B) and (C) are out. Choices (D) and (E) both involve the idea that isolating and culturing can be used to give an estimate of the total number of species. We know that the thermal method is superior; does the passage say that the isolating and culturing method can give an estimate? It yielded an estimate that was 200 times lower than the actual number, but that means that the thermal method did not "confirm" it, so (D) is out, and there is no support for (E). That leaves us with (A). Indeed, the first clause of (A) is the author's main point. The second clause of (A) can be faintly inferred from the fact that the thermal method does not allow the examination of individual species, only a count of the species, while culturing something involves isolating it and growing it, in which case direct study would be possible.

The correct answer is (A).


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