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Chapter 4 : Trying To Make Sense of the Microbial Census
Based upon observations with powerful telescopes, astronomers teach us that the Milky Way has more than 1012 stars and that 1012 similar-size galaxies account for the more than 1024 stars in the universe—roughly equivalent to the number of grains of sand on Earth. The number of microbes on Earth eclipses the number of stars in the universe by many orders of magnitude. And the collective biomass of single-cell organisms on Earth outweighs all of the plants and animals combined. Given the massive number of microbes with seemingly unlimited metabolic diversity, the accumulation of mutations during the past 3.5 billion years could have led to enormous numbers of distinct microbial populations that exhibit high levels of genetic diversity and phenotypic variation. Closely related taxa have nearly identical rRNA sequences, while microbes that diverged from each other hundreds of millions of years ago exhibit greater levels of nucleotide variation. It became possible to infer objective dichotomous evolutionary branching patterns that describe taxonomic relationships for cultured microbes. The expense of sequencing full-length rRNA genes has constrained most molecular inventories of microbial populations. The promise of discovering new phylotypes among the lower-abundance taxa fostered experimental designs where low-resolution procedures—e.g., restriction fragment length polymorphisms—identify putatively distinct clones for DNA sequencing. The combination of rapid gene isolation technology afforded by polymerase chain reaction (PCR) and automated DNA sequencing capabilities soon opened another window on microbial diversity.
Key Concept Ranking
- Restriction Fragment Length Polymorphism