believed to be the common ancestors of all organisms [ http://instruct.uwo.ca/biology/284/intro.html ]. Microbes not only grow virtually everywhere
but also are present in abundance. In contrast to the relatively small number of
humans (6 x 109), populations of
terrestrial and marine bacteria are immense, 5 x 1030 and 1.2 x 1029,
respectively. In fact, the human body contains ten times more bacterial
cells than human cells. Microbes carry out innumerable transformations of matter
that are essential to life and thus have an enormous effect on climate and the
geosphere. Much of our knowledge base in biology and molecular biology has
derived from microbial studies.
scientists realized in the early 1990’s that that only a small fraction (1%) of
microbes in natural communities was known, these communities became the focus of
many studies. Several years later, however, it is still shocking to realize the
depth of our ignorance, which is well illustrated by the story of the SAR11
clade. When the technique of ribotyping (cloning and sequencing 16S rRNA genes)
was first used to survey natural ecosystems, SAR 11 was one of the first groups
of novel microbes to be discovered (Giovannoni et. al., 1990). We now know that
the highest percentage of bacterial 16S ribosomal genes present in all oceanic
and coastal waters are from members of the SAR11 clade, making this group “one
of the most successful clades of organisms on the planet” (Morris et al, 2002;
Giovannoni et. al., 2005). In some waters, SAR11 comprise up to 50% of the total
surface bacterial community and approximately 25% of the sub-euphotic zones. On
average, members of the SAR11 clade account for about one third of all cells in
surface waters. Both the great abundance and global distribution of SAR11
suggests that its members are instrumental in metabolizing oceanic dissolved
organic matter (DOM), but the specific roles of these bacteria in biogeochemical
cycles is still unknown (Malmstrom et al., 2004; Malmstrom et al, 2005).
Although most members of the SAR11 clade remain uncultured, a few have recently
been cultivated in the laboratory in seawater, though growth in synthetic medium
remains elusive. As a result of several subsequent studies, SAR 11 was placed
of Proteobacteria. However, members of the SAR11 group show less than 82%
sequence similarity to cultivated
(Rappe et al., 2002.). One SAR11 isolate,
Pelagibacter ubique. has the smallest genome (1.3 x 106 base
pairs) of any known free-living cell in nature capable of independent
replication (Rappe et al., 2002; Giovannoni et al. 2005). Although surprisingly
the small P. ubique genome encodes
almost all basic functions characteristic of a-Proteobacteria, this genome
contains little, if any, nonfunctional or redundant DNA and very short
intergenic DNA regions, averaging only three bases in length (Giovannoni et al.
2005). It seems certain that many more surprises await from future studies of
The basis for classification by ribotyping is the sequence of the 16S ribosomal
RNA gene in prokaryotes and the 18S gene in eukaryotes. The 16S and 18S rRNA
genes were selected for classification and identification of microbes because
these genes are universal and essential; all living organisms must synthesize
proteins to survive (Woese and Fox, 1977). These genes are also well suited for
this purpose because they contain both conserved and variable regions, as is
evident in the nucleotide sequence of the 16S gene shown in the Figure on the
following pages. Sequences that are highly conserved are shown in brown,
conserved regions are red, variable regions are black, highly variable sequences
are blue and sequences that are > 75% variable are green. The map locations of
some common PCR primers are also shown in the Figure, which was adapted from
Baker et al, 2003. Conserved sequences are found in the 16S genes of all members
of a domain, bacteria or archaea, and are used as universal PCR primers. In
contrast, variable sequences are characteristic of certain genera, species or
strains and are useful as more specific PCR primers.
Baker, G.C., Smith, J.J., and D.A. Cowan. 2003. “Review and analysis of domain-specific 16S primers.” J. Microbiol. Meth. 55: 541 – 555.
Giovannoni, S.J., Britschgi, T.B., Moyer, C.L. and K.G. Field. 1990. “Genetic diversity in Sargasso Sea bacterioplankton.” Nature 345: 60 – 63.
Giovannoni, S.J., Tripp, H.J., Givan, S., Podar, M., Vergin, K.L., Baptista, D., Bibbs, L., Eads, J., Richardson, T.H., Noordewier, M., Rappe, M.S., Short, J.M., Carrington, J.C., and E.J. Mathur. 2005. “Genome streamlining in a cosmopolitan oceanic bacterium.” Science: 309: 1242 – 1245.
Malmstrom, R.R., Kiene, R.P., Cottrell, M. T., and D. L. Kirchman. 2004. “Contribution of SAR11 bacteria to dissolved dimethysulfonioproprionate and amino acid uptake in the North Atlantic Ocean.” Appl. Environ. Microbiol. 70: 4129 - 4135.
Malmstrom, R.R., Cottrell, M. T., Elifantz, H., and D. L. Kirchman. 2005. “Biomass production and assimilation of dissolved organic matter by SAR11 bacteria in the northwest Atlantic Ocean.” Appl. Environ. Microbiol. 71: 2979 - 2986.
Morris, R.M., Rappe, M.S., Connn, S.A., Vergin, K.L., Siebold, W.A., Carlson, C.A., and S.J. Giovannoni. 2002. “SAR11 clase domoinates ocean surface bacterioplankton communities.” Nature 420: 806 – 810.
Rappe, M.S. Connon, S.A., Vergin, K.L., and S.J. Giovannoni. 2002. “Cultivation of the ubiquitous SAR11 marine bacterioplankton clade.” Nature 418: 630 – 633
Woese, C. R. and G. E. Fox. 1977. “Phylogenetic Structure of the Prokaryotic Domain: The Primary Kingdoms.” Proc. Natl. Acad. Sci. USA. 74: 5088 - 5090.
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