Archaea branch into two groups, Crenarchaeota and Euryarchaeota, initially
distinguished by differences in their SSU rRNA and protein trees. Further
comparative genomics support the profound divergence between Crenarchaeota and
Euryarchaeota, since a number of genes present in euryarchaeal genomes are missing
altogether in crenarchaeal ones and vice versa. These differences are not trivial, and
suggest different strategies for cellular processes such as maintenance of chromosome
structure, replication, and division. These differences are more fundamental than the
ones usually observed at the phylum level, and it may be more appropriate to consider
Crenarchaeota and Euryarchaeota as subdomains. The marine bacteria are themselves
deeply divided in the rRNA phylogeny (Fig. 5.4), showing a suite of 11 well-
separated groups. One of those, the Proteobacteria, divides into five major subgroups,
designated alpha-, beta-, gamma-, delta-, and epsilon-proteobacteria. Bacteria are
everywhere in natural systems, many of them not requiring extremes of any sort. They
make up the bulk of water-column prokaryotic cells in surface water.
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