Characterization of marine prokaryotic communities via DNA and RNA.

We know very little about species distributions in prokaryotic marine plankton. Such information is very interesting in its own right, and ignorance of it is also beginning to hamper process studies, such as those on viral infection. New DNA- and RNA-based approaches avoid many prior limitations. Here we discuss four such applications: (1) cloning and sequencing of 16S rRNA genes to produce lists of what types of organisms are present; (2) quantification of these individual types in marine samples by nucleic acid hybridization, including single cell fluorescence; (3) quantitative comparison by DNA-DNA hybridization of entire microbial communities in terms of shared common types, without knowledge of community components; and (4) finding cultures that are representative of native communities. Several previously uncharacterized types of bacteria and archaea (probably including novel phyla) are present in marine plankton. Evidence from both the Atlantic and Pacific suggests that as-of-yet uncultivated archaea may dominate the deep sea, and thus may be the most abundant group of organisms on Earth. Such archaea are in surface waters as well, and can be visualized with fluorescent probes and enriched at room temperature with addition of organic nutrients. Community hybridization shows that variability of microbial community compositions in time and space is high. Although most native bacteria do not grow in culture, some proteobacterial cultures appear by genomic hybridization to be representative of certain communities. These and other results indicate the utility of DNA- and RNA-based methods.