A cluster of marine bacteria within the alpha-3 subclass of
Proteobacteria was found to account for up to 28% of the 16S rDNA
sequences in seawater samples from the coast of the southeastern U.S.  Two
independent oligonucleotide probes targeting 16S rDNA of this "marine alpha"
cluster indicate that the group dominates bacterioplankton communities in
estuarine and nearshore regions of the southeastern U.S. coast.  Marine
alpha bacteria decline predictably in abundance with decreasing salinity
along estuarine transects and are not detectable in low salinity
(5o/oo) or freshwater samples.  Sequences of 16S rDNA obtained
from seawater by PCR using one group-specific oligonucleotide as a primer
confirm that the oligonucleotide targets only members of this phylogenetic
cluster.  Likewise, sequences of 16S rDNA obtained from seawater by PCR
using several different pairs of non-specific primers show an unusually high
abundance of marine alpha sequences (52 to 84%) among the clones and
indicate a PCR bias toward the group.  Members of the marine alpha group
were readily cultured from coastal seawater, accounting for 40% of the
colonies isolated on low-nutrient marine agar based on hybridizations with
the group-specific 16S rDNA probe and sequence analysis.  This is the first
description of a numerically dominant cluster of coastal bacteria,
identified by molecular techniques, that can be readily cultured and studied
in the laboratory.