NUMERICAL DOMINANCE OF A GROUP OF MARINE-BACTERIA IN THE ALPHA-SUBCLASS OF THE CLASS PROTEOBACTERIA IN COASTAL SEAWATER

A cluster of marine bacteria within the alpha-3 subclass of the class Proteobacteria accounted for up to 28% of the 16S ribosomal DNA (rDNA) sequences in seawater samples from the coast Of the southeastern Unit ed States, Two independent oligonucleotide probes targeting 16S rDNA o f this ''marine alpha'' cluster indicate that the group dominates bact erioplankton communities in estuarine and nearshore regions of the Sou theastern U.S. coast; Marine alpha bacteria decline predictably in abu ndance with decreasing salinity along estuarine transects and are not detectable in low-salinity (5 parts per thousand) or freshwater sample s. Sequences of 16S rDNA obtained from seawater by PCR with 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 with several different pair s of nonspecific primers show an unusually high abundance of marine al pha sequences (52 to 84%) among the clones; which possibly indicates a PCR bias toward the group, Members of the marine alpha group were rea dily cultured from coastal seawater, accounting for 40% of the colonie s isolated on low-nutrient marine agar, based on hybridizations with t he group-specific 16S rDNA probe and on sequence analysis. This is the first description of a numerically dominant cluster of coastal bacter ia, identified by molecular techniques, that can be readily cultured a nd studied in the laboratory.