MEASUREMENT OF RIBOSOMAL-RNA VARIATIONS IN NATURAL COMMUNITIES OF MICROORGANISMS ON THE SOUTHEASTERN UNITED-STATES CONTINENTAL-SHELF

The development of a clear understanding of the physiology of marine p rokaryotes is complicated by the difficulties inherent in resolving th e activity of various components of natural microbial communities. App lication of appropriate molecular biological techniques offers a means of overcoming some of these problems. In this regard, we have used di rect probing of bulk RNA purified from selective size fractions to exa mine variations in the rRNA content of heterotrophic communities and S ynechococcus populations on the southeastern U.S. continental shelf. H eterotrophic communities in natural seawater cultures amended with sel ected substrates were examined. Synechococcus populations were isolate d from the water column by differential filtration. The total cellular rRNA content of the target populations was assayed by probing RNA pur ified from these samples with an oligonucleotide complementing a unive rsally conserved region in the eubacterial 16S rRNA (heterotrophs) or with a 1.5-kbp fragment encoding the Synechococcus sp. strain WH 7803 16S rRNA (cyanobacteria). The analyses revealed that heterotrophic bac teria responded to the addition of glucose and trace nutrients after a 6-h lag period. However, no response was detected after amino acids w ere added. The cellular rRNA content increased 48-fold before dropping to a value 20 times that detected before nutrients were added. Variat ions in the rRNA content from Synechococcus spp. followed a distinct d iel pattern imposed by the phasing of cell division within the irradia nce cycle. The results indicate that careful application of these appr opriate molecular biological techniques can be of great use in discern ing basic physiological characteristics of selected natural population s and the mechanisms which regulate growth at the subcellular level.