DETERMINATION OF ACTIVE MARINE BACTERIOPLANKTON - A COMPARISON OF UNIVERSAL 16S RIBOSOMAL-RNA PROBES, AUTORADIOGRAPHY, AND NUCLEOID STAINING

We compared several currently discussed methods for the assessment of bacterial numbers and activity in marine waters, using samples from a variety of marine environments, from aged offshore seawater to rich ha rbor water, Samples were simultaneously tested for binding to a fluore scently labeled universal 16S rRNA probe; H-3-labeled amino acid uptak e via autoradiography; nucleoid-containing bacterial numbers by modifi ed DAPI (4',6-diamidino-2-phenylindole) staining; staining with 5-cyan o-2,3-ditolyl tetrazolium chloride (CTC), a compound supposed to indic ate oxidative cell metabolism; and total bacterial counts (classical D API staining), taken as a reference, For the universal-probe counts, w e used an image intensifying and processing system coupled to the epif luorescence microscope, All of the above-mentioned methods yielded low er cell counts than DAPI total counts, Universal-probe counts averaged about half of the corresponding DAPI count and were highly correlated to autoradiography counts (r(2) = 0.943; n = 7). Nucleoid-containing cell counts could be lower than DAPI counts by as much as 1 order of m agnitude but sometimes matched autoradiography or probe counts. CTC co unts were 2 orders of magnitude below DAPI counts, Universal 16S rRNA probe counts correlated well,vith autoradiography results, indicating a population with at least minimal metabolic activity, The greater var iability of the nucleoid-containing cell counts calls for further inve stigation of the processes involved, and CTC counts were well below th e range of the other methods tested.