DISTRIBUTION OF BACTERIAL-POPULATIONS IN A STRATIFIED FJORD (MARIAGERFJORD, DENMARK) QUANTIFIED BY IN-SITU HYBRIDIZATION AND RELATED TO CHEMICAL GRADIENTS IN THE WATER COLUMN

The vertical distribution of major and intermediate electron accepters and donors was measured in a shallow stratified fjord. Peaks of zero valence sulfur, Mn(TV), and Fe(III) were observed in the chemocline se parating oxic surface waters from sulfidic and anoxic bottom waters. T he vertical fluxes of electron accepters and donors (principally O-2 a nd H2S) balanced within 5%; however, the zones of oxygen reduction and sulfide oxidation were clearly separated. The pathway of electron tra nsfer between O-2, and H2S was not apparent from the distribution of s ulfur, nitrogen, or metal compounds investigated. The chemical zonatio n was related to bacterial populations as detected by ethidium bromide (EtBr) staining and by in situ hybridization with fluorescent oligonu cleotide probes of increasing specificity. About half of all EtBr-stai ned cells were detectable with a general oligonucleotide probe for all eubacteria when digital image analysis algorithms were used to improv e sensitivity. Both EtBr staining and hybridization indicated a surpri singly uniform distribution of bacteria throughout the water column. H owever, the average cell size and staining intensity as well as the ab undance of different morphotypes changed markedly within the chemoclin e. The constant overall cell counts thus concealed pronounced populati on shifts within the water column. Cells stained with a delta 385 prob e (presumably sulfate-reducing bacteria) were detected at the chemocli ne at about 5 x 10(5) cells per ml, and this concentration increased t o 2 x 10(5) cells per ml beneath the chemocline. A long slim rod-shape d bacterium was found in large numbers in the oxic part of the chemocl ine, whereas large ellipsoid cells dominated at greater depth. Applica tion of selective probes for known genera of sulfate-reducing bacteria gave only low cell counts, and thus it was not possible to identify t he dominant morphotypes of the sulfate-reducing community.