• Mixed culture;
  • coexistence of aerobes and anaerobes;
  • glucose-fermenting bacteria;
  • chemostat enrichments

Abstract The occurrence and properties were studied of glucose-metabolizing bacteria present in the anaerobic sediment 5–10 cm below the surface of an estuarine tidal mud-flat. Of all these bacteria (104– 105 per g wet sediment) 80–90% were facultatively anaerobic species. Chemostat enrichments on glucose under aerobic, oxygen-limited and alternately aerobic-anaerobic conditions also yielded cultures dominated by facultative anaerobes. One of the dominant species, tentatively identified as a Vibrio sp., was studied in more detail under oxygen-limiting conditions. Fermentative and respiratory metabolisms were found to operate simultaneously, and the ratio between the two was regulated by the extent of oxygen limitation. A small fraction of the acetate formed under such growth conditions was shown to be subsequently respired. A co-culture was established of the Vibrio sp. and a sulfate-reducing bacterium (Desulfovibrio HL21) in an aerated chemostat. The importance of these observations is discussed in relation to the role of facultative anaerobes in anaerobic habitats.