Nitrogen Fixation by Marine Agar-degrading Bacteria

Abstract

Several strains of agar-degrading bacteria capable of fixing N2 were isolated from seawater and eelgrass-bed sediment in Aburatsubo Inlet, Kanagawa, Japan, during the summer of 1986. All strains were Gram-negative, facultatively anaerobic, and required NaCl for growth. They were straight or slightly curved rods and were motile in liquid medium by means of a single polar flagellum. These characteristics as well as the G+C contents of their DNA (44·7–46·1 mol%) placed them in the family Vibrionaceae. These strains produced extracellular agarase on agar medium, yielding reducing sugars and acids as the end products. They expressed significant nitrogenase (acetylene reduction) activities after a few hours of incubation under anaerobic conditions. They utilized combined nitrogen sources both aerobically and anaerobically, but fixed N2 only under anaerobic conditions. Neither yeast extract nor vitamins were required for N2 fixation. These strains were demonstrated to fix N2 anaerobically using agar as the sole carbon source.