A cyanobacterial recombination study, involving an efficient N2-fixing non-heterocystous partner

Many filamentous cyanobacteria fix atmospheric nitrogen under natural conditions in specialized anaerobic compartments, heterocysts, interspersed between vegetative cells, which provide protection to the O-2-sensitive nitrogenase. A few unicellular cyanobacterial strains are also known to fix nitrogen aerobically at a slower rate. Filamentous cyanobacteria lacking heterocysts are not known so far to fix nitrogen. We describe the isolation and purification of a non-heterocystous filamentous cyanobacterium from the fronds of the water-fern Azolla, fixing nitrogen at 18.7 +/- 0.2 n moles ethylene mu g Chl. a(-1) h(-1) when grown in nitrogen-free medium at a low level of oxygen between two layers of agar. This strain of Anabaena azollae has been designated as het(-) nif(+) (nonheterocystous and nitrogen-fixing), and is found to be easily and effectively preserved in nitrogen-free medium in standard synthetic cyanobacterial nutrient medium (pH 8.5) at a continuous light intensity of 2800 Ix at 25 +/- 1 degrees C. This het(-) nif(+) strain is an effective donor of the nif(+) marker to a het(+) nif(-) strain of another cyanobacterium, Nostoc muscorum, when both are grown together in a recombination study.