A cyanobacterial recombination study, involving an efficient N-2-fixing non-heterocystous partner

Many filamentous cyanobacteria fix atmospheric nitrogen under natural condi tions in specialized anaerobic compartments, heterocysts, interspersed betw een vegetative cells, which provide protection to the O-2-sensitive nitroge nase. A few unicellular cyanobacterial strains are also known to fix nitrog en aerobically at a slower rate. Filamentous cyanobacteria lacking heterocy sts are not known so far to fix nitrogen. We describe the isolation and pur ification 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 bee n designated as het(-) nif(+) (nonheterocystous and nitrogen-fixing), and i s found to be easily and effectively preserved in nitrogen-free medium in s tandard synthetic cyanobacterial nutrient medium (pH 8.5) at a continuous l ight 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 ano ther cyanobacterium, Nostoc muscorum, when both are grown together in a rec ombination study.