Role of chemotaxis in establishing a specific nitrogen-fixing cyanobacterial-bacterial association.

A specific association with the cyanobacterium Anabaena oscillarioides was established by positive bacterial (pseudomonad) chemotaxis to Anabaena oscillarioides heterocysts. This association enhanced nitrogen fixation in A. oscillarioides, and positive chemotaxis was particularly strong during periods of active nitrogen fixation. Addition of compounds known to elicit positive chemotaxis in pseudomonads interfered with the establishment of the association, while removal of these compounds led to reestablishment of the association. Anabaena oscillarioides excretion products, some of which are exuded from heterocyst-vegetative cell junctions, are likely to be responsible for positive chemotactic responses. Chemotaxis-controlled associations such as this one explain in part why aquatic bacterial-algal and bacterial-particle associations occur sporadically and are heterogeneously distributed in time and space.