Role of heterotrophic bacteria in promoting N2 fixation byAnabaena in aquatic habitats.

Anabaena species are commonly colonized by bacteria, especially during N2-fixing blooms. Generally these associations do not represent bacterial attack on algal hosts. Instead, the algal N2-fixing capabilities are increased in the presence of the bacteria. Possible mechanisms promoting the mutual growth of algae and attached bacteria were investigated by observing specific sites of bacterial attachment, by noting reduced microzones created by the bacteria, and by locating sites of bacterial uptake of organics representative of algal excretion products.Attached bacteria show preference for typical algal excretion products and their growth is enhanced by such products. In return, enhancement of algal nitrogenase activity occurs when bacteria create O2-consuming microzones around the nitrogenase-bearing heterocysts.