John F. Carr Roann E. Ogawa 1969 <p>A series of experiments on heterocyst production in <i>Anabaena variabilis</i> provides some strong indirect evidence for the role of heterocysts in nitrogen fixation. Of the algae tested (<i>Anabaena variabilis, A. inaequalis, A. cylindrica, A. flos-aquae, Tolypothrix distorta, Gloeotrichia echinulata, Aphanizomenon flos-aquae, Oscillatoria sp.</i>, and <i>Microcystis aeruginosa</i>), only those with heterocysts grew in a nitrate-free medium. Growth in the nitrate-free medium was accompanied by an increase in heterocysts. Heterocyst formation in <i>A</i>. variabilis was evident 24 hr after transfer from a nitrate-containing to a nitrate-free medium. The number of heterocysts was altered by changes in the nitrogen source. Numbers were lowest when NH₄-N was used as a nitrogen source and highest when nitrogen (N₂-N) was derived from the atmosphere. Heterocyst numbers could also be regulated by controlling the concentration of NO₃-N in the medium. Heterocyst production depended on the absence of combined nitrogen and the presence of phosphate. Data are presented on the occurrence of blue-green algae (with heterocysts) in Lake Erie and the environmental conditions apparently necessary for them to become dominant.</p> application/pdf 10.4319/lo.1969.14.3.0342 en Wiley The influence of nitrogen on heterocyst production in blue-green algae article