THE HETEROCYSTS OF BLUE‐GREEN ALGAE (MYXOPHYCEAE)

Summary 1. Heterocysts are found in many species of filamentous blue‐green algae. They are cells of slightly larger size and with a more thickened wall than the vegetative cells. 2. Structural details of the heterocyst are: the presence of three additional wall layers, the absence of granules, sparse thylakoid network throughout, except at the poles where a dense coiling of membranes occurs. Other characters include the two pores at opposite poles ‘plugged’ with refractive material called the polar granule. 3. Peculiarities in the pigment composition of the heterocyst include an abundance of carotenoids and absence of phycobilins, and a short‐wave form of chlorophyll a . 4. Unique glycolipids and an acyl lipid, not found in the vegetative cells of the algae or in other plant cells, are associated with the heterocyst. The glycolipids constitute the laminated layer of the wall and probably regulate diffusion of substances through it, whereas the acyl lipids are supposed to function as carriers and intermediates in the biosynthesis of the wall. 5. The heterocysts develop from vegetative cells, and the visible changes during differentiation include cell enlargement, synthesis of additional wall layers, disappearance of granules and reorientation and synthesis of the thylakoids. 6. Heterocysts are formed sequentially with characteristic cellular spacing during the growth of cultures in medium free from combined nitrogen. 7. Various sources of combined nitrogen inhibit heterocyst formation when supplied in the culture medium. Ammonium salts are among the most powerful inhibitors. Heterocysts are formed simultaneously and within a short period after transference of ammonia‐grown non‐heterocystous filaments to ammonia‐free medium. 8. Incompletely differentiated heterocysts or proheterocysts are found in cultures grown in the presence of combined nitrogen. If two or more proheterocysts are close together generally a single one develops to maturity after a competitive interaction in medium free from combined nitrogen. This indicates that heterocyst formation is completed in two phases: phase I, synthesis and conservation of macromolecules, which takes place during growth in ammonia‐containing medium: and phase 11, morphological differentiation of the heterocyst which is unaccompanied by growth in cell number. In the ammonia‐free medium phase 11 quickly succeeds phase 1 and the whole process appears as a continuum. 9. Heterocyst formation shows a definite requirement for light. Red light favours heterocyst formation, whereas green and blue light do not. The effects of light seem to be mainly due to photosynthesis, although some effects may be morphogenetic. 10. Studies with metabolic inhibitors have revealed the involvement of photosynthesis, respiration and protein synthesis in heterocyst formation. Photosynthesis provides carbon skeletons, whereas ATP is most probably supplied by oxidative metabolism. 11. Various functions have been assigned to the heterocyst from time to time. Their role in akinete formation is suggested by (i) the formation of akinetes adjacent to the heterocysts and (ii) prevention of sporulation by detachment of the heterocysts from the vegetative cells (potential akinetes). Despite substantial evidence for such a role, it is not applicable to all akinete‐forming genera. 12. Heterocysts are now widely believed to be the site of nitrogen fixation in blue‐green algae. The main facts in favour of such a role are: (i) fixation of nitrogen by all heterocystous algae, (ii) inhibition of heterocyst formation by combined nitrogen and (iii) direct observations on acetylene reduction by isolated heterocysts. 13. Some non‐heterocystous and unicellular algae, and vegetative cells of heterocystous algae fix nitrogen under microaerophilic conditions suggesting that absence of oxygen favours nitrogenase activity. Heterocysts lack the oxygen‐evolving photo‐system 11, possess oxidative enzymes, and reduce externally supplied tetrazolium salts ‐ all indicating that they are the most suitable sites for harbouring nitrogenase in aerobic conditions. 14. Heterocysts probably originated in the Precambrian in response to the earth's changing environment and seem to be the first example of morphological differentiation in the plant kingdom.