The function of nitrogen fixation in Anabaena (cyanobacterium) is performed by
(2) Heterocyst is a specialised cell found in nitrogen-fixing cyanobacteria. These are enlarged cells with thick cell walls and they lack chlorophyll, giving them a colourless appearance. They are the site of nitrogen fixation for which they produce the enzyme nitrogenase. They lack photosynthetic activity (which would produce oxygen), together with the thick cell wall are thought to maintain the anoxic conditions in heterocysts that are essential for the activity of nitrogenase. Heterocysts are connected by plasmodesmata to surrounding cells, on which they depend for nutrients.