I. Overview
I.A. Spaced heterocysts: the sites of nitrogen fixation in filaments of Anabaena

• Cyanobacteria can do photosynthesis (CO2 to sugar)
• Some cyanobacteria can do N₂ fixation (N2 to ammonia and ultimately protein)
• These cyanobacteria can thrive with just water, light, air, and a bit of salt.

How does the cyanobacterium Anabaena solve this problem, both producing O₂ by photosynthesis and also fixing N₂? They do so in a most unbacterial way -- growing as multicellular filaments, consisting of cells linked like beads on a string. There are two types of cells (Fig. 1):

• Vegetative cells: Capable of photosynthesis but not N₂ fixation. Most cells are of this type.
• Heterocysts: Able to fix N₂ but not perform photosynthesis. About one cell in 10 is a heterocyst.

Fig. 1: Heterocyst in filament of Anabaena. Green, photosynthetically competent vegetative cells surround (and feed) pale heterocyst.

Fig. 2: Pattern of spaced heterocysts. Blue cells in this filament of Anabaena PCC 7120 are heterocysts stained with Alcian blue, a dye that binds to heterocyst-specific polysaccharide

Heterocysts are very expensive to make and to operate, and so they are not present in Anabaena filaments unless required. When the cyanobacterium is grown in the presence of a nitrogen source like ammonia or nitrate, then its filaments have only vegetative cells. However, if Anabaena finds itself in a medium with no source of nitrogen, then its filaments look like that shown in Fig. 2.

Anabaena grown with nitrogen source	Anabaena shifted to medium without nitrogen source
Filaments have only vegetative cells  No N2-fixation	Heterocysts appear in 12-18 hours  Filaments fix of N2  Spacing of heterocysts is not random.

Notice that the heterocysts in Fig. 2 are well spaced. Spacing is certainly a good idea. It ensures that expensive heterocysts feed as many vegetative cells fixed nitrogen as possible. But how does Anabaena do it? WE can see the pattern, looking down from above, but how do THEY -- dumb bacteria -- count?

(To learn more about spacing in Anabaena, click here)

Consider what we have here! Anabaena, a bacterium, is capable of:

• Differentiation, like animal cells
• Patterns of differentiation, like animals cells
• Fast differentiation in a test tube, in response to an easily controlled parameter, most unlike animal cells