2-36 Heterocysts are differentiated cells that specialize in nitrogen fixation

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• Some cyanobacteria species form specialized structures for the fixation of nitrogen.

Some photosynthetic filamentous cyanobacteria are capable of forming specialized structures called heterocysts. These are rounded structures distributed at regular intervals along the string of vegetative cells as shown in Figure 2-53 or at one end. Heterocysts evolved to solve the problem of performing plant-like photosynthesis (which produces oxygen) and at the same time fixing N₂ to ammonia (a process that involves enzymes that are inactivated by O₂). The single focus of the heterocysts is to fix N₂, while the rest of the cells perform photosynthesis (and divide), thus keeping the two processes separate. Heterocysts develop a surface that is impermeable to gasses, and begin synthesizing large amounts of nitrogenase, the protein that fixes N₂. Importantly, these heterocysts maintain some permeability to the cells on either side. Neighboring cells take N₂ from the atmosphere and pass it along to the heterocyst, which reduces it to NH₃ and returns fixed nitrogen to its neighbors. The neighboring cells also take up and utilize O₂, but they prevent it from reaching the heterocyst. Heterocysts are essentially specialized organs for the "multi-cellular organism" represented by a chain of cyanobacterial cells and they are only formed when nitrogen is limiting. The regulation of this developmental cycle is intriguing and serves as a simple example of multicellular development in a unicellular organism.

This ends our survey of the cellular structure of bacteria. In the remaining sections we take a look at the major structural differences that distinguish the archaea and eukaryotes from the bacteria.

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