About the Function of Heterocysts

Nitrogen is an essential element for the growth of almost all forms of life, especially plants. Although nitrogen is is abundantly available in the air as a gas, it is unusable by plants and animals until it is turned into nitrate or ammonia, which is then used to make proteins and other important compounds. Some plants use special cyanobacteria, called heterocysts, in order to convert nitrogen in the air to a biologically available form.

Identification

• Heterocysts are special nitrogen-fixing cells formed in certain kinds of cyanobacteria during times of low nitrogen. These specialized cells, in order to convert nitrogen in the air to a form the bacteria can use, have to keep oxygen out of the cell. This requires several changes in the cell itself, including extra walls to keep oxygen out. In addition, since the cell needs to be oxygen-free for nitrogen fixation to happen, it is unable to produce some of the compounds it needs to produce energy and stay alive. As a result, these cells engage in a trade with other nearby cells, exchanging usable nitrogen for compounds it can turn into energy. Once a regular cyanobacteria cell turns into a heterocyst, it cannot turn back. It is thought that heterocyst formation is based on the amount of ammonia and nitrate that are available.

Significance

• Heterocysts often form a symbiotic relationship with certain kinds of blue-green algae. This means that the two organisms work together for their mutual benefit. Heterocysts provide the algae with ammonia and nitrates, and the algae in turns provides the heterocysts with other nutrients. In this form of symbiosis, the heterocysts can form the majority of the bacteria, and heterocyst formation is no longer regulated by the amount of ammonia and nitrates, but based on signals that the plant provides. This form of relationship is not uncommon between plants and bacteria, as it allows plants to get nutrients they could not otherwise obtain.

Features

• Heterocyst formation is marked by several changes to the cell, including the formation of three additional cell walls. Heterocysts also begin to produce certain proteins that are involved in nitrogen fixation, and degrade proteins that produce oxygen. Finally, the cell produces more glycolytic enzymes, which enables to cell to produce energy without oxygen. Finally, in order to ensure that no oxygen is present in the cell, the heterocyst cell produces proteins which scavenge any remaining oxygen.

Types

• In addition to the symbiosis between heterocysts and blue-green algae, symbiosis between nitrogen fixating bacteria and plants occurs with other species as well. The most common plant that uses this technique is the legume family, which includes beans, peanuts, clover, and alfalfa. These plants use a type of bacteria called rhizobia in their roots to produce nitrogen.

Significance

• Nitrogen fixation, by heterocysts and other symbiotic bacteria, is an important part of ecology and is critical for preventing the depletion of usable nitrogen, not just for the individual plants that utilize this symbiosis, but for ecological systems as a whole. Nitrogen fixation is the basis for such practices such as crop rotation, in which farmers rotate fields between nutrient-consuming plants such as corn with other, more replenishing crops, such as beans and legumes. Without heterocysts and other nitrogen-fixing bacteria, the nitrates and ammonia necessary for sustaining life would be very limited, thus allowing only certain kinds of plants to grow and severely restricting the variety of fauna and flora that populate the planet.