13b. Bacteria and other microorganisms - The nitrifying bacteria

The nitrifying bacteria consist of both those that oxidize ammonia (NH3) to nitrite (NO2), and those that oxidize nitrite (NO2) to nitrate (NO3). These bacteria are obligate chemolitotrophs, which means that they can only grow on an inorganic energy source (NH3 or NO2), and an inorganic carbon source (CO2).
They are characterized by an extensive cytomembrane system, as shown in the cell at left, Nitrosomonas europaea.





Different strains of ammonia oxidizing bacteria show cytomembranes in different arrangements, as in this strain of Nitrosomonas. The enzyme that oxidizes ammonia is called ammonia monoxygenase and is located on the outer surface of the cellmembrane and cytomembranes. It carries out the following reaction:

NHS + O2 + 2e + 2H ---> NH2OH = H2O

NH2OH + H2O + 1/2O2 ---> NO2 + 2 H2O + H







The nitrite oxidizing bacteria, typified by Nitrobacter agils also have extensive membranes. In this case peripheral cytomembranes that are localized in only one end of the cell.










Other nitrite oxidizing bacteria, such as this Nitrococcus sp. have another way of extending the cell membrane into the cell. In this case the membrane system appears as randomly arranged membranous tubes, again all connected directly to the cell membrane. The enzyme involved in nitrite oxidation is called nitrite oxidase and it too is localized on the outer side of the cell membrane and the cytomebranes and tubes. It carries out the following reaction:

NO2 + 1/2 O2 ---> NO3













This is an ultrathin section of a marine Nitrosomonas, an ammonia oxidizing bacterium. An extensive peripheral cytomembrane system is clearly evident, and it always originates from the cell membrane.















This illustration shows the relationship of the cytomembranes to the cell membrane. The cell membrane invaginates inward forming the extensive cytomembranes shown in the ultrathin sections. It also diagramatically shows how the outer surface oif the membrane in in contact with the outside environment of the cell. In other words the outside environment can reach deep with the cell itself without really being inside the cytoplasm of the cell.
In the case of ammonia oxidation, it means that the oxidation of ammonia occurs on the outer (or external side of the membrane. Thus when ammonia is oxidized to nitrite, a toxic substance, the nitrite is immediately flushed out of the cell into the environment and never comes into contact with the inner cytoplasm of the cell. However, the energy gained from the reaction can be transferred through the membrane where it can be used in the cell.


Another view of the marine ammonia oxidizer, Nitrosomonas sp. This freeze-etching clearly shows the complex hexagonal structure of its cell wall, a characteristic of marine ammonia oxidizers.
















A freeze -etching of Nitrosococcus marinus, another ammonia oxidizer. Within this cell are extensive peripheral cytomembranes.














A thin section of Nitrosocystis oceanus showing another variation of the extensive cytomembrane system. In this case, the membranes are bundled in the center of the cell.

















This is a freeze-etching of Nitrosocystis oceanus showing once again the extensive centrally located cytomembrane system
















This freeze-etching shows another way that the cell membrane can extend itself in the cell. In this ammonia oxidizer, Nitrosouva aequatorialis, the cell membrane has invaginated into the cell creating compartments within the cell. As in the other bacteria, this results in the logarithmic increase of membrane surface area. Since the oxidation of ammonia to nitrite is very inefficient in producing energy for the cell, an increase in membrane surface area means that more enzyme can be operational at the same time, thus increasing the amount of energy being produced.




Another image of Nitrosouva aequatorialis; this time an ultrathin section again showing the compartmentalization of the cell by the cell membrane.

These micrographs were all taken by me or my colleagues at the Woods Hole Oceanographic Institution.