Nitrobacter is a genus of mostly rod-shaped, gram-negative, and chemoautotrophic bacteria.Nitrobacter plays an important role in the nitrogen cycle by oxidizing nitrite into nitrate in soil. Unlike plants, where electron transfer in photosynthesis provides the energy for carbon fixation, Nitrobacter uses energy from the oxidation of nitrite ions, NO2-, into nitrate ions, NO3-, to fulfill their energy needs. Nitrobacter fixes carbon dioxide via the Calvin cycle for their carbon requirements.Nitrobacter have an optimum pH between 7.3 and 7.5, and will die in temperatures exceeding 120°F (49°C) or below 32°F (0°C).Some sources regard Nitrobacteraceae to be the family of the genus Nitrobacter. Species in the genus Nitrobacter include Nitrobacter winogradskyi, Nitrobacter hamburgensis, Nitrobacter vulgaris and Nitrobacter alkalicus. According to Grundmann, Nitrobacter seem to grow optimally at 38°C and at a pH of 7.9, but Holt states that Nitrobacter grow optimally at 28°C and grows within a pH range of 5.8 -8.5 and has an pH optima between 7.6 and 7.8. Nitrobacter belongs to the α-subclass of the Proteobacteria.Nitrobacter may either be rod shaped, pear-shaped or pleomorphic. Cells normally reproduce by budding (Holt, 1993). Carboxysomes which aid carbon fixation are found in lithoautotrophically and mixotrophically grown cells. Additional energy conserving inclusions are PHB granules and polyphosphates. When both nitrite and organic substances are present, cells can exhibit biphasic growth, first the nitrite is used and after a lag phase, organic matter is oxidized. Chemoorganotrophic growth is slow and unbalanced thus more poly-β- hydroxybutyrate granules are seen that distort the shape and size of the cells.Nitrobacter play an essential role in aquaponics. Nitrosomonas bacteria first convert ammonia into nitrites. Nitrobacter convert the nitrites into nitrates, which are readily absorbed by the plants.