The genus Nitrobacter represents a group of rod-shaped, gram-negative bacteria that play a role in the global nitrogen cycle. These organisms are classified as chemoautotrophs, meaning they derive their energy from the oxidation of inorganic chemical compounds rather than from sunlight or organic matter. Found abundantly in soil and aquatic environments, Nitrobacter performs a specialized function necessary for maintaining ecological balance. They convert a hazardous nitrogen compound into a safer form that can be used by plants.
The Chemical Reaction: Converting Nitrite to Nitrate
The core function of Nitrobacter is the oxidation of nitrite ($\text{NO}_2^-$) into nitrate ($\text{NO}_3^-$), a chemical reaction that provides the minimal energy required for the bacterium’s survival. This process is catalyzed by a specific enzyme complex known as nitrite oxidoreductase (NXR), which facilitates the addition of a single oxygen atom to the nitrite molecule. The resulting energy yield from this conversion is relatively low, measured at only about $-74$ kilojoules per mole of nitrite oxidized. This low energy return explains why these bacteria exhibit a slow growth rate compared to many other microorganisms.
Nitrobacter uses the energy released from nitrite oxidation to fix carbon dioxide ($\text{CO}_2$) from the environment. They accomplish this carbon fixation through the Calvin cycle, similar to plants, which allows them to synthesize the organic molecules necessary for cell structure and reproduction. The end product, nitrate, is a highly soluble compound that completes the biological conversion of nitrogenous waste, making nitrogen available for consumption by plants in both terrestrial and aquatic ecosystems.
Where Nitrobacter Thrives: Environmental Conditions
As an obligate aerobe, Nitrobacter must have high levels of dissolved oxygen present in its environment to perform the oxidation reaction that sustains its life. Without sufficient oxygen, the bacteria cannot fully process the nitrite, leading to a breakdown in the nitrogen cycle. Optimal activity occurs within a moderate temperature range, typically between $25$ and $30$ degrees Celsius ($77$ and $86$ degrees Fahrenheit). Temperatures outside this range can significantly slow down or completely halt their metabolic processes.
The $\text{pH}$ level of the surrounding water or soil is an important factor. They function best in neutral to slightly alkaline conditions, with an optimal $\text{pH}$ range often cited between $7.3$ and $7.5$. Environments that become too acidic, falling below a $\text{pH}$ of $6.0$, can severely inhibit the bacteria’s growth and ability to convert nitrite.
Nitrobacter in Action: Establishing Biological Filters
The conversion capabilities of Nitrobacter are utilized in closed aquatic systems, such as aquariums, aquaculture farms, and municipal wastewater treatment plants, where they form the backbone of biological filtration. In these environments, waste breaks down into ammonia, which is then converted into highly toxic nitrite. A stable population of Nitrobacter is necessary to quickly convert this nitrite, preventing it from reaching lethal concentrations for aquatic life.
The process of establishing a robust Nitrobacter population in a new system is commonly referred to as “cycling” the tank. Because these bacteria are non-motile, they must colonize a solid, inert surface. Biological filter media, often called bio-media, is specifically designed to provide a massive surface area where the bacteria can anchor and reproduce. This colonization is a slow process, given that Nitrobacter species can take approximately $13$ to $15$ hours to double their population under optimal circumstances.
The slow growth rate means that a sudden increase in waste, such as adding too many fish at once, can quickly overwhelm a developing filter, causing a spike in nitrite levels. Effective biological filtration relies on the total surface area available for the bacteria to attach, which directly determines the capacity of the system to process nitrogenous waste.
The Complete Process: Working Alongside Nitrosomonas
The entire process of nitrification is a two-step sequence that requires a synergistic relationship between Nitrobacter and another group of microorganisms, primarily those from the genus Nitrosomonas. Nitrosomonas are the first responders in the nitrogen cycle, oxidizing ammonia ($\text{NH}_3$) and ammonium ($\text{NH}_4^+$) that originate from organic waste. The oxidation of these compounds yields the intermediate product, nitrite ($\text{NO}_2^-$).
This nitrite is the sole food source that Nitrobacter consumes for energy. The activity of the Nitrobacter population is directly dependent on the prior operation of the Nitrosomonas population. The sequential action of the two microbial groups ensures that nitrogenous compounds are processed from highly toxic ammonia, through the nitrite stage, and finally into the less harmful nitrate that can be assimilated by plants.