Which microbes are typically involved in nitrification and denitrification in freshwater systems?

Which microbes are typically involved in nitrification and denitrification in freshwater systems?

• A. Nitrification by photosynthetic algae; denitrification by fungi.
• B. Nitrification by chemoautotrophic bacteria such as Nitrosomonas and Nitrobacter; denitrification by heterotrophic bacteria such as Pseudomonas.
• C. Nitrification by archaea; denitrification by archaea.
• D. Nitrification by anaerobic bacteria; denitrification by aerobic bacteria.

Answer: (B)

In freshwater nitrogen cycling, the distinct roles of microbes are key: nitrification is the aerobic oxidation of ammonia to nitrite and then to nitrate, while denitrification is the anaerobic or low-oxygen reduction of nitrate to nitrogen gas. The classic players are chemoautotrophic bacteria for nitrification and heterotrophic bacteria for denitrification. Nitrosomonas oxidizes ammonia to nitrite, and Nitrobacter oxidizes nitrite to nitrate; both obtain energy from these oxidation steps and fix carbon independently of light. Denitrification is carried out by heterotrophic bacteria such as Pseudomonas, which use nitrate as the terminal electron acceptor under low-oxygen conditions and reduce it to N2 gas, often using organic carbon as a fuel source. This combination is why the option with nitrification by chemoautotrophic bacteria (like Nitrosomonas and Nitrobacter) and denitrification by heterotrophic bacteria (like Pseudomonas) is the best choice. Nitrification is not driven by photosynthetic algae, and fungi are not the typical denitrifiers in this context. Archaea can participate in nitrification, but the classic freshwater scenario emphasizes bacterial players. Nitrification requires oxygen, so anaerobic bacteria would not carry it out, and while some aerobic bacteria can participate in denitrification, the standard denitrifiers are facultative or anaerobic.

In freshwater nitrogen cycling, the distinct roles of microbes are key: nitrification is the aerobic oxidation of ammonia to nitrite and then to nitrate, while denitrification is the anaerobic or low-oxygen reduction of nitrate to nitrogen gas. The classic players are chemoautotrophic bacteria for nitrification and heterotrophic bacteria for denitrification. Nitrosomonas oxidizes ammonia to nitrite, and Nitrobacter oxidizes nitrite to nitrate; both obtain energy from these oxidation steps and fix carbon independently of light. Denitrification is carried out by heterotrophic bacteria such as Pseudomonas, which use nitrate as the terminal electron acceptor under low-oxygen conditions and reduce it to N2 gas, often using organic carbon as a fuel source.

This combination is why the option with nitrification by chemoautotrophic bacteria (like Nitrosomonas and Nitrobacter) and denitrification by heterotrophic bacteria (like Pseudomonas) is the best choice. Nitrification is not driven by photosynthetic algae, and fungi are not the typical denitrifiers in this context. Archaea can participate in nitrification, but the classic freshwater scenario emphasizes bacterial players. Nitrification requires oxygen, so anaerobic bacteria would not carry it out, and while some aerobic bacteria can participate in denitrification, the standard denitrifiers are facultative or anaerobic.