Understanding Nitrification Micro-organisms

Nitrification is a crucial step in the nitrogen cycle. It's the biological oxidation of ammonia (NH₃) or ammonium (NH₄⁺) to nitrite (NO₂^-) followed by the oxidation of the nitrites to nitrate (NO₃^-). This process is vital for converting ammonia, which can be toxic to plants and aquatic life, into nitrates, which are readily absorbed by plants as a nutrient.

Key Micro-organisms for Nitrification

The question asks to identify the micro-organisms responsible for nitrification. This process is primarily carried out by specific groups of bacteria and archaea.

• Nitrosomonas and Nitrobacter: This pair represents the most common and well-studied nitrifying bacteria.
  • Nitrosomonas species are ammonia-oxidizing bacteria (AOB). They convert ammonia (NH₃) or ammonium (NH₄⁺) into nitrite (NO₂^-). The simplified reaction is:

    $ 2NH_4^+ + 3O_2 \rightarrow 2NO_2^- + 4H^+ + 2H_2O $

  • Nitrobacter species are nitrite-oxidizing bacteria (NOB). They take the nitrite produced by Nitrosomonas and oxidize it into nitrate (NO₃^-). The simplified reaction is:

    $ 2NO_2^- + O_2 \rightarrow 2NO_3^- $

  Because both steps are essential for complete nitrification (ammonia to nitrate), these two genera are correctly identified as the micro-organisms responsible for the process.

Evaluating Other Options

Let's look at why the other options are not the primary agents of nitrification:

• Nostoc and Anabaena: These are types of cyanobacteria (blue-green algae). They are primarily known for nitrogen fixation, converting atmospheric nitrogen gas (N₂) into ammonia. They are not involved in the oxidation of ammonia or nitrite.
• Rhizobium and Azotobacter: These are also important nitrogen-fixing bacteria. Rhizobium lives symbiotically in the root nodules of legumes, while Azotobacter is a free-living nitrogen fixer. Their role is in converting atmospheric N₂ to ammonia, not in nitrification.
• Clostridium and Pseudomonas:
  • Clostridium species are typically known for fermentation and some can be involved in denitrification (converting nitrates back to nitrogen gas), especially under anaerobic conditions.
  • Pseudomonas species are metabolically versatile and can be involved in various processes, including denitrification and decomposition, but they are not the primary nitrifying bacteria.

Therefore, Nitrosomonas and Nitrobacter are the correct micro-organisms responsible for nitrification.