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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Nitrifying Bacteria
Nitrifying bacteria are tiny but mighty microorganisms that play a crucial role in the nitrogen cycle. Without these microscopic helpers, plants wouldn't be able to access the nitrogen they need to grow and our ecosystems would collapse. These special bacteria transform nitrogen compounds from one form to another, making them available for plants to use.
Key Definitions:
Nitrifying bacteria take something that would be toxic waste (ammonia) and transform it into a valuable plant nutrient (nitrates). It's like turning rubbish into treasure! These bacteria are found in soil, freshwater and marine environments worldwide.
Plants cannot directly use atmospheric nitrogen (N₂) or ammonia (NH₃). They need nitrogen in the form of nitrates (NO₃⁻). Nitrifying bacteria are essential because they create this plant-friendly form of nitrogen.
Nitrification is a two-stage process carried out by different groups of bacteria. Each group has a specific job in the nitrogen transformation assembly line.
Ammonia-oxidising bacteria (like Nitrosomonas) convert ammonia (NH₃) to nitrites (NO₂⁻):
NH₃ + O₂ → NO₂⁻ + H⁺ + H₂O
This first step releases energy that the bacteria use to grow and reproduce.
Nitrite-oxidising bacteria (like Nitrobacter) convert nitrites (NO₂⁻) to nitrates (NO₃⁻):
NO₂⁻ + O₂ → NO₃⁻
This second step completes the nitrification process, creating nitrates that plants can absorb through their roots.
Nitrifying bacteria are classified based on the chemical transformations they perform. Let's meet the main players:
Examples: Nitrosomonas, Nitrosococcus, Nitrosospira
These bacteria convert ammonia to nitrites. They're the first workers on the nitrification assembly line.
Examples: Nitrobacter, Nitrospira, Nitrococcus
These bacteria convert nitrites to nitrates. They complete the nitrification process, creating plant-friendly nitrogen.
Example: Nitrospira inopinata
Recently discovered "comammox" bacteria can perform both steps of nitrification by themselves - converting ammonia directly to nitrates!
Nitrifying bacteria have some unique features that make them special in the microbial world:
Nitrifying bacteria are some of the slowest-growing bacteria known! While E. coli bacteria can double their population in just 20 minutes, nitrifying bacteria might take 8-24 hours to double. This slow growth makes them vulnerable to environmental changes.
Several environmental conditions can affect how well nitrifying bacteria do their job:
Nitrification works best between 25-30°C. It slows down dramatically below 5°C or above 40°C.
Nitrifying bacteria need water to function, but too much water limits oxygen availability. They work best in moist but not waterlogged soil.
Most nitrifying bacteria prefer neutral to slightly alkaline conditions (pH 7-8). Acidic soils (below pH 6) significantly reduce nitrification.
As aerobic organisms, nitrifying bacteria require oxygen. In waterlogged or compacted soils with low oxygen, nitrification is limited.
Many pesticides, heavy metals and other pollutants can inhibit nitrifying bacteria, reducing nitrification rates in contaminated soils.
Plant roots create favourable environments for nitrifying bacteria by improving soil structure and providing organic matter.
The nitrogen cycle is a complex process that involves several steps and different microorganisms. Nitrifying bacteria are just one part of this cycle, but they play a crucial role.
1. Nitrogen fixation: Atmospheric N₂ is converted to ammonia by nitrogen-fixing bacteria.
2. Ammonification: Decomposers break down organic matter, releasing ammonia.
3. Nitrification: Nitrifying bacteria convert ammonia to nitrites and then nitrates.
4. Assimilation: Plants absorb nitrates and use them to make proteins.
5. Denitrification: Denitrifying bacteria convert nitrates back to atmospheric N₂.
Without nitrification, nitrogen would remain trapped in forms that plants can't use. This would lead to:
Farmers need to understand nitrifying bacteria to manage their soil fertility effectively:
Beneficial practices:
Harmful practices:
Rice paddies present a unique challenge for nitrification. The flooded conditions create an oxygen-poor environment where nitrifying bacteria struggle to function. To manage this, farmers often drain their paddies periodically during the growing season. This practice, called "mid-season drainage," allows oxygen to enter the soil, promoting nitrification and improving nitrogen availability for the rice plants. Studies show this can increase rice yields by 10-15% while reducing methane emissions from the paddies.
Beyond agriculture, nitrifying bacteria are important in several environmental applications:
Nitrifying bacteria might be microscopic, but their impact is enormous. These tiny organisms:
Without nitrifying bacteria, the nitrogen cycle would break down, plants would struggle to grow and ecosystems would be dramatically altered. These humble microorganisms are truly unsung heroes of our planet!
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