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Unlock This CourseNitrogen is essential for all living things - it's needed to make proteins and DNA. However, most organisms can't use nitrogen gas directly from the air. This is where special bacteria come to the rescue! These tiny microorganisms act like nature's recycling team, converting nitrogen between different forms that plants and animals can actually use.
Two main groups of bacteria are the stars of this show: nitrifying bacteria (which help make nitrogen available to plants) and denitrifying bacteria (which return nitrogen to the atmosphere). Together, they keep the nitrogen cycle spinning and ensure life on Earth can continue.
Key Definitions:
Nitrogen makes up about 78% of our atmosphere, but it's locked in a form (N₂) that most living things can't use directly. Plants need nitrogen to make chlorophyll (for photosynthesis) and proteins. Animals get their nitrogen by eating plants or other animals. Without bacterial helpers, this vital element would remain trapped and unusable!
Nitrifying bacteria are like tiny chemical factories that work in two stages to convert ammonia into plant-friendly nitrates. This process is called nitrification and happens in soils, water and even sewage treatment plants.
Nitrification doesn't happen all at once - it's a carefully orchestrated two-step dance performed by different types of bacteria working together.
Bacteria like Nitrosomonas convert toxic ammonia (NH₃) into nitrite (NO₂⁻). This happens when dead plants and animals decompose, or from animal waste. The bacteria get energy from this conversion process.
Different bacteria called Nitrobacter take over and convert the nitrite into nitrate (NO₃⁻). Nitrates are the form of nitrogen that plants love most - they can absorb it easily through their roots.
Plants absorb nitrates from soil through their root systems. They use this nitrogen to build proteins, nucleic acids and chlorophyll. Without this process, plants would struggle to grow properly.
In healthy farmland soil, nitrifying bacteria are constantly at work. A single gram of fertile soil can contain millions of these bacteria. Farmers sometimes add organic matter like compost or manure, which provides ammonia for nitrifying bacteria to convert. This natural process can provide up to 50kg of usable nitrogen per hectare per year - that's like getting free fertiliser from nature!
While nitrifying bacteria help make nitrogen available, denitrifying bacteria do the opposite - they convert nitrates back into nitrogen gas and return it to the atmosphere. This might seem counterproductive, but it's actually essential for preventing toxic build-up of nitrates in soil and water.
Denitrifying bacteria are most active in waterlogged soils where oxygen levels are low. They use nitrates instead of oxygen for respiration, breaking them down in the process.
In flooded fields, marshes, or poorly drained soils, oxygen becomes scarce. Denitrifying bacteria switch to using nitrates for energy, converting them through nitrite and nitric oxide, finally producing nitrogen gas that bubbles back to the atmosphere.
Ocean sediments are major sites of denitrification. Here, bacteria process nitrates from marine organisms and agricultural runoff, helping prevent ocean acidification and algal blooms.
Natural wetlands act like giant water filters. Denitrifying bacteria in wetland mud remove excess nitrates from water flowing through, cleaning it naturally before it reaches rivers and lakes.
Water treatment plants deliberately create low-oxygen zones where denitrifying bacteria can remove nitrates from wastewater, preventing pollution of natural water bodies.
Understanding how nitrifying and denitrifying bacteria fit into the bigger picture helps us appreciate their crucial roles in maintaining life on Earth.
The nitrogen cycle is like a giant recycling system with several key players working together.
1. Nitrogen fixation converts atmospheric N₂ to ammonia
2. Nitrification converts ammonia to nitrates (our focus bacteria!)
3. Assimilation - plants absorb nitrates
4. Decomposition returns nitrogen to soil as ammonia
5. Denitrification returns nitrogen gas to atmosphere
In a healthy forest, the nitrogen cycle operates like clockwork. Fallen leaves decompose, releasing ammonia. Nitrifying bacteria in the soil convert this to nitrates. Tree roots absorb these nitrates for growth. In waterlogged areas near streams, denitrifying bacteria prevent nitrate build-up by converting excess back to nitrogen gas. This balance has been maintained for millions of years, supporting diverse forest ecosystems worldwide.
Human activities have dramatically affected these bacterial processes, sometimes helping and sometimes hindering their natural work.
Modern farming both relies on and disrupts natural nitrogen cycling processes.
Synthetic fertilisers provide instant nitrates but can overwhelm natural bacterial processes. Excess nitrates can leach into groundwater or run off into rivers, causing pollution.
Heavy machinery compacts soil, reducing air spaces where nitrifying bacteria live. This slows down the natural conversion of ammonia to nitrates, affecting plant nutrition.
Organic methods work with bacterial processes by adding compost and avoiding chemicals that harm soil bacteria. This supports healthy populations of both nitrifying and denitrifying bacteria.
Scientists and farmers are finding new ways to work with these bacterial processes rather than against them.
Modern farmers use soil testing to measure bacterial activity and adjust fertiliser application accordingly. This reduces waste and environmental impact while maintaining crop yields. Some farms now add specific bacterial cultures to boost soil health naturally.
Research is developing new ways to enhance beneficial bacterial activity. Scientists are creating bacterial inoculants - like probiotics for soil - that boost nitrification in depleted soils. Meanwhile, constructed wetlands use denitrifying bacteria to clean agricultural runoff, protecting water quality while maintaining the natural nitrogen cycle balance.