1) Decomposers (Putrefying Bacteria) These aerobic bacteria live in the soil. When an organism dies decomposers digest the proteins and DNA that are found in the cells of the organism and produce ammonium ions (NH4+) as a waste product. Decomposers will also break down the molecules in animal faeces and will decompose the urea in animal urine into ammonium ions.
2) Nitrifying Bacteria These are also aerobic bacteria that live in the soil. They get their energy by converting ammonium ions into nitrates (via an intermediate ion called a nitrite). Nitrifying bacteria are essential for the Nitrogen cycle because the nitrates they form are the ions that plants will absorb through their roots. Nitrates will be used by the plant to make amino acids, proteins and DNA and these can pass up food chains.
3) Nitrogen-fixing Bacteria The picture now gets a little more complicated. Nitrogen-fixing bacteria can convert atmospheric nitrogen found in air spaces in the soil into ammonium ions (see diagram above). These bacteria live in the soil and respire aerobically like all the bacteria mentioned so far. Some nitrogen-fixing bacteria have formed a symbiotic relationship with a family of plants that includes peas, beans and clover. These plants are called leguminous plants and have small nodules in their roots that contain the nitrogen-fixing bacteria. The relationship is a mutualistic one as both parties benefit. The plant benefits as these bacteria produce nitrate ions that the plant can absorb and use to make proteins and the bacteria benefit as they are protected from soil predators and have a stable environment in which to live.
4) Denitrifying bacteria
These are anaerobic bacteria that thrive in conditions where there is little oxygen in the soil. This often happens when the soil becomes water-logged so all the air spaces are flooded. Denitrifying bacteria are “bad news” for Nitrogen cycle as they get their energy by taking nitrates from the soil and converting them into nitrogen gas. This obviously reduces the nitrate available to the plants with roots in the soil and this is one of the reasons farmers like to keep soil well aerated for their crops.
I will post a quiz on Nitrogen cycle on Zondle later this evening and so if you want to test your understanding of this potentially tricky topic, I would suggest you have a go at my quiz. There are also plenty of past paper questions on Nitrogen Cycle in the red question booklet.
I will make a blog post on each of the three “cycles” you need to know about for iGCSE. By far the most complicated is the Nitrogen cycle, so we might as well start there….
The first bit of understanding you need to is be clear the difference between how energy moves through an ecosystem and how matter (i.e. atoms) are exchanged between organisms and their environment. Energy in the ecosystems moves in a linear flow: there is no recycling of energy. The energy comes in at one end (in the producers through the process of photosynthesis) and is ultimately all lost as heat to the environment through the process of respiration. There is no possible way energy can be recycled. The “circle of life” that students like so much from Disney certainly does not apply here…. People find this idea very difficult to appreciate. All the time students will tell me that the energy in dead plants and animals goes into the soil and is then absorbed through the roots of plants: “it’s the circle of life sir” they earnestly tell me. And in the words of the late, great Amy Winehouse, I say “NO,NO,NO”…
Matter on the other hand is recycled through the ecosystem. The individual atoms that make up your body (H,O,C,N,S etc.etc,) have all been in other organisms and indeed will be again in the future. You took them in through your food and use these atoms to build the molecules that make up your cells. But ultimately all these atoms will leave your body either through metabolic processes or when you die and are decomposed. You could draw up a cycle for any of the atoms that are found in living things but your specification only requires you to understand two. How are carbon atoms cycled – the Carbon cycle – and how are nitrogen atoms cycled – the Nitrogen cycle. (You will also look at the Water cycle as well…..)
Things to understand about the Nitrogen Cycle:
1) Which molecules in living things contain nitrogen atoms?
Well the answer is fairly simple. Proteins are polymers of amino acids. Amino acids all contain an -NH2 group (amine group) and so Nitrogen is found in proteins. The bases in DNA (Adenine, Cytosine, Guanine and Thymine) are described as nitrogenous bases and so they contain nitrogen too.
2) Where are nitrogen atoms found in the ecosystem other than in the molecules of living organisms?
This is more complicated. Nitrogen gas makes up 78% of the atmosphere so clearly there is a lot of nitrogen in the air. In the soil, there will be urea from the urine of animals and urea contains nitrogen. There are also a range of ions found in the soil that contain nitrogen: the two most important are ammonium NH4+ and nitrate, NO3-. (I don’t know how to do subscript and superscript in WordPress and so you will have to excuse the rather ugly molecular formulae)
3) How do nitrogen atoms move from the abiotic (non-living) parts of the ecosystem and into the organisms?
There is only one way nitrogen atoms can move from the abiotic environment and into the organisms in an ecosystem. This is via plants that can absorb nitrate ions from the soil in their roots. This is a slight simplification but it will do at the moment. Look at the diagram above and find the arrow that shows assimilation of nitrates from the soil into plants.
4) How many different kinds of soil bacteria are involved in cycling nitrogen?
You need to know about four different kinds of bacterial that live in the soil that play a role in recycling nitrogen. Use the diagram above and your notes to describe the role each of these organisms play in the cycling of nitrogen atoms in the ecosystem.
- Decomposers (Putrefying Bacteria)
- Nitrifying Bacteria
- Nitrogen-Fixing Bacteria
- Denitrifying Bacteria
I am off to have my supper: another post about these bacteria will appear here later tonight or tomorrow. Please don’t read it until you have tried to write down a paragraph on each of the four types of bacteria in the bullet point list above.