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How microbes affect global processes

In Week 1, you learnt about the many different types of nutrients microbes need to grow and reproduce, and how nutrient availability and environmental conditions affect where microbes live. In this Step, you’ll explore the key roles microbes play in global processes that affect food chains, our atmosphere and climate.

It is worth thinking about viruses for a moment. Cellular microbes are recognised for the vital role they play in decomposing dead organisms and recycling nutrients in the food chain, but the role of viruses in global nutrient cycling is often overlooked because they don’t have their own metabolism. Viruses kill trillions upon trillions of organisms (mainly bacteria and archaea in our oceans) every day, and microbiologists are only just beginning to understand how this impacts global nutrient cycling.

The global carbon cycle

Microbes affect the flow of carbon in food chains and influence our climate by altering the levels of carbon dioxide (CO2) and methane (CH4), potent greenhouse gases, through the processes of photosynthesis, respiration and methanogenesis.

Wood and fossil fuels burning, arrow to carbon dioxide, to photosynthes, to cyanobacteria, algae, plants, arrow through consumption to animals, arrow up through respiration and back to carbon dioxide. In this cycle, from animals, a separate arrow down to soil and water microbes through excretion decomposition, to assimilation back up to plants, which cycles down to soil and water microbes as well as a separate arrow to sequestered and back up to wood and fossil fuels. Arrow from carbon dioxide to photosynthesis, to respiration to carbon dioxide. Carbon dioxide to dissolved carbon dioxide to photosynthesis to cyanbacteria, algae, plants under sea arrow through consumption to marine microbes and aquatic animals arrow to mineralized and separate arrow to sequestered which goes to wood and fossil fuels burning. Arrow also from cyanobacteria, algae, plants to decomposition to DOC. DOC arrow down to marine microbes and aquatic animals through assimilation, arrow up to dissolved carbon dioxide through respiration soil and water microbes to respiration to carbon dioxide.

Figure 1: Microbes play a vital role in the global carbon cycle. DOC: Dissolved organic carbon © University of Reading

In our oceans, photosynthetic microbes (cyanobacteria and algae, such as diatoms and coccolithophores) fix more CO2 than all the plants on land and produce oxygen as a by-product. When they die, some of the carbon they fixed is released into the ocean as dissolved organic carbon (DOC) that acts as a food source for non-photosynthetic marine microbes. They release some of the carbon back into the ocean as dissolved CO2 via respiration, some is recycled back to DOC when they are killed by viruses and the rest enters the marine food web when they are eaten by other marine organisms. Any DOC that is not consumed by marine microbes remains in the ocean carbon reservoir and slowly sediments to the ocean floor where it mineralizes to form rock: a carbon sink that lasts for millions of years. This process is called the Microbial Carbon Pump (MCP).

A set of drawings by Ernst Haeckel

Figure 2: Ernst Haeckel published beautiful drawings of diatoms in his book Kunstformen der Natur (Art forms of nature) in 1900. Diatoms are photosynthetic protists (algae) that live in aquatic environments. They have tough cell walls, composed mainly of silica, with intricate patterns © Ernst Haeckel [Public Domain]

Diatoms are photosynthetic protists (algae) that live in aquatic environments. They have tough cell walls, composed mainly of silica, with intricate patterns.

Some microbes (called methanogens) produce methane as a by-product of metabolism. All known methanogens are archaea; most are obligate anaerobes and many are extremophiles.

Diagram showing the placement of the abomasum, small intestine, esophagus, reticulum, omasum and rumen

Figure 3: Methanogenic archaea live in the rumen, a section of the ruminant digestive tract. © Pearson Scott Foresman [Public domain]

Several species of methanogen, such as Methanosarcina, live in the digestive tract of ruminant animals (cows, sheep, goats). The increased demand for meat production worldwide has led to a huge increase in methane from agriculture contributing to global climate change. You can find out more about this topic in another University of Reading online course on FutureLearn called the Future of Farming.

The global nitrogen cycle

Dinitrogen (N2) is the most abundant gas in the atmosphere (78%), yet only nitrogen-fixing microbes are able to use it. Some nitrogen-fixing bacteria (eg Rhizobium) form mutualistic symbioses and donate almost all the nitrogen they fix to their legume plant hosts in the form of ammonia, others are free-living in the environment. Nitrifying microbes transform ammonia to nitrites, or nitrites to nitrates, which are then absorbed by plants. Denitrifying bacteria are responsible for the loss of nitrogen from the biosphere, converting nitrates to N2.

Atmospheric nitrogen arrow to nitrogen-fixing bacteria living in legume root nodules. Arrow also points to (ammonification) Ammonium arrow goes to (Nitrification) Nitrates arrow goes on to Nitrates

Figure 4: Microbes play a vital role in the global nitrogen cycle © University of Reading

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Small and Mighty: Introduction to Microbiology

University of Reading

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