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An introduction to symbiosis

General overview of symbiosis, where it's found and the variety of roles that symbionts play.
The word ‘symbiosis’ come from the Greek for ‘living together’, and is used across biology to describe close associations across the animal kingdom, from the bobtail squid that uses fluorescent symbiotic bacteria to help it avoid predators, to humans that take probiotics in order to feed their good bacteria. But symbioses are more ancient than humans and squid. A symbiosis is thought to have kickstarted all complex life on earth. Around 1.5 billion years ago, two cells came into close contact, an alphaproteobacterium and an archaeon. The bacterium was captured by the archaeon, and this would go on to become fixed within the host and form the organelle of modern eukaryotes known as mitochondria. This process is known as endosymbiosis.
A similar endosymbiosis event signalled the origins of chloroplasts, too. But this time, the captured microbe was likely a species of cyanobacteria. From here, all complex eukaryotic life evolved. Symbioses establish continuously and repeatedly. For example, a huge range of bacterial species are found within insects. They influence how well the insect is able to survive and reproduce . Some species of bacteria, like Wolbachia, are even able to affect their own spread through an insect population by exerting a range of effects
including: cytoplasmic incompatibility, feminisation, male killing and sterilisation. Scientists hope that these symbiotic bacteria, often essential for the insect, might be harnessed to reduce the spread of disease or the crop damage caused by pests. But it’s not just within the animal kingdom that symbioses are found. Plants have been harnessing the power of microbes for millions of years, and the nitrogen cycle is one of the clearest examples of this.
There are four distinct stages of the nitrogen cycle: ammonification, nitrification, nitrogen fixation and denitrification. You’ll find that microbes are vital at every stage. In ammonification, ammonia is produced from organic compounds that contain nitrogen, like urea, proteins or vitamins. These compounds may come from the decomposition of dead organisms. In the second stage of the cycle, ammonium ions are converted to nitrate ions. This is carried out by nitrifying bacteria in the presence of oxygen. There are two main types of bacteria involved in the third stage, nitrogen fixation. The bacteria can either be free-living, or mutualistic, meaning they are in symbiosis with the plant.
These mutualistic microbes live in the root nodules of beans and peas and have a two-way symbiotic relationship with the plant. They provide the plant with amino acids during nitrogen fixation, and receive carbohydrates from the plant in return. The fixing of nitrogen is energy consuming but the bacteria provide a benefit to the plant by doing the hard work for them. The final stage, denitrification, is dominated by anaerobic bacteria that convert nitrate in the soil into gaseous nitrogen.

Mutually beneficial relationships can lead to something wonderful. The pivotal development of symbiosis between prokaryotic archaea and bacteria led to the creation of single-celled eukaryotes… cells that were more efficient at harvesting energy, and so could grow larger and more complex. Single celled eukaryotes could then specialise further and develop into multicellular plants and animals. Symbiosis is all around us and has played a major role in shaping the organisms of today.

In this video Rebecca Hall tells us something about the importance of symbiosis in biology.

Here are some questions to consider while you’re watching the video. We’d be happy to hear your views in the comments once you’ve watched the video

  • How do you think scientists might have figured out that mitochondria were once free-living bacteria?
  • If insects benefit by having gut symbionts provide them with metabolites, how might these microbes benefit in return?
  • Do you think there could be a cost to the host associated with having symbiont bacteria?
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The Biology of Bugs, Brains, and Beasts

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