Living Together: Symbiosis

Look closely at a tree trunk or a rock in a Virginia park. Some of the gray-green crusty patches you see are lichens, and each lichen is a partnership of very different living things growing as one body.

One partner is a fungus. The other is a tiny green alga or a cyanobacterium, a kind of bacterium that can make its own food. Why would two different living things share one body? And do living things that live close together always help each other? First, decide what you think now.

Explore: pair up two species

In this simulation you see two species and a bar for each population over 20 weeks. You choose how living together affects each species: helped, unaffected or harmed. Then you watch the bars change and name the relationship. You can also pick a real pair, such as the fungus and alga in a lichen, a lichen on a tree, or a tick on a deer.

Try this.

1. Pick the lichen fungus and alga. Predict whether both bars will rise, then run it.
2. Set species A to helped and species B to harmed. Predict what happens to each bar, and what to call the pair.
3. Set both species to harmed. Predict whether this matches any kind of symbiosis, then check the message.

Explain: what symbiosis means

Symbiosis is a close relationship between individuals of different species living together. The partners stay together for a long time. A bird that eats one seed and flies off is not in a symbiosis with the plant.

A lichen is a living thing made of a fungus and an alga or a cyanobacterium living as one body. They stay two different species, but they share one body. Here is what each partner gives:

• The fungus forms the outside of the lichen. It shelters its partner and helps keep it moist.
• The lichen body, made mostly of the fungus, soaks up water from rain and from the air.
• The alga or cyanobacterium makes sugar by photosynthesis. That food feeds both partners.

This two-partner picture is a model. In 2016, scientists found yeast cells, another kind of fungus, living inside some lichens too. What the yeast does there is still being studied.

Explain: three kinds of symbiosis

Scientists sort symbiosis by asking what each partner gains or loses. There are three kinds.

• Mutualism is symbiosis in which all the partners benefit.
• Commensalism is symbiosis in which one partner benefits and the others are unaffected.
• Parasitism is symbiosis in which one partner benefits and the others are harmed.

In parasitism, the host is the living thing that the other partner lives on or in. The parasite is the partner that benefits by feeding on its host.

Look at the two lichen rows. The fungus and alga inside the lichen are one relationship. The lichen and the tree are a different one. Experts say ordinary lichens on bark almost always do not harm the tree.

The soybean bacteria turn nitrogen from the air into a form the plant can use. You met them in Nitrogen for Life.

Blacklegged ticks live in Virginia. A tick can hold onto its host and drink blood for several days. Some hosts get irritated skin around a bite.

Explain: why partnerships last

A parasite is not the same as a predator. A predator kills and eats its prey quickly. A parasite usually lives on or in its host and feeds over a long time, often while the host stays alive. You can compare them in Predators and Prey.

Partners do not help each other on purpose. A fungus cannot decide to be kind. Over many generations, partners that lived together survived and reproduced better than ones that did not. Their offspring kept living the same way, so the relationship carried on.

That is how symbiosis supports the survival of different species. A lichen can grow on bare rock, where the alga alone might dry out and the fungus alone could not make its own food.

Elaborate: does a root fungus help a plant grow?

Many plants grow with a fungus on their roots. The fungus brings minerals from the soil into the roots. The plant passes food it made to the fungus. Does the plant really grow more with this partner?

These are means from a pot experiment by Asrar and Elhindi, published in 2011. They grew Jubilee marigolds in a greenhouse at King Saud University in Riyadh, Saudi Arabia. Seeds were sown in September 2009. The shoots were weighed as dry mass six weeks after the seedlings were moved into pots. The table shows only plant growth, not how many plants lived.

1. At 25% water, how many grams more did plants with the fungus have? What percent is that of the mass without the fungus?
2. As the water drops from 100% to 25%, what happens to shoot mass in both groups?
3. At which water level is the gap between the two groups smallest?

Design problem

Plan an investigation to test whether a root fungus helps bean plants grow. Name your variables, your control, what you keep the same, how many plants you use, what you measure in metric units, and how you keep it safe.

Evaluate: check your thinking

Those gray-green crusty patches can be lichens: a fungus and an alga or cyanobacterium living as one body. Both partners gain, so that is mutualism. Living close together does not always help, though. A lichen on bark seems to leave the tree unaffected, and a tick harms the deer it feeds on.

Science practices you used: predicting and testing with a model, sorting examples into groups, reading a data table and finding a difference and a percent, planning a fair test with a control and many plants, and naming the limits of one study.