Mutualisms can improve the acquisition of water, nutrients, and places to live

We can categorize mutualisms in several ways. For example, some mutualists are generalists, which means that one species interacts with many other species. Other mutualists are specialists, which means that one species interacts with either one other species or a few closely related species. When two species provide fitness benefits to each other and require each other to persist, we call them obligate mutualists. We saw an example of this in Chapter 1 when we discussed the tubeworms and chemosynthetic bacteria that live together near deep-sea hydrothermal vents. Tubeworms provide a place for bacteria to live and bacteria provide food for the tubeworms; neither species can survive without the other. In contrast, facultative mutualists provide fitness benefits to each other, but the interaction is not critical to the persistence of either species. For example, a group of tiny insects known as aphids suck the sap from plants and produce a droplet rich in carbohydrates that is consumed by several species of ants. The ants gain a source of food and in exchange they protect the aphids from predators. Although both groups benefit, it is a facultative mutualism because each can persist without the other. A mutualism between two species can be composed of two obligate mutualists, one obligate and one facultative mutualist, or two facultative mutualists.

One of the most common functions of mutualisms is to help species acquire resources they need, such as water, nutrients, and a place to live. In previous chapters we have discussed a few examples of such mutualisms. In Chapter 1 we saw that lichens are composed of a fungus living with either green algal cells or cyanobacteria (see Figure 1.13). This fungus provides the algae with water, CO₂ from fungal respiration, and nutrients and, in exchange, the algae provide the fungus with carbohydrates from photosynthesis. Similarly, in Chapter 2 we discussed how corals provide a home for photosynthetic algae known as zooxanthellae. As you can see in Figure 17.1, the coral catches bits of food with its tentacles and during digestion the coral emits CO₂, which the algae use during photosynthesis. The algae then produce sugars and O₂, some of which can be consumed by the coral. Other animals also incorporate symbiotic algae into their bodies. Similarly, in Chapter 2 we discussed how the eggs of spotted salamanders incorporate algae in the tissues of the embryo. Similarly, when the green sea slug (Elysia chlorotica) consumes algae, it stores the chloroplasts from the algae inside its tissues and thus gains most of its energy through photosynthesis (Figure 17.2). In this section, we will review how numerous species of plants, animals, fungi, and bacteria interact in mutualisms to gain water, nutrients, and places to live.

Figure 17.1 A mutualism between coral and zooxanthellae. A coral grabs food with its tentacles, which contain stinging cells, and pulls the food into its mouth. Zooxanthellae algae live along the surface of a coral’s tentacles where they obtain sunlight for photosynthesis. Corals provide a place for algae to live and emit CO₂ that algae use in photosynthesis. As algae photosynthesize, they provide sugars and O₂ that corals consume.

Figure 17.2 The green sea slug. When the slug first hatches from an egg, it is brown. However, as it begins to eat algae, it stores the chloroplasts from the algae in its own tissues. As the slug accumulates a large number of chloroplasts, its body turns green and it is able to acquire most of its energy through photosynthesis rather than through herbivory.

Photo by Nicholas E. Curtis and Ray Martinez, University of South Florida.

Acquisition of Resources in Plants

Although plants obtain water and soil minerals through their root systems, ecologists have learned that many plants also rely on mutualisms with fungi and bacteria to help them obtain nutrients.

Plants and Fungi

Fungi that surround plant roots and help plants obtain water and minerals are known as mycorrhizal fungi. The network of fungal hyphae provide plants with minerals such as nitrogen and phosphorus and with water from the surrounding soil. Plants provide the fungi with the sugars produced by photosynthesis. Because fungi can increase the amount of minerals obtained by the plants, they are able to increase the plants’ tolerance to both drought and salt stress. They can also help plants combat infections from pathogens. This mutualism is valuable to many species of plants, including plants that humans use for food and materials.

Mycorrhizal fungi can be divided into either endomycorrhizal fungi or ectomycorrhizal fungi. Endomycorrhizal fungi, illustrated in Figure 17.3a, are characterized by hyphal threads that extend far out into the soil and penetrate root cells between the cell wall and the cell membrane. One fungal species can commonly infect multiple plant species. There are several types of endomycorrhizal fungi. The most common type is arbuscular mycorrhizal fungi, which comprise a group of mycorrhizae that infects a tremendous number of plants including grasses and apple, peach, and coffee trees. Arbuscules are branching hyphal structures found within plant cells that help the fungus provide nutrients to the plant.

Figure 17.3 Mycorrhizal fungi. Mycorrhizal fungi can be categorized as endomycorrhizal fungi or ectomycorrhizal fungi. (a) Endomycorrhizal fungi have hyphae that penetrate the root cells of plants and reside between the cell wall and cell membrane. (b) Ectomycorrhizal fungi have hyphae that do not penetrate the root cells but instead grow between the root cells of plants.

Ectomycorrhizal fungi, illustrated in Figure 17.3b, are characterized by hyphae that surround the roots of plants and enter between root cells but rarely enter the cells. These fungi are currently known to live only in mutualistic relationships with trees and shrubs. Species of ectomycorrhizal fungi also tend to form mutualistic relationships with fewer plant species than do endomycorrhizal fungi.

The mutualistic relationship between plants and mycorrhizal fungi goes back more than 450 mya to the time when plants first evolved to live on land. This ancient interaction between the ancestral plants and fungi probably explains why so many modern species of plants and fungi continue to interact as mutualists. Researchers currently estimate that mutualism between plants and fungi involves more than 6,000 species of mycorrhizal fungi and 200,000 species of plants, which is about two-thirds of all plant species.

Plants and Bacteria

In some cases, mutualistic interactions between plants and bacteria convert unusable forms of minerals into forms that plants can use. One of the best-known examples of this is the group of bacteria in the genus Rhizobium that live in a mutualistic relationship with numerous species of legumes, including important crops such as beans, peas (Pisum sativum), and alfalfa (Medicago sativa). When legumes detect the presence of Rhizobium bacteria in the soil or when bacteria enter the plant through an opening in the root, the plant develops small nodules that surround the bacteria on the roots and provide them with a place to live (Figure 17.4). Plants also provide the bacteria with the products of photosynthesis. In exchange, the bacteria do something the plant cannot do; they convert atmospheric nitrogen—a form of nitrogen plants cannot use—into ammonia, a form of nitrogen plants can readily use. This mutualism can be quite valuable to plants, especially when they are living in areas of low soil fertility. We will discuss this phenomenon in more detail in Chapter 21.

Figure 17.4 Root nodules that contain Rhizobium bacteria. Legumes such as this bean plant can enter into a mutualistic relationship with Rhizobium bacteria. The plant provides the bacteria with sugars from photosynthesis and a root nodule in which the bacteria can grow. In return, the bacteria convert atmospheric nitrogen to ammonia, a form of nitrogen that the plant can use.

Photo by Nigel Cattlin/Science Source.

Acquisition of Resources in Animals

Animals also use a variety of organisms to help them obtain food and habitat. These interactions range from protozoans living in animals to mutualisms between animals.

Animals and Protozoans

Termites are a group of insects that consume wood, which is difficult to digest since it is composed largely of lignin and cellulose. To assist in this effort, species of protozoa that are able to consume lignin and cellulose live in termite guts. The protozoa receive a home in the termite gut with a constant source of food from wood that the termite consumes and, in exchange, the termite receives nutrients from the waste products of protozoan digestion. Many other animals also contain microbes in their digestive system. Humans, for example, host hundreds of species of microbes—bacteria, fungi, and protozoa—that largely seem to be beneficial. In fact, a person’s digestive system contains 10 times more bacterial cells—from more than 500 bacterial species—than the total number of human cells in that person’s body.

Mutualisms Between Animals

Mutualisms for acquiring resources can also occur between two species of animals. A fascinating example occurs between humans and a bird known as the greater honeyguide (Indicator indicator). For centuries, people in Africa have consumed the honey produced by bees, but locating the beehives is a challenge. While the greater honeyguide likes to consume bee larvae and bee’s wax, it has a hard time getting into beehives. The local people and the honeyguide both obtain resources by working together. Over time, local people have learned to use calls to attract the attention of the bird and then follow the bird to the beehive. Along the way, the bird stops to perch in nearby trees. As the bird gets closer to the beehive, it flies shorter distances from one perch to the next and perches lower in trees, as shown in Figure 17.5. Local people have learned how to interpret the bird’s behavior and they follow it to the hive. When they find the hive, they scoop out the honey and leave pieces of the honeycomb with beeswax and bee larvae on the ground for the honeyguide to consume. It is thought that the honeyguide may have originally evolved this behavior as a mutualism with other honey-consuming mammals, such as the honey badger (Mellivora capensis).

Figure 17.5 The greater honeyguide. When the greater honeyguide leads humans or other honey-consuming mammals to a beehive, the bird benefits by consuming the discarded honeycombs that contain beeswax and bee larvae. Humans have learned that as the greater honeyguide gets close to a beehive, (a) the bird flies shorter distances between stops and (b) at each stop it perches lower on trees. Error bars are standard deviations.

Data from H. A. Isack and H.-U. Reyer, Honeyguides and honey gatherers: Interspecific communication in a symbiotic relationship, Science 243 (1989): 1343–1246.

Some animals provide a habitat for other animals in exchange for reciprocal benefits. For example, alpheid shrimp live in the ocean and have very poor vision. They burrow into the sand and allow a group of fish known as gobies to share their burrows. In contrast to shrimp, gobies have excellent vision and are able to see shrimp predators. In exchange for receiving a burrow, a goby allows a shrimp to stay in close contact by permitting the shrimp to place an antenna on it once the goby leaves the burrow. If the goby sees a predator, it warns the shrimp by twitching. The shrimp detects the twitching via its antenna and heads back into the burrow for protection (Figure 17.6).

Figure 17.6 Habitat mutualist. The alpheid shrimp (Alpheus randalli) digs a burrow that it shares with a fish known as the pinkbar goby (Amblyeleotris aurora) in the waters off Indonesia. In exchange for this habitat, the goby allows the shrimp to place an antenna on its body so that the shrimp can feel the goby twitching when it sees an approaching predator.

Photo by Jim Greenfield/imagequestmarine.com.