The 5 Big Time Mutualisms

 The Effect of Global Climate Change on the 5 Big Time Mutualisms

1. Mycorrhizal fungi growing on the roots of land plants

What are mycorrhizal fungi?

    Mycorrhizal fungi are found on the root systems of the vast majority of terrestrial plants. This type of fungi colonize the root system of a host plant and form a symbiotic relationship with the plant. Symbiosis is a mutually beneficial relationship between two organisms. The mycorrhizal fungi increases the water and nutrient, mainly phosphorus, uptake for the host plant ("The Truth About Phosphates and Mycorrhizal Fungi", 2013). In return, the plant provides the fungi with the sugar and energy needed for survival. For example, a large source of energy that a host plant can supply to its mycorrhizal fungi is fixed carbon. 

How does global climate change effect mycorrhizal fungi and their host plants?

    It is speculated that due to the warming climate, there is an increase in soil temperatures. This can be extremely impactful because most nutrient cycling occurs in soil and it is where the mycorrhizal fungi are located. Soil structure is also particularly important as it is the location of water intake and biogeochemical cycling processes, which can greatly affect both the plant and the mycorrhizal fungi (Rillig et al., 2002). Warming temperatures may alter the composition of mycorrhizal fungi, and therefore change the relationship between the fungi and the host plant (Carrenho et al., 2020).

Figure: Dry soil shown on the left and more hydrated soil shown on the right.
Both with mycorrhizal fungi and host plant.

2. Symbiotic nitrogen fixation

    All life depends on fixed nitrogen, but most of the nitrogen in our atmosphere is in the form of unfixed nitrogen gas, N2. Nitrogen fixation occurs through an enzyme called nitrogenase. This enzyme converts atmospheric N2 to ammonia, NH3. Ammonia is used as a nitrogen source for plants and promotes plant growth as well as increases and improves the production of plant seeds and fruits (Caines, 2018). A fun fact is that all nitrogenase on earth is in the form of bacteria ("Nitrogenase", 2022). Nitrogen fixation occurs on land and in the sea. For example on land, rhizobia bacteria in the root nodules of legume hosts fixes nitrogen. On the other hand in the sea, planktonic bacteria are one of the major fixers of nitrogen. 

Rhizobia bacteria in the root nodules of legume hosts.

Trichodesmium is an example of planktonic bacteria. 

Photo credit: John Waterbury, Woods Hole Oceanographic Institution.

What is the first evidence of nitrogen fixation?

    The first evidence of nitrogen fixation is from cyanobacteria, a marine organism. Cyanobacteria are the only prokaryotes that are able to perform oxygenic photosynthesis. They first occurred around 3.5 billion years ago and are found almost everywhere. They are able to fix both carbon and nitrogen. Cyanobacteria are found in many different aquatic habitats and can prosper in diverse and extreme habitats, such as freshwater lakes, oceanic areas, and even in hot springs (Herrero et al., 2001).

What are the global climate change effects on cyanobacteria specifically?

    Global warming enhances toxic algal blooms, also known as eutrophication, in freshwater and marine ecosystems. When there is large nutrient and fertilizer runoff due to heavy precipitation events, it helps promote the growth of cyanobacteria (“Climate Change and Cyanobacteria (Blue-Green Algae)”, 2018). Additionally, warming ocean temperatures promotes cyanobacterial growth as cyanobacteria growth rate is optimized at higher temperatures. Warming ocean temperatures also increases stratification, in which warm surface waters float on top of colder water layers. This allows for nutrients to travel to the top of the water column from the bottom and helps promote cyanobacterial blooms (“Climate Change and Cyanobacteria (Blue-Green Algae)”, 2018). 

How does global climate change effect symbiotic nitrogen fixation in general?

    Through the Haber-Bosch process and climate change, we are overwhelming denitrification processes. This symbiotic relationship between plants and nitrogen fixing bacteria can be negatively affected as nutrient cycling processes are overwhelmed. The Haber-Bosch process is when nitrogen is fixed artificially to be used in fertilizers ("Haber process", 2022). The process of denitrification sends nitrous oxide back into the atmosphere. 

3. Pollination of land plants by animals

    About 80-90% of plants rely on animals to carry their pollen from the male plant to the female plant ("Animal Pollination", n.d.). In order to entice animals to transfer their pollen from one plant to another, plants use nectar which is a concentrated sugar solution. This benefits the animal because they can gain energy from consuming nectar, and it also benefits the plant because it can help ensure future generations. Common examples of these animal carriers are bees, hummingbirds, bats, and even humans. In other forms of pollination, plant pollen can be dispersed by wind or water. 

How does global climate change effect the pollination of land plants by animals?

    Global climate change effects both plant species and species interactions between plants and animals. This can be detrimental because pollination by insects is crucial and necessary for the success of 75% of all food crops ("What is a pollinator?", n.d.). Without insects, may plant species populations would decline.

    Global warming may also affect the livelihoods of animal species. For example, bumblebees and butterflies have a climate niche in which they are able to survive, and global warming may lead to narrower climate niches or local and regional extinction of animal species (Kerr et al., 2015). Warming may also lead to earlier pollination and growing times for land plants, which may not coincide with some animal's hatching time (University of Würzburg, 2019). 

4. Seed dispersal of land plants by animals

    Seeds can be dispersed from land plants through wind, water, gravity, and animals. Seed dispersal is important for the viability, livelihood, and biodiversity of many terrestrial plants. Also some plants rely on seed dispersal to decrease intraspecific competition. Intraspecific competition is competition between individuals of the same species. Animals are attracted to the fruit of the land plant and after consuming it, will leave the seed among their waste product. This allows for the seed to germinate and grow. Some examples of seed dispersal of land plants by animals are through birds, squirrels, humans, and ants.

What is myrmecochory?

    Ants are great at dispersing seeds as worker ants bring back seeds to their ant colony, a term called myrmecochory ("Myrmecochory", 2022). The elaiosome is a structure of dead cells and lots of lipids/fats that is attached to a seed (Ellis, 2021). This structure provides a lot of nutrients such as starch, proteins, and vitamins to those that consume it. A reason that seeds of plants may have this structure is to entice animals like ants to aid in dispersal (Ellis, 2021). After the worker ants bring the seed back to their colony, the elaiosome is consumed and the rest of the seed is discarded somewhere in the ant colony ("Myrmecochory", 2022). This allows for the seed to begin to germinate and eventually sprout. This type of relationship between the ants and plant seeds is mutualistic, meaning that both species benefit in some way. This phenomenon is extremely interesting as more than 3,000 plant species around the world exhibit myrmecochory ("Myrmecochory", 2022). 

The white part of the seed shown is known as the elaiosome.

What is the effect of global climate change on seed dispersal of land plants by animals?

    Due to rapid climate change that is ongoing in our world today, plant populations and the dispersal of their seeds have been negatively impacted as species need to move longer distances in order to stay in their climate niche. It is also difficult because seed dispersal is dependent on animal populations, which are also negatively impacted by climate change. This is because the warming climate may lead to a decline in overall biodiversity. 

5. Coral symbioses

    Coral reefs are extremely important ecosystems and many marine organisms rely on coral reefs. These amazing reefs provide habitat for many marine species and can also provide shoreline protection by acting as a buffer for waves and storms (NOAA, 2014). This can help immensely in preventing coastal erosion. When coral reefs are degraded or destroyed, coastal communities can be greatly damaged by wave action and effects from storms. Many cities and towns are located near the coast, which can be negatively impacted if coral ecosystems and this natural barrier are damaged. 

Coral reefs protect the shore line. Photo credit: Malcom Peacey/Flickr

 through a Creative Commons license.

    Many food webs are also largely dependent on coral reef systems. A term that is applicable to both terrestrial and aquatic ecosystems is trophic cascade. These are indirect interactions between organisms that can affect and alter entire ecosystems (Silliman et al., 2012). For example, a top-down trophic cascade is essentially when the population of the top predator controls the population of the primary producer ("Trophic Cascade", 2021). When populations of predators are large enough to reduce the abundance of their prey, they can then increase the populations of a primary producer that the prey usually feeds on (a positive indirect effect). On the other hand, if populations of predators are too low, then the prey population thrives and the primary producer population decreases. For bottom-up cascades, the primary producer population controls the populations of higher trophic levels. Some examples of primary producers are phytoplankton, zooplankton, terrestrial and aquatic plants, and bacteria ("Trophic Cascade", 2021). A classic example of a trophic cascade that many may have heard of is the example with sea otters, sea urchins, and kelp. Sea otters are a top predator and consume their prey, sea urchins. Kelp are the main diet of sea urchins and when populations of sea urchins are not kept in check, many kelp forests can become quickly destroyed. As populations of sea otters are growing, sea urchin populations are decreasing and more kelp forests are being conserved.

An example of a three-level trophic cascade. An indirect effect is indicated

by a dotted line, while a direct effect is indicated with a solid line.

What are zooxanthellae?

    The livelihood of coral reefs is largely and solely dependent on their symbiosis with zooxanthellae. This marine organism is a type of algae that lives inside coral polyps, a colony of tiny creatures that comprise a coral reef structure (NOAA, 2013). Coral polyps are related to anemones and jellyfish. Coral reefs and zooxanthellae have a symbiotic relationship as 90% of the energy needed for corals are from fixed carbon done by zooxanthellae, and this algae is the reason for the unique colors of different corals. Additionally, zooxanthellae allows for the coral to produce calcium carbonate from their photosynthesis products (NOAA, 2013). This is especially important because coral reefs require calcium carbonate to build their hard skeletons. Meanwhile, the coral provides the algae with a protected environment as well as nutrients and compounds needed for photosynthesis.  

                

The image on the left shows zooxanthellae and the image on the right shows the

 coloration that this algae provides for corals.

What other coral symbioses are there?

    Crustaceans and corals are another example of a symbiotic and mutualistic relationship. Tiny crustaceans inhabit coral structures because these structures provide protection and nutrients to these small organisms (ReefCause, 2021). In return, the crustaceans protect coral structures from predation, are able to remove unwanted sediments and algae, and are able to repel predators through chemical signals. Crustaceans are able to use their claws to snip at looming predators of corals which deters them from trying to consume the coral structure (Glynn, 1980). 

What are the global climate change effects on coral symbioses?

    One of the largest global climate change effects on coral symbioses is coral bleaching. Coral bleaching occurs when zooxanthellae are expelled out of the coral due to the stress of warming ocean temperatures (Hancock, n.d.). This expulsion of the algae causes the coral to lose all of its color and turn white, therefore becoming "bleached". Coral bleaching causes marine biodiversity to decline as many marine organisms depend on coral reef ecosystems. This can be extremely detrimental because some at-risk species may be facing extinction if coral reef ecosystems collapse (Hancock, n.d.). Decreasing coral populations also impact humans as many communities rely on reef fish for food security. If these food webs are impacted by global climate change on coral reefs, then humans will also be negatively impacted. Long-term bleaching causes coral populations to decline greatly and the recovery of these reefs can take decades. 

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