How many symbionts does it take to make a happy coral?

 McIlroy, S. E., terHorst, C. P., Teece, M., & Coffroth, M. A. (2022). Nutrient dynamics in coral symbiosis depend on both the relative and absolute abundance of Symbiodiniaceae species. Microbiome, 10(1), 1-13.

Corals get their energy from photosynthetic algae that live inside their tissues. In turn, these symbiotic microbes receive nutrients—such as carbon and nitrogen—from the host coral, creating a mutually beneficial relationship. The success of the symbiosis depends on the efficient exchange of nutrients between coral and algae. However, not all symbionts are created equal! Many different species of algae colonize corals, and corals often host a diverse community of different microbes rather than just a single species. This means that the presence or absence of a given symbiont isn’t the only factor that might impact the host—the composition of the symbiont community matters as well. Community-scale features, such as the relative abundance of different species, might have important consequences for the host as well. 

Symbionts do not exist passively, but interact and compete with one another for nutrients. For example, symbionts can shift their metabolism when exposed to a potential competitor, investing more energy towards their own needs and less towards producing energy for the coral. This competition between different species can create complicated dynamics, which scientists are only beginning to understand. From the coral’s perspective, these competitive interactions might negatively affect the quality of the symbiosis. Natural selection might promote corals to limit associations with “selfish”  symbionts in favor of others, thus avoiding conflict and maintaining stability. Some organisms, such as plants with symbiotic fungi, can choose to give more resources to certain symbionts. Far from passive coexistence, plants reward  symbionts that  cooperate with or benefit them, and conversely, punish those that act “selfishly”. It is unclear if corals can do the same.

Briareum asbestinum. Image source: Karol Jakubec, via Wikimedia Commons. 

Do the symbionts have chemistry?

In a recent study, biologists wanted to understand how the composition of the symbiont community—i.e. the relative abundance of different species of algae—affected the exchange of carbon and nitrogen between hosts and symbionts. To do this, they exposed corals to two different species of algae in different proportions and densities. One of the species was known to be more “selfish” than the other. The researchers used a technique called “stable isotope analysis”, which allowed them to track the exchange of carbon and nitrogen. To understand this technique, it’s important to remember that atoms can come in different flavours depending on their weight—these atomic flavours are called isotopes. Atomic weight isn’t dependent on pounds, but on the number of particles called protons or neutrons. For example, nitrogen, which has 7 protons, can either come with either 7 or 8 neutrons. This results in nitrogen with a molecular weight of either 14 or 15, with nitrogen-14 being much more common. In this case, the scientists gave the coral a bunch of nitrogen-15, which they were then able to trace over time against a background of nitrogen-14. 

Living with roommates can be complicated

Cells of symbiotic algae (Symbiodinium). Image source: Todd LaJeunesse, via Wikimedia Commons, CC-BY-2.0

The scientists noted a variety of differences between the two groups of coral, depending on the relative proportions of the two symbiont species as well as their density (the amount of total symbionts). The usefulness of a symbiont to a host wasn’t easily generalizable. For example, communities with a lot of one species (Breviolum minutum) produced more organic carbon for both the host and the symbiont… but only above a certain density threshold. Patterns of nitrogen usage were even more complicated. One algae (Symbiodinium microadriaticum) provided more nitrogen to the host at low densities, but became more “selfish” and provided less nitrogen at higher densities. Therefore, for denser algal communities, B. minutum provided the most nitrogen and the greatest benefit to the host.

So, what does this all mean? This suggests that no one microbe can be uniformly characterized as “selfish” or “beneficial”— different species provide different benefits under changing conditions. The complex interactions observed between microbes and corals suggest that corals could have a far more dynamic relationship with their symbionts. For example, corals could regulate their symbiont communities to maximize success by switching between different symbionts throughout their decades-long lives. Usually, corals take up symbionts from the external seawater as young adults. At this early stage, algal densities might be low and S. microadriaticum might provide the most nitrogen. However, as the coral grows and the density of symbiotic microbes increases, it may want to switch to B. minutum to maximize the benefits of symbiosis. 

Maintaining healthy relationships isn’t just a matter of getting along for corals—it can be crucial to their survival. Many tropical reefs are nutrient-poor, or “oligotrophic.” In such conditions, nutrients—in this case nitrogen—often limit the amount of photosynthesis that can take place. Therefore, corals benefit from maintaining healthy relationships between their microbial tenants.



Cover photo by Jim Maragos/U.S. Fish and Wildlife Service, via Wikimedia Commons, CC-BY-2.0.

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