Coral reefs are incredibly biodiverse ecosystems, harboring sponges and seastars, octopi and sharks. The reefs themselves are built up over thousands of years by hard corals with the help of their symbiotic partner, Symbiodinium. Symbiodinium are basically a single-celled algae that lives within the coral tissue – a symbiont. Corals harbor Symbiodinium from herbivores and a harsh environment, and provide their symbionts with nutrients such as phosphorus and nitrogen. In return, the Symbiodinium photosynthesize, creating useable carbon from sunlight and giving nutrients back to the coral. Thus, both species benefit from the interaction. Ecologists call this type of positive-positive interaction a mutualism.
There are hundreds of coral species and potentially hundreds of Symbiodinium “types”. Each coral and Symbiodinium has its own characteristics: where it likes to live, how it responds to the environment, and how cooperative it is with mutualistic partners. Because of this variability, each potential pairing has different characteristics and some partnerships might be more effective than others. For instance, certain Symbiodinium are better photosynthesizers, while others can withstand environmental disturbances like increased water temperatures and prevent corals from bleaching. As a starting point, we need to know which pairings are possible and why.
In a new article in the journal PLoS One, I use social network theory to ask:
- Which corals have very few Symbiodinium partners and which corals have many?
- Are there specialist (few partners) and generalist (many partners) Symbiodinium?
- Do any coral traits influence interaction patterns within coral-Symbiodinium social networks?
To answer these questions, I worked with a research group at the Hawaii Institute of Marine Biology (there are very few coral reefs in Davis, CA). Hollie Putnam, Erik Franklin, Michael Stat, and Ruth Gates had assembled a large database of coral-Symbiodinium interactions that they named GeoSymbio. The data within GeoSymbio comes from locations around the world, including Hawaii, Thailand, the Caribbean, and the famous Great Barrier Reef in Australia. We created social networks to show the connections between corals and Symbiodinium, much in the same way that I have connections to my friends on Facebook.
We found that most corals have very few Symbiodinium partners, but that a handful of corals have many, many symbionts. This was important because it’s generally believed that most corals have few symbiont options to choose from. We also found that transmission mode was important in these communities: young corals get their Symbiodinium from their parents or from the environment, but not both. Symbiodinium types were usually passed on through only one of these strategies, nearly dividing the networks in half. This is like saying that my family members are “friends” on Facebook and my friends are “friends” on Facebook, but there are no connections between these groups.
Our research is important to understanding the basic interaction patterns between corals and Symbiodinium. With a greater understanding of this important mutualistic interaction, we’ll be better equipped to protect coral reef communities – and the species they support.
You can find our paper here.