Hello all! The day has finally come. My very last blog post on our favorite zooxanthellae, Symbiodinium. This week we will be discussing what I find to be the most interesting thing about these tiny algae, its role in a successful coral reef. In my last post, I went over how coral and Symbiodinium work together in mutualistic symbiosis to cycle essential nutrients for each other’s metabolism (NOAA, 2017). These two organisms must maintain a balance in the delivery of nutrients to one another based on the environmental conditions at any given time. Coral even have the ability to regulate the number of Symbiodinium present in their cells to provide for their metabolic needs. This arrangement seems to work out pretty well until coral gets stressed. What stresses out a coral you ask? Current events.
(I had to do it. I’m sorry)
Let’s get back to business. So, the real culprit is something called photo-oxidative stress. The gist is that sea temperatures rise above what the algae and coral are accustomed to. The excess heat and light damage the photosynthetic machinery in Symbiodinium. Ordinarily, the alga would produce usable oxygen for the coral to put into cell respiration. However, under photo-oxidative stress, a toxic form of oxygen called reactive oxygen species (ROS) leaks out of the algae and into the host cells, causing damage. The coral kicks out the Symbiodinium to try and protect itself from this toxic oxygen (Nielsen, Petrou, & Gates, 2018).
This is where we bring in the term “coral bleaching.” If you will remember, in my very first blog post I talked about how it is the zooxanthellae that usually give coral reefs their bright pigmentation (Coral Polyps. n.d.). If a host coral has to expel too many of its algae counterparts, it loses its color and therefore looks bleached. The coral cannot survive without their symbiotic partners and the reef will eventually die if the ocean temperatures do not regulate.
https://www.abc.net.au/news/2017-04-27/coral/8476536
This leads to the question “what is causing this?” NOAA reports that between 2014 and 2017, 75% of the ocean’s reefs were stressed enough to induce bleaching and for 30% it was bad enough to cause them to die completely. The temperature of the ocean is not constant. It is usually expected that there will be a rise in ocean temperatures every few years because of phenomena like El Niño. What is interesting about this most recent “ocean heat wave” was that it did not coincide with a regularly scheduled climate event (Scott, & Lindsey, 2018).
https://www.climate.gov/sites/default/files/SotC2017_CoralHeatStress_2015-2016_large.jpg
I do not want to get into a long, divisive global warming debate but the fact of the matter remains that the ocean temperatures are rising and the coral populations are drastically declining. So, everyone, please do yourself, the coral, and the Symbiodinium a favor and reduce, reuse, recycle.
I hope you all enjoyed my blog and maybe even learned something along the way.
-L
Coral Polyps. (n.d.). Retrieved April 24, 2019, from https://coral.org/coral-reefs-101/coral-reef-ecology/coral-polyps/
Nielsen, D. A., Petrou, K., & Gates, R. D. (2018, February 20). Coral bleaching from a single cell perspective. Retrieved April 24, 2019, from https://www.nature.com/articles/s41396-018-0080-6
NOAA Ocean Service Education. (2017, July 6). Retrieved March 19, 2019, from https://oceanservice.noaa.gov/education/kits/corals/media/supp_coral02bc.html
Scott, M., & Lindsey, R. (2018, August 01). Unprecedented 3 years of global coral bleaching, 2014–2017. Retrieved April 24, 2019, from https://www.climate.gov/news-features/understanding-climate/unprecedented-3-years-global-coral-bleaching-2014–2017