The Science

Theory for coral bleaching: The current theory for coral bleaching is that high temperatures induce stress in the Symbiodinium algae living within the coral tissue. This temperature stress leads to what is called oxidative stress – the production of oxygen radicals by cells. These oxygen radicals, particularly superoxide (O₂^-), are very reactive and can cause damage to the organisms. This excess superoxide somehow triggers the coral to release or “kick out” Symbiodinium from the tissue. The superoxide then induces a biochemical reaction in the coral that leads to death (programmed cell death). Many aspects of this theory however have been based on indirect evidence – in particular, measurements of enzymes (proteins) that are produced when high amounts of superoxide are around. However, there could be a number of alternate scenarios for coral bleaching, and in fact, the key trigger(s) underpinning coral bleaching are unclear.

What is superoxide?: Did you even wonder why blueberries and dark chocolate are good for you? We’re told they contain antioxidants, so you should eat them. Antioxidants are chemicals or proteins that break down and remove toxic oxygen radicals, like superoxide. We make superoxide all the time. You’re doing it right now. It’s just an unfortunate consequence of breathing oxygen (O₂). When we (and other oxygen-breathing organisms) are breathing, sometimes we make mistakes and instead of breaking oxygen all the way to water, we make superoxide inside our cells. Superoxide at high concentrations is toxic so we need to get rid of it to live and be healthy. Our bodies are fighting these toxic chemicals all the time by producing natural antioxidant enzymes and molecules to get rid of them. Eating blueberries and dark chocolate just help join in the fight! Excess production of superoxide in the body has been linked to cancer and ageing – so eat those blueberries!

But, superoxide gets a bad rap. It’s not always bad. And, instead, it is an essential chemical all organisms need – but only at low concentrations. If you’ve ever cut the stem of a plant, its response to that is to overproduce superoxide (an “oxidative burst”) to try to fix the torn tissue. Also, cells use superoxide to divide and grow – for instance, mushrooms use superoxide to change the way it is growing. It is also a way that tiny organisms (micro-organisms) communicate with each other. They use superoxide instead of words. So, there needs to be a delicate balance to maintain health – both at the cellular and ecosystem level.

Testing the coral bleaching theory:  Over the past year, our team has refined methodology to measure superoxide fluxes from aquaria held corals (larvae and adult), and laboratory cultures of Symbiodinium and coral-associated bacteria. Through these controlled experiments, we have obtained a number of lines of evidence suggesting that our current understanding of coral bleaching is inaccurate. These data indicate that there are several sources of superoxide within corals and that their response to temperature differs. All signs point to Symbiodinium algae not being the dominant source. Within natural coral communities, therefore, it is unclear who is actually making the superoxide that is inducing the breakdown of the symbiosis at elevated seawater temperatures. In fact, it is not clear what the role of superoxide is within these communities – is superoxide production the cause or effect of bleaching? It is clear that we need to revisit the current model proposed for coral bleaching or else our ability to predict and ideally mitigate the demise of coral reefs will remain unattainable.

What we are doing in Hawaii: The challenge with identifying the link between superoxide and coral bleaching is the lifetime of superoxide. Superoxide is very reactive and has a lifetime of only a couple minutes in natural waters. That precludes us from collecting a sample, taking it to the lab and making a measurement. And, there are no instruments currently available that can be put in the water to make direct measurements. To circumvent this, we modified and optimized a widely used lab-based method for deployment in the field. In reality what we are doing is turning the boat into a lab and using a high flow injection system into the instrument. Two of us are operating the instrumentation on the boat and two are in the water positioning tubing at various locations in the reef and communicating with those on the boat. The key limitation is still the lifetime of superoxide. Since the instrument is on the boat, we only have about 1 minute to get the water from the position that the tubing is being held into the instrument. With our current set-up and using a boat with a low profile, that limitation now is about 1-2 meters. Since many of the corals that are bleaching in Hawaii are shallow corals, we have a unique opportunity to take this approach to the field and measure superoxide being formed from corals during a natural bleaching event. Our goal is to measure superoxide being released from healthy-looking and bleached or bleaching corals and then link this to the health of the corals and any signs of stress as well as the relationship between the coral and the community of microorganisms living within the coral tissue.

- Colleen Hansel

It Takes a Team!

Our team collecting in situ superoxide measurements in a shallow reef in Kaneohe Bay, Hawaii. Amy is in the water holding a tube next to a coral, Laura is holding the boat steady, Tong is operating the instrument measuring the superoxide (FeLume, Waterville Analytical), and Colleen (not pictured) is operating the pump and recording the timing of sampling. 

(image: Colleen Hansel)