AsianScientist (Apr. 24, 2018) – Climate change may lead to coral bleaching in an unexpected way: by making corals’ symbiotic algae ‘greedy.’ These findings, by researchers at the University of Hong Kong (HKU), have been published in The ISME Journal.
In the symbiotic relationship of corals and algae, corals provide refuge and nutrients for algae to grow. In return, algae recycle waste from corals through photosynthesis, converting it into useful sugar and proteins for corals to survive.
Corals exposed to warm water burn more energy, a natural sign of stress in a marine invertebrate. Their partner alga, called Symbiodinium, are thought to suffer and escape the coral host when ocean temperatures exceed 32ºC in a process that reveals coral’s bright white skeleton underneath: also known as bleaching. Without their algae, which provide up to 95 percent of the energy needs, corals would starve and die.
Dr. David Baker and his team from the HKU Swire Institute of Marine Science revealed that instead of suffering with their coral hosts, Symbiodinium were even happier at higher temperatures. In fact, they were less affected by warm water and even exploited their coral host for their own survival.
Considering carbon and nitrogen as the ‘currency’ traded in the symbiotic relationship, the algae were increasing their sugar and protein ‘profits’ at the expense of the coral host. At the same time, the coral lost their carbon ‘savings account’ as a result of increased metabolism and respiration. With such an imbalance in the coral-alga economy, the coral eventually loses out.
“The corals are burning a lot of their stored energy to keep their partners happy, while the algae are hoarding more resources to use for reproduction… like a landlord keeping the lights on for a tenant who’s running a good business but won’t pay their rent,” said Baker.
“Just a small increase in cell division has a big effect. It’s too much of a good thing. Too many symbionts cells can worsen coral bleaching,” said Baker’s PhD student Ms. Jane Wong, a co-author of the study.
More discouraging, it did not seem to matter what species of algae the corals were hosting. Even a ‘friendly’ type of alga became parasitic. This finding casts doubt on the expectation that unique species of Symbiodinium might save corals in a warmer world.
The team points out that their experiments were conducted with high nutrient concentrations, which is a state that is now common on coral reefs. As many reefs warm, they are also becoming more polluted by sewage and sedimentation from coastal development all over the world. This combination of factors is likely worsening corals’ demise, which is why Baker and his team suggest nutrient pollution management should be a part of the conversation on climate change.
The article can be found at: Baker et al. (2018) Climate Change Promotes Parasitism in a Coral Symbiosis.
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Source: University of Hong Kong; Photo: Till Röthig.
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