Invasive microbe aids coral survival but puts reefs at risk

By now it’s common knowledge that climate change and human activities are reshuffling the distributions of plant and animal species all over the world – an invasive plant takes root here, a bird expands its range northward there, and so on. Less obvious, at least so far, is how that reshuffling may also take place at a scale invisible to the naked eye.

But microbes, too, can be invasive, and even if they aren’t disease-causing pathogens they can have dramatic effects on ecosystems. According to research published yesterday in the Proceedings of the National Academy of Sciences, an invasive microbe from the Indo-Pacific has colonized corals throughout the Caribbean, making them better able to tolerate high temperatures but less efficient at building reefs.

In the study, a team of researchers led by Todd LaJeunesse of Penn State University scanned the DNA of microbes found in samples of coral tissue collected from the eastern Indian Ocean near Thailand, the western Pacific near the island nation of Palau, and eight locations around the Caribbean Sea.

The microbes are photosynthetic, single-celled organisms called zooxanthellae, which live inside the bodies of reef-forming corals, providing energy and nutrients to the corals in exchange for other nutrients and a safe place to live. The study focused on one species of zooxanthellae named Symbiodinium trenchii.

S. trenchii populations in the Indo-Pacific are much more genetically diverse than those in the Caribbean, the researchers found. Samples from a single Indo-Pacific reef are likely to contain more genetic diversity than is found in S. trenchii across the entire Caribbean. In fact, a single lineage of the microbe accounted for 42 percent of all samples collected in the Caribbean. And Caribbean S. trenchii samples didn’t contain any unique DNA.

All these results support scientists’ suspicions that S. trenchii is not native to the Caribbean. It was probably introduced sometime within the last several decades, perhaps via ballast water in a ship coming through the Panama Canal, or perhaps multiple times in different places. In any event, the microbe has spread rapidly through the region.

During unusually hot weather or other stressful conditions, corals may expel their zooxanthellae, a process known as “bleaching,” which can eventually be fatal for the corals. Studies of a mass bleaching event in the Caribbean in 2005 revealed that S. trenchii often colonized corals that had lost their usual microorganisms, or symbionts. The invasive S. trenchii often persisted in corals for up to several years, but in the meantime these corals were able to thrive at temperatures a couple of degrees Celsius higher than is comfortable for those harboring native species of Symbiodinium.

At first, this sounds like good news, and it’s tempting to conclude that the newly arrived microbe could help buffer corals against the effects of climate change. But in the new study, the researchers also measured the rate at which one of the most important reef builders in the Caribbean, mountainous star coral (Orbicella faveolata), lays down the calcium carbonate that forms the structure of a coral reef. Coral specimens colonized with S. trenchii build new reef at roughly half the rate of those that harbor native Symbiodinium species, they found.

S. trenchii is a selfish symbiont, the researchers say, keeping more of the energy it produces through photosynthesis for itself compared to native species, so less energy is available to the corals for reef building. This stingy strategy probably also explains why S. trenchii is so opportunistic and able to colonize corals so widely. As the Caribbean warms further in the coming years, S. trenchii is likely to become a bigger player in the region’s coral reef ecosystems, the researchers say – “for better or worse,” or possibly some measure of both.

More broadly, scientists have recently pondered trying to manipulate coral symbionts in order to make the animals more heat tolerant and help them adapt to a warming climate. But the new study suggests that this strategy is likely to be tricky to pull off in practice. Symbiosis – the kind of close, mutually beneficial relationship that corals and zooxanthellae have – isn’t fungible. A relationship that evolved over thousands or even millions of years may have no quick fix. – Sarah DeWeerdt | 2 June 2015

Source: Pettay D.T. et al. 2015 Microbial invasion of the Caribbean by an Indo-Pacific coral zooxanthella. Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1502283112

Header image: Mountainous star coral (Orbicella faveolata), one of the most important reef builders in the Caribbean. Credit: NOAA via Wikipedia.

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