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The Microbe That Invaded Caribbean Coral Reefs

Think of giant pythons from southeast Asia, ending up in the Florida everglades and suffocating any small mammal they could find. Think of cane toads from South America, relentlessly marching over Australia, swallowing bird eggs and native frogs. Think of rats from pretty much any mainland country, stowing away onto pristine islands and eating their way through the helpless local birds. These are all classic examples of invasive species.

Here is another, and it’s very different. It’s a microscopic alga called Symbiodinium trenchii. Unlike the python or the cane toad or the rat, this tiny brown bauble seems fairly benign—even beneficial. It lives in the cells of corals and provides them with food, by harnessing the sun’s energy to make sugars. It typically does this in its native waters in the Indo-Pacific Ocean. But somehow, it recently found its way to the Caribbean, on the other side of the world. And there, it displays all the characteristics of an invasive species.

Tye Pettay from Pennsylvania State University has shown that S.trenchii has spread through a large number of Caribbean corals. It provides its hosts with nutrients but is less generous than the native coral-associated algae that is has displaced. It is especially common in populations that have been ravaged by heat or pollution or disease. It looks for all the world like an opportunistic infection, of the kind that takes hold in people whose immune systems have been weakened. “It’s all over the Caribbean and it’s not going away,” says Todd LaJeunesse, who led the study.

Colony of the symmetrical brain coral, Pseudodiploria strigosa, is a common reef–building coral in the Caribbean. When under stress, these animals may harbor a species of mutualistic microbe, which was recently discovered to be introduced from the Indo-Pacific. This microscopic symbiont, a dinoflagellate, increases the stress tolerance of native corals, but may significantly reduce the animal’s rate of calcification, thus diminishing their role as reef builders (coral image by Robin T. Smith, Science Under Sail. Symbiodinium mastigote image was provided by Sung Yeon Lee and Professor Hae Jin Jeong, Seoul National University).
Brain coral, Pseudodiploria strigosa (Credit: Robin T. Smith, Science Under Sail) with Symbiodinium algae (inset; credit: Sung Yeon Lee and Hae Jin Jeong, Seoul National University).

Corals are stinging colonial animals that form partnerships with many species of Symbiodinium algae. These allies—these symbionts—provide them with the energy they need to construct their impressive reefs. But if oceans get too hot, the corals evict their symbionts, losing a source of both energy and colour. That’s why they are said to be “bleached”. If they stay too long in this condition, they die. Solitude is no life for a coral.

But there’s a way out. Some Symbiodinium species make their coral hosts more tolerant to heat and other stressful conditions. If a coral can swap its algal partner for a hardier one, it could survive.

This is what happened in 2005. That year, the Caribbean experienced exceptionally high temperatures; as a result, more than 80 percent of its corals bleached. It was a catastrophe, but for S.trenchii, it was an opportunity. In the months before the bleaching event, LaJeunesse found this species in less than 1 percent of the corals. During the event, he saw it in 20 percent of them. “We found it in the most severely stressed animals,” he says. “We have never seen it behave like this elsewhere.”

Through almost a decade of work, LaJeunesse’s team has confirmed that S.trenchii does indeed behave very differently in separate parts of the world. In the Indo-Pacific, it exists as a genetically diverse population, which has probably been around hundreds of thousands of years—if not millions. It is not alone, either. S.trenchii is part of a lineage of Symbiodinium called “Clade D”, which arose in the Indo-Pacific Ocean and diversified into many species, each of which associates with certain types of coral.

In the Caribbean, things are very different. S.trenchii is the only member of Clade D around, and it lives inside a wide variety of coral hosts. This population has absurdly little genetic diversity. Even across hundreds of kilometres of oceans, individual cells of S.trenchii are almost identical. It’s like looking at a sea of clones.

S.trenchii must have hitched a ride to the Caribbean on some kind of ship, perhaps as recently as a few decades ago. It then spread across the entire region, perhaps taking advantage of the rough times that the local corals were experiencing. “The poor Caribbean has been trashed with sea surface temperature anomalies and pollution and a huge human population,” says LaJeunesse. “It’s severely degraded. If it were pristine, if we went back 100 years, I’m not sure S.trenchii would be so successful.”

But we can’t go back, and it is successful. “It is what it is,” says LaJeunesse. “There’s nothing we can do about it.” And indeed, we might not want to do anything about it. There’s a tendency to view all invasive species as villains, but they’re not all like cane toads or Burmese pythons. There’s some evidence that these invaders can have positive effects on their new homes. Take S.trenchii. You could argue that its invasion benefited the corals, by allowing them to weather the warm spell of 2005.

This silver lining comes with a cloud. Pettay’s experiments showed that S.trenchii is a more selfish partner than the native algae of the Caribbean. It produces just as much sugar as its peers, but it hands over much less to its coral partners. As a result, the corals can only build their rocky reefs at half their usual rate. It might be better for them to have a stingier partner than no partner at all, but in the long-term, incompatibilities with S.trenchii might ultimately harm them and the reefs that they build. For now, no one knows where the balance of benefits and risks lies.

LaJeunesse wants to find out. He wants to know how S.trenchii affects the growth rates of its coral hosts. Also, what makes it such a good invader? And why is it hardier than other related species? “It’s the only member of Clade D whose genome has been duplicated,” he says. “It’s speculative, but maybe that’s something to do with it.”

He also wants other coral biologists to pay more attention to the microbial side of the coral-algal symbiosis. Many of them talk about corals “choosing” or “shuffling” their symbionts, as if the symbionts were passive halves of their own partnership. “It drives me crazy,” he says LaJeunesse. “I think people work on corals because they like corals, so they take the host’s point of view.”

But the symbionts are incredibly complex, too. They aren’t just bacteria; they are very complex organisms. They have as much DNA in their cells as you do in yours, and even more genes. Humans and corals have just over 20,000 genes in their genome; Symbiodinium has between 40,000 and 50,000, and we have no idea what around half of those do. “Who is the master of this house?” asks LaJeunesse.

Reference: Pettay, Wham, Smith, Iglesias-Prieto & LaJeunesse. 2015. Microbial invasion of the Caribbean by an Indo-Pacific coral “zooxanthella”. PNAS http://dx.doi.org/10.1073/pnas.1502283112

8 thoughts on “The Microbe That Invaded Caribbean Coral Reefs

  1. By saying the genome has been duplicated, does this refer to polyploidy? Is it just one copy or multiple?

  2. genetic data shows two copies of every locus we looked at. The genomes of Symbiodinium have lots of repetitive areas but this seems to be a case of ploidy change rather than localized gene duplication.

  3. “But somehow, it recently found its way to the Caribbean, on the other side of the world. And there, it displays all the characteristics of an invasive species.” Although the effects of the alga, Symbiodinium trenchii, are not clear yet, it does show the potential negative signs of an invasive species, which is why it needs to be watched over time. This type of alga is causing the coral to grow slower, and in the future the effects of this species may be long term and not easily reversed, it may cause other species extinction, or it may alter the ecosystem completely. Although this invasive species have benefitted the corals before, it should not be disregarded for it can still be a danger to the ecosystem. Experiments need to be done and data needs to be kept on the corals and the Symbiodinium trenchii to make sure the delicate balance of the ecosystem is not altered too much by the invasive species. #UCIGCB2015

  4. As I read the title “The Microbe that Invaded Caribbean Coral Reefs” I thought that the article would be about the microbe being a negative invasive species. Although we learned in lecture that some invasive species can be good for their new environment, I was still shocked from the effects the Symbiodinium trenchii had on the Caribbean reefs. One aspect I found very interesting about this microbe is that depending on the weather conditions it reacts different with the coral. An important benefit the Symbiodiniun trenchii provides corals with is “[making] their coral hosts more tolerant to heat and other stressful conditions. If a coral can swap its algal partner for a hardier one, it could survive.” This microbe is a great example of a benign invasive species that can help species in danger due to climate change keep a steady life-style. #UCIGCB2015

  5. “Through almost a decade of work, LaJeunesse’s team has confirmed that S.trenchii does indeed behave very differently in separate parts of the world. ” While it has been proved that S. trenchii is beneficial to species in certain parts of the world, more efforts need to be focused on eradicating it from the areas that it hinders. As an invasive species in the Caribbean, the alga is on track to making coral go extinct if not dealt with soon. Because this can lead to a loss of habitat for many other marine species, tourism, species diversity, and mangrove protection, we need to act now to save the Caribbean from the pitfalls of S. trenchii.

  6. “It lives in the cells of corals and provides them with food, by harnessing the sun’s energy to make sugars.” The alga, Symbiodinium trenchii demonstrates unlikely effects of an invasive species and seems to be beneficial for the coral. This is crucial because in recent years, nitrogen pollution in the ocean has led to a decline of coral reefs and sea grasses. However, its long term effects should be studied cautiously to ensure it is not harmful to the coral which provides food and diversity in the ocean’s ecosystem. #UCIGCB2015

  7. Zooxanthellae (the algae in question) are unicellular protists, and hence eukaryotes (nucleated cells), but with rather unusual genomes and nuclei. Genome size is only the beginning of their weirdnesses.

  8. In the never ending battle to out compete neighboring corals – won’t the slower growth rate result in an eventual return to near pre-2015 percentages of native and invasive Zooxanthellae?

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