Coral Reefs: Invasive Microbe May Protect Them, But at a Cost

Coral reefs worldwide are taking a beating from global warming, and while new research shows that a certain species of invasive microbe may protect them, it comes at a cost.

The symbiotic, single-celled micro-alga Symbiodinium trenchii, which comes from the Indo-Pacific, has spread across the Caribbean Sea and now lives within the cells of native coral animals. And while it improves the resilience of coral communities to heat stress, it also hinders corals' ability to build reefs.

"The results raise a potentially contentious issue about whether this invasion is relatively good or bad for the long-term productivity of reef corals in the Atlantic Ocean and the ecosystems they support," Todd LaJeunesse, an associate professor of biology at Penn State, said in a press release.

Corals and photosynthetic algae have long benefitted from one another as their relationship has evolved over millions of years. Corals get their energy and nutrients from algae, and in turn algae obtain nutrients and protection by living in the tissues of corals.

However, this symbiotic balance is now at risk thanks primarily to factors such as climate change and ocean acidification, which threaten to destroy even the iconic Great Barrier Reef.

"Coral reefs are highly important to the biosphere, and they also have enormous economic and societal value in the form of tourism, recreation and coastal protection, and as a source of food and pharmaceuticals," noted LaJeunesse. "Currently, these ecosystems are threatened by synergistic effects of diminished water quality, increased temperature and reduced ocean alkalinity."

To determine whether these invasive algae are a new threat to add to this growing list, LaJeunesse and his team used DNA sequencing techniques to document S. trenchii's spread. Though their analyses indicate that the introduction of S. trenchii to the Caribbean was relatively recent, in a short span of time it has managed to do some harm.

S. trenchii flourishes within coral colonies during periods of increased sea-surface temperatures. It replaces the more sensitive native species of algae, which are expelled by their hosts when the environment becomes too warm - a stress response known as coral bleaching.

Although, S. trenchii eventually is replaced by native species of algae after environmental conditions return to normal.

During these warming periods, S. trenchii does help corals withstand the heat, providing them with thermal tolerance under conditions up to 3.6 degrees Fahrenheit higher than normal. However, the team also found that for Mountainous Star corals in particular, S. trenchii cuts their calcification rate in half - the process by which reefs are built.

"Our results indicate that S. trenchii may not translocate as many nutrients to the coral host as do native species, which is why we see reduced calcification rates among the corals," said LaJeunesse.

"Invasive species pose major threats to biodiversity, ecosystem function and economic well-being," LaJeunesse concluded. "Growing evidence indicates that microbes, which include micro-algae, are being successfully introduced to new places around the world, but we still have little understanding of the negative or positive outcomes from such introductions. This work highlights how microbial introductions, many of which may be unknown to science, can affect ecosystem stability and function - in this case, reduced calcification of corals in the Caribbean."

The findings were published in the journal Proceedings of the National Academy of Sciences.

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