Invasive alga protects corals from climate change but harms reef-building abilities

Symbiodinium trenchii, a single-celled invasive species of alga that has become prevalent throughout the Caribbean, has proven to be a mixed blessing for the corals it inhabits. While S. trenchii helps these corals survive in warming waters, it also severely impacts their ability to calcify and build reefs.

Coral and algae have enjoyed a symbiotic relationship for millennia. Algae provide nutrients to the coral, and in return they are sheltered in the calcified corals’ tissues. This mutually beneficial relationship has struggled to survive in decreasing water quality, however, as warming waters, pollution, acidification, and other factors lead to coral expelling algae in a process called coral bleaching. Because coral reefs provide critical habitat to countless marine species and are therefore central to human economies and general ecosystem stability, coral bleaching is an extremely dangerous phenomenon.

S. trenchii is much more likely to survive conditions that usually result in coral bleaching. A team of researchers from Penn State, the University of Delaware, and the Universidad Nacional Autonoma de Mexico found that, even in areas where low water quality had resulted in corals expelling native algae, S. trenchii remained. The invasive alga was able to help corals survive temperatures of up to 2 degrees Celsius (3.6 degrees Fahrenheit) higher than normal.

This capacity for survival, however, is not entirely good news. Researchers believe that S. trenchii is able to survive higher temperatures because it transfers fewer nutrients to its host than native species. While this leaves more nutrients for the alga itself and aids its – and its host’s – short-term survival, it also limits the host coral’s ability to calcify and grow. Some host corals, including Mountainous Star Coral, saw 50% reductions in calcifying rates.

These findings show that researchers and conservationists should not be focusing solely on stress-tolerant algae species in their search for a way to help corals cope with worsening conditions in the oceans. The benefits may end up being outweighed by the costs. The findings also highlight the need for further research into invasive microbes and their impact on the ecosystems they are invading.


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