Invasive algae could help save corals

Close your eyes and imagine having a neighbor who brings you cookies during the holidays. Now imagine that they also bring sparklers and chicken wings over during Independence Day. Unfortunately, they sometimes blare terrible music until 2 a.m. and call you late at night asking to “borrow” a couple of eggs to make the cookies they’ll later bring to you. Not all bad, not all good. That’s exactly the relationship currently happening between Caribbean corals and Symbiodinium trenchii, a brown microscopic alga that was recently found in the Caribbean Sea. Originally native to the Indo-Pacific Ocean, Symbiodinium trenchii is a genetically diverse species.

Oftentimes, the symbiotic relationship between corals and algae seems to fall into mutualism (when both species benefit from each other), commensalism (one benefits, the other is not harmed) or parasitism (one benefits, the other is harmed). In fact, zooxanthellae, the name for the best-known microbe associated with corals, is the alga that gives coral polyps the ability to produce their beautiful and sturdy structures. It’s a type of Symbiodinium that’s classified as having a mutualistic relationship with its coral hosts.

However, Symbiodinium trenchii, the species recently found stretching across the Caribbean, is an oddball of the endosymbiotic dinoflagellates. It also seems that it’s both beneficial and harmful to the Caribbean corals in which it was first found.
According to Pennsylvania State University’s Dr. Tye Pettay, Symbiodinium trenchii provides its hosts with nutrients as it converts the sun’s energy to make sugars, but it doesn’t give away as many nutrients as the native algae that is usually found in the Caribbean corals. Although reef building will still occur amid the invasive species, the corals will take a longer amount of time to build their structures. That’s the not-so-cool part.

However, certain Symbiodinium species are beneficial because they allow their coral hosts to tolerate rising temperatures from factors like climate change, irregular weather patterns or other stressors. This opportunistic action could allow for a higher species survival in the Caribbean as temperatures continue to rise, and it’s different from that of most algae found in corals, which are expelled once waters become too warm and corals become stressed – also called “bleaching”.

Leader of the 10-year study Dr. Todd LaJeunesse, associate professor of biology at Pennsylvania State University, found the most evident benefits in the relationship between S. trenchii and the Caribbean corals in 2005, a year when, according to data by LaJeunesse, the Caribbean Sea was hit with high sea surface temperatures about four degrees Celsius warmer than usual. Although the year resulted in more than 80 percent of bleached corals in the Caribbean, it also revealed a perpetual fact: One month before the corals all bleached, LaJeunesse saw only one percent of S. trenchii in the corals. However, during the bleaching event, he found S. trenchii in 20 percent of the corals, specifically in the animals with the highest stress, seen via bleaching activity.

LaJeunesse’s study on S. trenchii also revealed that the alga found in its native Indo-Pacific waters, amid its genetic diversity, still practices symbiosis with a variety of coral species. However, the Caribbean S. trenchii is found in many species of corals despite not having much genetic variation. Perhaps the Caribbean S. trenchii has yet to adapt to the Caribbean waters. It is still unknown how they reached the Caribbean.

Research shows that S. trenchii’s opportunistic endeavors during times of rising sea surface temperatures are beneficial to Caribbean corals. This is incredibly helpful as scientists continue to research coral bleaching, especially with increasing concerns over the impact of climate change on warm water corals. However, S. trenchii still an invasive species that could cause the corals more harm in the long run, especially as it impacts other species in the ecosystem. Though there is more research to be done on S. trenchii, it’s clear that its invasiveness in the Caribbean is something much like that neighbor you imagined – an annoyance you can’t ignore.