Beneath the wavy surface lies a smooth opportunist, a scavenger for life, a haggler of space. Divert your eyes to the waters of the Indo-Pacific, two bodies of water connected but separate, which give life to more than 3000 species of fish and 500 species of reef building corals. Look among them and you will see the beauty of earth’s genetic variation; a tapestry of evolutionary diversity. In these same waters, decorating the sides of coral animals all along the reefs, sits a rather prudent-looking micro-alga with a name that is anything but. Symbiodinium trenchii; an algae that like it’s brethren, latches on to tropical corals and forms a symbiotic relationship in which the coral provides a place to stay, while the algae produces food using sunlight that then is shared with the coral. The relationship lasts as long as it’s beneficial, however, there are times when the surface temperature rises too high for the coral’s comfort and the stress causes it to rid itself of its algae counterpart in a process known as bleaching. In its native waters, Symbiodinum trenchii exist unquestioned because of its unique ability to resist higher surface temperatures, but recently it seems this unique quality has led it to wander and explore the ocean blue.
What better place to go than the tropical water of the Caribbean? And though its beaches and landscape are a treasure to the eye; Symbiodinium trenchii has found its way here for very different reasons. The coral here, like everywhere else, goes through bleaching during the warmer times of the year and in 2005 this was especially true, with over 80 percent of its corals bleached. “Heat stress during the 2005 event exceeded any observed in the Caribbean in the prior 20 years, and regionally-averaged temperatures were the warmest in at least 150 years,” said Mark Eakin, coordinator of NOAA’s Coral Reef Watch Program. This news comes as no surprise to anyone who has kept themselves informed with the issue of global warming, but it does carry some worry for these corals, as prolonged bleaching can lead to increased mortality and disease. For S. trenchii, however, the news came in the form of an open invitation and it took no time for this alga to reach the bleached corals of the Caribbean. Found in less than 1 percent of corals prior to this incident, S. trenchii showcased its strength and persistence by attaching to 20 percent of corals in the months to come. This fast-paced spread is not surprising as S. trenchii is the only alga of its kind in these waters. A member of the Symbiodinium lineage “Clade D,” this alga is a very diverse population native to the Indo-Pacific, which has allowed it to live inside a wide variety of coral hosts. Interestingly, this now-Caribbean population has very little genetic diversity, despite the wide range of corals it has managed to link up with, implying that this alga has only recently been introduced to these waters. And as more and more algae were beaten by the heat, S. trenchii was more than happy to take its place, and now it’s here to stay.
At first glance, this surprising relocation of algae appears to be a natural attempt at homeostasis, a necessary use of S. trenchii’s unique resistance to temperature, which allowed for some these otherwise-bleached corals to survive. However, a careful study by Tye Pettay from Pennsylvania State University has shown the invasive nature of the alga; though S. trenchii allows coral animals to survive in higher temperatures, it drastically hinders its ability to grow and expand. The problem is rooted in S. trenchii’s greedy nature, manufacturing just as many nutrients as the native algae, yet sharing far less of them with its coral host.
The question now revolves around the effects that this will have on the coral and the ecosystem it serves; as the “rainforests of the sea,” the decline and loss of these beautiful reefs has significant social, cultural, economic and ecological impacts on people and communities throughout the world. Todd LaJeunesse, who led the study, accurately summarized the situation in which we’ve found our coral reefs, stating that, “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 sea is a large place, full of life and full of mystery; pleasing to the eye, confusing to the mind and unknown to the majority of us above the surface. This event only furthers our idea that we are far from fully understanding the relationships that form under the water, while simultaneously showcasing our ability to research and analyze in order to access and make sense of what we don’t fully comprehend. In this way, the dynamic between confusion and discovery remains symbiotic; feeding off of each other as long as it remains beneficial.