Harvesters of light
They fan out into lily-pad-shaped disks, branch haphazardly like the antlers of deer, and hold fast to the sea floor in squat little spheres. Corals come in many shapes and sizes—and this diversity in form is driven by sunlight.
"Light is the most important factor modulating how corals grow," says Roberto Iglesias-Prieto, professor of biology. "Over time corals have evolved various shapes that help them to maximize the amount of light they receive."
Iglesias-Prieto, along with Professor of Biology Todd LaJeunesse, will demonstrate how the various shapes of corals serve to capture sunlight at Penn State's "The Art of Discovery" booth at the 2019 Central Pennsylvania Festival of the Arts. The scientists will give festival-goers the opportunity to shine laser pointers onto coral skeletons and witness firsthand how the different shapes scatter the light, thereby increasing the surface area to enhance the capture of light energy.
According to LaJeunesse, light is essential for the survival of corals because it is their primary source of energy. "Corals occur in very nutrient-poor environments," he says. "They get around this problem by maintaining a symbiotic relationship with photosynthetic algae."
Known colloquially as zooxanthellae, these tiny algal symbionts live inside the cells of corals and provide the animals with energy converted from sunlight.
"Together, corals and zooxanthellae are the world's most efficient light harvesters—far better than plants," says Iglesias-Prieto. "They can absorb the same amount of light as a green plant but with an investment of an order of magnitude less chlorophyll, which is the most expensive thing for a primary producer to create."
This light-harvesting symbiotic partnership is important because it forms the basis of one of the world's most diverse ecosystems, providing habitat for millions of species of reef-dwelling organisms. Additionally, coral reefs provide environmental and economic services to hundreds of millions of people. For example, they drive a multi-billion-dollar tourism industry, with the reefs in the Florida Keys alone estimated to have an asset value of nearly $8 billion. They also protect shorelines from wave action, preventing erosion and property damage. Furthermore, pharmaceutical drugs have been and continue to be developed from animals and plants that live on coral reefs.
Given that coral reefs provide more than $400 billion in goods and services and support such immense biodiversity, it is concerning that many reef ecosystems worldwide are collapsing as a result of overfishing, pollution, and climate change.
To slow or prevent such loss, scientists must understand the biology of the organisms that create these ecosystems. According to LaJeunesse, that means first knowing which species of algae and coral you're dealing with. Unfortunately, until recently, researchers were unable to discern among the species of algae.
"Back in the 1970s scientists thought there was only one species of symbiont co-occurring with all the corals around the world," he says. "Over time, in their minds, that number grew to a handful of species. My recent work suggests that there are possibly thousands of species of zooxanthellae with large differences in their abilities to endure temperature stress, as well as high and low light environments"
Over the past decade, LaJeunesse has used genetic tools to formally describe species. "I wanted to build a solid foundation for this field of study, and this begins with good systematics and taxonomy," he says.
LaJeunesse notes that knowing which species of zooxanthellae occur with which species of coral is enabling scientists to better study the organisms' ecology, physiology, and evolution.
Such species-level knowledge is helping Iglesias-Prieto to examine the differences among algal symbiont species in their abilities to capture sunlight. For example, in one project, he and his students are using a fluorometer to monitor the amount of light absorbed by different species, not just from the top of the animal that is exposed to the sun, but also from the sides and bottom of the animal, through the process of "backscattering."
"Corals contain symbionts in all of their tissues, not just the ones on their top surfaces," says Iglesias-Prieto. "The shape of the coral skeleton facilitates the capture of sunlight when it bounces off of the sea floor or other objects and hits these less accessible surfaces."
Normally, he says, this process is helpful to corals, but sometimes it can contribute to the organisms receiving too much light.
"When primary producers are exposed to excessive light, they tend to reduce their pigmentation," says Iglesias-Prieto. "Think of a houseplant placed in a sunny window; its leaves are lighter in color than the same plant would be if it was placed in a darker spot. In corals, however, reducing pigmentation on the side of the animal that is facing the sunlight sometimes is not enough to protect it from too much light because of an increase in backscattering from the animal skeleton."
This, he says, can lead to coral bleaching, a phenomenon in which—when stressed by light, temperature, or disease—the algae are expelled, and the animal starves to death.
"Understanding how corals and their symbiotic algae have co-evolved over time to maximize light absorption is important in determining how these important organisms will respond to changes in their environment, especially those that are taking place as a result of human activities," says Iglesias-Prieto.