Even with little light from the sun, mesophotic corals can still thrive in Auau channel
A recent study off Maui’s west coast sheds some light on deep-water reefs
In the Auau Channel between Maui and Lanai lies one of the largest mesophotic coral reefs, a patchwork of different plate-shaped coral along with fish, which all thrive in an unconventional dark environment of less than 1 percent of light from the surface.
“It’s a really unbelievable place,” said University of Washington researcher Jacqueline Padilla-Gamino, who was part of a study from 2010 to 2011 of the deep-ocean reef a quarter- to half-mile off the Lahaina coastline. “What is surprising is that, in theory, these corals should not be there because there’s so little light. Now, we’re finally understanding how they function to be able to live there.”
The assistant professor at the University of Washington School of Aquatic and Fishery Sciences explains that corals have symbionts or microscopic seaweed living inside their tissues. The seaweed perform photosynthesis and obtain energy.
But without much sunlight can photosynthesis still happen?
“What we are finding, hell yeah. They can still use photosynthesis,” Padilla-Gamino animatedly explained via phone from her university office Thursday.
She and other researchers found that different corals in the mesophotic zone, in depths of 100 to at least 500 feet, have different mechanisms to adapt to the low light. Some change the type and size of the microscopic seaweed and other corals change the amount of pigments at deeper depths.
These changes allow corals to acquire and maximize the light available to perform photosynthesis and obtain energy, said Padilla-Gamino, who did her doctorate degree studies in oceanography at the University of Hawaii at Manoa from 2005 to 2011.
During the mesophotic study, scientists also discovered that corals at deeper depths likely are eating more organisms, such as zooplankton, for energy — more than their counterparts in shallower water.
This spring, Padilla-Gamino and colleagues, including those from the University of Hawaii, College of Charleston, University of California, Berkeley and other institutions shared their findings in the journal Limnology and Oceanography.
Padilla-Gamino, the lead author, said not many studies have been done on the mesophotic coral systems because they are hard to get to; researchers use submersible watercraft or need sophisticated diving equipment to swim to the ocean life.
The mesophotic coral systems are found in tropical and subtropical regions at depths from 100 feet to more than 490 feet, under low light conditions according to the National Oceanic and Atmospheric Administration. The term mesophotic comes from the words “meso,” which means middle, and “photic,” meaning light.
The mesophotic coral reef in the Auau Channel measures 4 square miles with reefs scattered and patchy throughout the channel, Padilla-Gamino said.
The coral in mesophotic reefs in Hawaii are shaped like plates, rather than the many armed nearshore coral. The larger, flat shape is used to take in all the light it can, Padilla-Gamino said.
“When you go sunbathing, you are not going to be standing up. You are going to be lying down, so you get as much light as possible,” she explained.
As coral bleaching continues to be a topic of concern with seawater temperature rise and land-based pollutants, Padilla-Gamino said she and the team did not encounter any bleaching in their study. She added, though, that this does not mean that mesophotic reefs are immune from coral bleaching.
As for land-based pollutants from historically large agricultural operations, Padilla-Gamino said she didn’t know if they have had an impact on the deep-water reefs because the study’s focus was on how the corals adapt to the low light conditions.
In a news release, Padilla-Gamino said knowing how the deep-sea coral reefs function is important because they appear to be hot spots for biodiversity and home to many species. Scientists say that more information about deeper reefs in the mesophotic zone is critical for preserving that habitat.
“We want to protect them for future generations,” Padilla-Gamino said.
She noted that the reefs can be damaged by undersea cables and anchoring.
During the ocean study, which ended eight years ago, Padilla-Gamino was on board two of the team’s eight submersible dives. The team used a submersible watercraft, “a very, very, tiny submarine,” that was tossed from the back of a larger boat, then disconnected once the submersible hit the water.
Padilla-Gamino said it could get claustrophobic in the 20 by 11 foot submersible; she experienced queasiness as it was lowered into the ocean.
“You feel the waves,” she said.
The submersible fits three people. The pilot is in the middle and the other two lie on their stomachs peering out of their own window.
“We cannot stand, we are lying on our tummy,” Padilla-Gamino said.
The dives lasted eight hours. Researchers used a robot arm to take specimens of coral, captured video footage and took photos.
“The more we can study this, the more information we can have about how life works. This is a remarkable system with enormous potential for discovery,” she said. “Our studies provide the foundation to explore physiological flexibility, identify novel mechanisms to acquire light and challenge current paradigms on the limitations of photosynthetic organisms like corals living in deeper water.”
Other co-authors are Celia Smith at the University of Hawaii at Manoa; Melissa Roth at UC Berkeley; Lisa Rodrigues at Villanova University; Christina Bradley at Salisbury University and Robert Bidigare and Ruth Gates at Hawaii Institute of Marine Biology.
The study was funded by the National Oceanic and Atmospheric Administration and the National Science Foundation.
* Melissa Tanji can be reached at email@example.com.