Search
A breath of fresh air
New research looks at coral resilience in low-oxygen conditions
Bathed in the violet glow of LEDs suspended over tanks bubbling nearby, Dr. Maggie Johnson selected one twig of stony coral, glued to a plastic wafer, from a small tub. Destined for a deep freezer to await further analysis, the pinky-sized piece was covered with healthy, glistening polyps.
After processing the sample, Johnson retrieved another bit of coral from a different tub, one of dozens awaiting a similar fate. But the appearance of this fragment differed markedly: instead of a uniform colony of living animals, the tissue had begun to die and recede, revealing ghostly white skeleton.
The killer: asphyxia.
These finger-like bits of staghorn coral (Acropora cervicornis) are part of a new three-year project underway at the Smithsonian Marine Station, in partnership with Dr. Andrew Altieri at the University of Florida. The research, funded by a NOAA Coastal Hypoxia Research Program grant, is among the first in-depth work to study how tropical coral species respond to hypoxia, or depletion of oxygen from the water.
Tropical reefs are already threatened by rising temperatures, acidification and increased pollution from sewage and agricultural runoff. But how oxygen deprivation may be contributing to stress in tropical reefs is virtually unknown.
“Particularly in areas that are already impacted, those are the places we can look to try to understand what another stressor means for those systems,” Johnson said.
Johnson, a postdoctoral researcher who studies ocean acidification and coral reef health, said the new investigation builds on a short series of experiments carried out at the Smithsonian Tropical Research Institute (STRI) in Bocas del Toro, Panama following a natural, days-long oxygen depletion there. Altieri, her advisor, had documented a low-oxygen event in 2010; Johnson and fellow STRI researcher Dr. Noelle Lucey had the rare opportunity to follow up with additional measurements and observations when it occurred again in 2017.
“It’s pretty moving to see firsthand what these acute, extreme events can do on a reef,” Johnson said. “It was like someone had come through with an eraser and just wiped out everything below 12 feet.”
Approximately 90 percent of the local corals died; some colonies were hundreds of years old. But not all succumbed.
“Some corals do survive,” Johnson said. “So we are trying to understand which species do okay, why, and what does that mean for the future of reefs?”
Dead zones on the rise
Hypoxia can develop under a number of conditions.
Warmer water holds less dissolved oxygen, which makes hypoxia more likely to occur. Increased sewage and fertilizer runoff also promote runaway proliferation of organisms like algae, and as they die, their decomposing tissues rob the water column of dissolved oxygen, suffocating any life unable to move to safer waters.
Without wind or strong currents to mix water together, it becomes stratified: layers of water with varying oxygen content. At Johnson’s Panama study sites, warm temperatures and low winds in summer promote stratification, allowing development of deep seated, low-oxygen layers. Unknown mechanisms can cause these layers to shift to shallower depths, resulting in the kind of die-offs Johnson witnessed in 2017.
But in general, the condition hasn’t been well studied in the tropics, where most of the world’s reefs are concentrated. A 2017 study by Altieri found that the bulk of research and documentation of “dead zones”—like the well-known annual event in the Gulf of Mexico—have been in temperate northern regions, and have been conducted by countries with greater scientific resources. The rapid human development of coastal zones in the tropics makes it even more imperative to understand the factors stressing reefs in those regions, Johnson noted.
The corals in Johnson’s experiments are species once common throughout the Florida Keys, and are the focus of restoration projects in degraded areas of the Keys and the Caribbean. By testing corals’ resilience, Johnson hopes to tease out the different ways in which corals respond to hypoxic stress. Not only will this aid in selection of species or genetic strains that are better suited for less-than-pristine environments, it will also help inform better site selection for restoration efforts.
Assessing coral resilience
In the warm waters of southern Florida and the Caribbean, normal seawater holds between 6.5 and 7 milligrams of dissolved oxygen per liter. In kitchen terms, that means that if oxygen were measurable like salt, it would barely fill the bottom of a teaspoon.
At SMS, Johnson controls the concentration of oxygen in each of the experimental treatments by bubbling nitrogen into a covered aquarium, forcing oxygen from the water. Working with four common species including A. cervicornis, Johnson will expose corals to a range of dissolved oxygen levels, from normal, to poor (4 mg/L, a level “which starts to make crustaceans uncomfortable,” Johnson said), to virtually oxygen-free. In the most oxygen-starved treatment, corals will be exposed to a mere 0.5 milligrams per liter.
Working closely with an army of assistants, including University of Florida doctoral student Sara Swaminathan and intern Emily Nixon, Johnson’s experiments have already yielded a few surprises. One of the first species used in test runs survived for weeks in near-anoxic conditions before showing any signs of stress. But other species made it only a few days in the low-oxygen treatments before half the samples died, the threshold for ending the experiment.
Though conclusive results are still months in the future, Johnson suspects at least some of the differences in resilience may concern symbiotic algae living in certain corals’ tissues, which may produce enough oxygen during daytime photosynthesis to make up for low oxygen levels in the surrounding water.
“It’s interesting, seeing who’s doing what, but that doesn’t tell us why those species are more resilient than others,” Johnson said. “We want to know what’s happening at a much finer scale.”
That means looking for differences in the corals’ health, physiology and mortality, as well as molecular and genetic mechanisms that are activated under varying physiological stressors.
Outside of the lab, Altieri, project lead on the study along with Johnson and SMS Director Valerie Paul, will be measuring oxygen concentrations and coral responses at field sites throughout the Keys at coral restoration sites.
Today, the reef at Bocas in Panama has only just started to recover – during a recent visit, Johnson spotted several new baby coral recruits in areas that had been denuded by hypoxia.
“We’re in uncharted territory with these experiments – no one has ever done anything like this before,” Johnson said. “It’s exciting to be able to start to fill in pieces of the puzzle for how to make coral restoration efforts more successful, and improve the health of near-shore coral reefs.”
--Michelle Z. Donahue
January 2020
*Full citation for map figure: Altieri, A. H., S. B. Harrison, J. Seemann, R. Collin, R. J. Diaz, and N. Knowlton (2017) Tropical dead zones and mass mortalities on coral reefs. Proceedings of the National Academy of Sciences of the United States of America, 114:3660-3665.