What happens to marine life when oxygen is scarce?

In September 2017, Woods Hole Oceanographic Institution postdoctoral researcher Maggie Johnson was conducting an experiment with a colleague in Bocas del Toro, off the Caribbean coast of Panama. After sitting on a calm, warm open ocean, they dived to find a peculiar layer of cloudy, foul-smelling water about 10 feet below the surface, with brittle stars and sea urchins, which usually lurk, perched on top. coral.

This unique observation prompted a collaborative study explained in a new article published today in Nature Communication analyze the cause of this misty layer of water and the impact it has on life at the bottom of the seabed.

“What we are seeing is hypoxic ocean water, which means there is little or no oxygen in this area. All the macroorganisms are trying to get away from this deoxygenated water, and those that don’t. can’t escape basically suffocate. I’ve never seen anything like this on a coral reef, “Johnson said.

The study takes a close look at the changes that occur in both coral reefs and microbial communities near Bocas del Toro during sudden hypoxic events. When water drops below 2.8 mg of oxygen per liter, it becomes hypoxic. More than 10% of coral reefs in the world are at high risk of hypoxia (Altieri et al. 2017, Tropical dead zones and mass mortalities on coral reefs).

“There is a combination of standing water due to low wind activity, warm water temperatures and nutrient pollution from neighboring plantations, which contributes to stratification of the water column. from there we see these hypoxic conditions forming that start to spread and encroach on shallow habitats nearby, ”explained Johnson.

Researchers suggest that oxygen loss in the global ocean is accelerating due to climate change and excess nutrients, but how sudden deoxygenation events affect tropical marine ecosystems is poorly understood. Previous research shows that rising temperatures can lead to physical alterations in coral, such as bleaching, which occurs when corals are stressed and expel algae that live in their tissues. If conditions don’t improve, bleached corals die. However, the real-time changes caused by decreasing oxygen levels in the tropics have rarely been observed.

Locally, hypoxic events can pose a more serious threat to coral reefs than warming events that cause massive bleaching. These sudden events impact all marine life requiring oxygen and can quickly kill reef ecosystems.

Investigators reported coral bleaching and mass mortality from this event, causing a 50% loss of live corals, which only showed signs of recovery a year after the event, and a drastic change in the seabed community. The shallowest measurement with hypoxic waters was about 9 feet deep and about 30 feet from the shore of Bocas del Toro.

What about the 50% of corals that have survived? Johnson and his fellow investigators have discovered that the coral community they observed at Bocas del Toro is vibrant and that some corals have the potential to withstand these conditions. This discovery paves the way for future research to identify the genotypes or species of corals that have adapted to rapidly changing environments and the characteristics that help them thrive.

Investigators also observed that microorganisms living in the reefs returned to a normal state within a month, unlike macroorganisms, such as brittle stars, which perished under these conditions. By collecting seawater samples and analyzing microbial DNA, they were able to conclude that these microbes did not necessarily adapt to their environment, but rather “waited” for their time to shine in these low-content conditions. oxygen.

“The take-home message here is that you have a community of germs; it has a particular makeup and plugs in, then suddenly all the oxygen is removed and you get a replacement of the community members. They thrive for a while, and eventually the hypoxia subsides, the oxygen returns and this community quickly reverts to what it was before due to the change in resources. It contrasts a lot with what you see with macroorganisms, ”said Jarrod Scott, co-author of the article. author and postdoctoral researcher at the Smithsonian Tropical Research Institute of the Republic of Panama.

Scott and Johnson agree that human activity can contribute to nutrient pollution and warming waters, which in turn leads to hypoxic ocean conditions. Activities such as coastal land development and agriculture can be better managed and improved, which will reduce the likelihood of deoxygenation events.

The study provides insight into the fate of microbial communities on a coral reef during an acute deoxygenation event. Reef microbes respond quickly to changes in physicochemical conditions, providing reliable insights into the physical and biological processes of nature.

The shift the team detected from the hypoxic microbial community to a community of normal conditions after the event subsides suggests that the reef microbe recovery pathway is independent and uncoupled from benthic macroorganisms. This can facilitate the restart of key microbial processes that influence the recovery of other aspects of the reef community.