What happens to marine life when oxygen is scarce?

Credit: CC0 Public DomainMaggie Johnson, a Woods Hole Oceanographic Institution postdoctoral researcher, was doing an experiment in Bocas del Toro on the Caribbean coast of Panama with a colleague. After sitting in the calm, open ocean, they went snorkeling to discover a strange layer of murky, foul-smelling, murky water approximately 10 feet below the surface. It was filled with sea urchins and brittle stars, which were usually hiding on tops of coral.This observation led to a collaboration study, published in Nature Communications today. It explains what the foggy water layer is and how it affects life on the seafloor."What we are seeing is hypoxic ocean water, which means that there is very little or no oxygen in the area. The deoxygenated waters are causing all macro-organisms to try and escape. Those that can't escape will essentially die. Johnson said Johnson, "I have never seen anything similar on a coral reef."This study examines the effects of sudden hypoxic events on coral reefs and microbiological communities in Bocas del Toro. Hypoxic water is when the oxygen level drops below 2.8 mg per liter. Hypoxia is a serious problem for coral reefs all over the globe. 2017 Tropical dead zones and mass mortalities in coral reefs"There are a mixture of stagnant water due to low wind activity, warm temperatures and nutrient polluting from nearby plantations. This causes a stratification in the water column. Johnson explained that hypoxic conditions can form from this and then infringe upon nearby shallow habitats.Researchers suggest that the loss of oxygen in our oceans is increasing due to climate change and excessive nutrients. However, it is not clear how these sudden deoxygenation events impact tropical marine ecosystems. Research has shown that rising temperatures can cause physical changes in corals, such as bleaching. This occurs when corals become stressed and expel the algae living within them. If the conditions aren't improved, bleached corals will die. The effects of decreasing oxygen levels in tropics on corals' health have rarely been seen.Hypoxic events can pose a greater threat to coral reefs at a local level than global warming events that cause widespread bleaching. These events can quickly kill reef ecosystems and impact all oxygen-requiring marine life.Investigators found evidence of coral bleaching and mass death due to the event. This caused a 50% loss in live coral and a dramatic shift in the seafloor community. The lowest measurement of hypoxic waters was approximately 9 feet deep, and 30 feet from Bocas del Toro's shore.What about the other 50% of corals that survived? Johnson and her colleagues discovered that Bocas del Toro's coral community is dynamic and that some corals can withstand these conditions. This discovery opens the door to future research that will identify the coral species and genotypes that have adapted to changing environments.Investigators also found that microorganisms in reefs were able to return to normal within one month. This was in contrast to macro-organisms like the brittle star, which died in such conditions. They were able to determine that the microbes were not adapting to their environment but were simply "waiting" to shine in low-oxygen conditions by collecting seawater samples and analysing microbial DNA.The take-home message is that you can have a microbe community. It has a specific composition and plugs along. But then, suddenly, all of its oxygen is gone and the community is replaced with new members. They thrive for a while and then hypoxia disappears, oxygen returns, and the community quickly shifts back to its previous state due to the changes in resources. This contrasts with what you see in macro-organisms," Jarrod Scott, co-author of the paper and postdoctoral fellow at Smithsonian Tropical Research Institute (Republic of Panama), said.Scott and Johnson both agree that human activity can lead to nutrient pollution, warming waters, and hypoxic ocean conditions. Deoxygenation events can be reduced by better management of activities such as coastal land development, farming, and other related activities.This study provides insights into the fate of coral reef microbe communities during an acute deoxygenation event. The rapid response of reef microbes to changes in physicochemical condition provides reliable indicators of biological and physical processes in nature.The shift in the community's hypoxic microbial condition to a normal state community was detected by the team after the event. This suggests that the recovery pathway of reef microbes can be independent from the benthic macro-organisms. This could allow for the restart of key microbe-related processes that can influence other aspects of the reef community's recovery.Further information: Maggie D. Johnson and colleagues, Rapid ecosystem-scale effects of acute deoxygenation in a Caribbean coral reef, Nature Communications (2021). Information from Nature Communications Maggie D. Johnson and colleagues, Rapid ecosystem-scale effects of acute deoxygenation in a Caribbean coral reef. (2021). DOI: 10.1038/s41467-021-24777-3