How will ocean anoxic zones interact with climate change?

Researchers at UC Santa Barbara have investigated the dynamics of ecosystems in parts of the ocean that have no dissolved oxygen to sustain animals or plants, which are known as ocean anoxic zones. In these areas, only microbes that are adapted to the environment can survive. 

“You don’t get big fish,” said study co-author Morgan Raven. “You don’t even get charismatic zooplankton.” Even though anoxic oceans may seem alien to organisms like ourselves that breathe oxygen, they are full of life, she added.

As a result of climate change, ocean anoxic zones are now expanding. Raven is interested in learning how this will affect the ocean as a carbon sink. “What happens to our carbon cycle as we get these large areas of the ocean that are oxygen-free?” 

In seawater that is rich in oxygen, carbon is moved around through the food web after being captured by phytoplankton that photosynthesize at the water’s surface.

“Most of the time they just get eaten by zooplankton,” said Raven. But if they are not eaten, the phytoplankton respire carbon dioxide and excrete organic carbon in the depths of the water. “It’s like a spinning wheel – CO2 goes to plankton, goes to CO2.”

In the absence of zooplankton and fish, more of the sinking organic carbon can survive and be deposited at depth, explained Raven. Sediments underneath anoxic zones generally have more organic carbon deposits, but experts do not know why this is the case.

To investigate, the team turned to a theory formed about a decade ago by geologist Don Canfield and colleagues at the University of Southern Denmark.

“They put out this idea that maybe inside of these zones, microbes are still eating organic carbon, but respiring sulfate.” This process is referred to as cryptic sulfur cycling.

The researchers tested large, fast-sinking organic particles collected from the Eastern Tropical North Pacific oxygen-deficient zone located off the northwestern coast of Mexico.

Raven said the aggregations of mostly dead phytoplankton, fecal matter, other small organisms, and bits of sand and clay get glued together in a sticky matrix. These particles were sent to the Stanford Synchotron Radiation Lightsource for analysis.

The results of the analysis, including evidence of the production of organic sulfur within the samples, demonstrate what Raven called “pickling” of the dead phytoplankton as they sink through the anoxic area.

“Phytoplankton grow in the surface ocean, but due to gravity, they sink.” As they fall through the anoxic region and undergo sulfurization, they create a shield that protects the carbon at their core. 

“Even when it gets to the sediment, bacteria there can’t eat these organic particles.” Raven explained that, just like the pickles we know and love, the preservation process makes the particles resistant to bacteria, which could explain why more organic carbon is found in the sediments below anoxic ocean zones.

According to the researchers, the sulfurization of organic carbon particles in anoxic ocean zones is actually an ancient process, despite the fact that it has just been confirmed in modern oceans. “It’s the same process that can also make petroleum,” said Raven. 

It is not yet clear how anoxic zones will interact with climate change. “Potentially as these zones expand, there could be a negative feedback – more CO2 in the atmosphere makes higher temperatures, which makes these zones bigger. These bigger zones then trap more CO2 and put it in the sediment and rocks.” 

Raven said that this feedback could help the Earth balance its carbon cycle over time, “but we need to know how this connects to everything else.”

The study is published in the journal Science.

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By Chrissy Sexton, Earth.com Staff Writer