Researchers are delving into the biogeochemistry of the anoxic areas of the ocean

Without dissolved oxygen to support animals or plants, oceanic anoxic areas are areas where only environmentally friendly microbes can live.

“You don’t have big fish,” said UC Santa Barbara biogeochemist Morgan Raven. “You don’t even get charismatic zooplankton.” But while anoxic oceans may seem foreign to organisms like us that breathe oxygen, they are full of life, she said.

These strange ecosystems are expanding due to climate change – a development that is worrying for fishing and for anyone relying on the oxygen-rich oceans. But what interests Raven is the changing chemistry of the oceans – the largest carbon sink on Earth – and how it could move carbon from the atmosphere to long-term reservoirs such as rocks.

“What happens to the carbon cycle as we get these large areas of the ocean without oxygen?” she said. This question was essential to the research of Raven and colleagues Rick Keil (University of Washington) and Samuel Webb (Stanford Linear Accelerator Laboratory) in a paper published in the journal Science.

“A spinning wheel”

In oxygen-rich oceans, carbon is largely displaced by food web processes that begin with carbon dioxide-fixing phytoplankton that photosynthesize on the surface of water.

“Most of the time they’re just eaten by zooplankton,” Raven said. But if they are not eaten by larger animals, they go to the depths where they breathe carbon dioxide and remove organic carbon.

“It’s like a spinning wheel – CO₂ goes to plankton, goes to CO₂“Raven said.

However, in the absence of zooplankton and fish, more of the sunken organic carbon can survive and be deposited at depth, she said. In fact, sediments in these anoxic areas generally have more deposits of organic carbon than their oxygen-rich counterparts. But, according to researchers, we lack a “full mechanistic understanding” of how this happens.

“It was a little mysterious,” Raven said.

The team had a clue in the form of a hypothesis formed about a decade ago by geologist Don Canfield and colleagues at the University of Southern Denmark.

“They put forward this idea that maybe inside these areas, microbes still eat organic carbon, but they breathe sulfate,” Raven said. Called “cryptic sulfur cycling”, the idea was somewhat difficult to accept largely because the products of this reduction in microbial sulphate (MSR) were difficult to detect and because other compounds in the area, such as nitrates, were more favorable. energy for metabolism.

However, according to the study, “there is molecular and geochemical evidence to suggest that MSR may occur in (oxygen-deficient areas), despite the abundance of dissolved nitrates.”

The researchers tested whether this enigmatic process could hide inside large,> 1 mm) fast-diving organic particles by collecting particles from the oxygen-deficient area of ​​the tropical Northeast Pacific, located roughly off the coast of northwestern Mexico.

“It’s just this polymeric, sticky thing,” Raven said of the aggregates of dead phytoplankton, feces, other small organisms, and pieces of sand and clay that are glued together into a “fluffy” matrix. The collection of these particles is in itself an achievement for researchers who comb the vast oceans for relatively small, diffuse particles.

“My colleagues at the University of Washington had this collection device that was really the thing that made this possible,” she said. The collected particles were sent to the Stanford Synchotron Radiation Lightsource for analysis.

Phytoplankton wall

The results of the analysis, such as evidence of organic sulfur production in the samples, demonstrate what Raven calls the “pickling” of dead phytoplankton as it sinks through the anoxic zone.

“Phytoplankton are growing in the surface ocean, but due to gravity, they are sinking,” she said. As they fall through the anoxic region, these organic aggregates undergo sulfurization, which has the effect of protecting the carbon in their nucleus from enzymes or other substances that would otherwise carry them.

“Even when it reaches the sediment, the bacteria there can’t eat these organic particles,” Raven said. And just like the pickles we know and love, the preservation process makes organic particles resistant to bacteria, she said, which could explain why more organic carbon is found in sediments beneath anoxic ocean areas. .

Sulfurization of organic carbon particles in anoxic oceanic areas, although recently confirmed in modern oceans, is actually an ancient process, Raven explained.

“It’s the same process that can produce oil,” she said, noting that where oil beds are, so is sulfur. It is possible that this process was widespread during the Cretaceous (145.5 to 65.5 million years ago), when the Earth was constantly tropical and the ocean was subject to geological and mass extinction events that have led to the burial of massive amounts of carbon and anoxic water. across the Atlantic.

“What I didn’t know is whether this is happening in these less extreme modern environments,” Raven said.

What remains to be seen is how these growing oxygen-depleted areas will interact with climate change.

“Potentially as these areas expand, there could be negative feedback – more CO₂ Higher temperatures rise in the atmosphere, making these areas larger, “Raven said.” These larger areas capture more CO₂ “This feedback could help the Earth balance its carbon cycle over time,” she said, “but we need to know how to connect this to anything else.”