Researchers delve into the biogeochemistry of anoxic zones in the ocean

Without dissolved oxygen to sustain animals or plants, anoxic zones in the ocean are areas where only microbes suitable for the environment can live.

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

These strange ecosystems are expanding thanks to climate change – a development that is important to fisheries and everyone who depends on oxygen-rich oceans. But what interests Raven is the changing chemistry of the oceans – Earth’s largest carbon sink – and how it could move carbon from the atmosphere into long-lasting reservoirs like rocks.

“What will happen to our carbon cycle if we get these large areas of the ocean that are oxygen-free?” she said. This question was central 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 at the water’s surface.

“Usually they just get eaten by zooplankton,” Raven said. But when not eaten by larger animals, they move to the depths where they inhale carbon dioxide and excrete organic carbon.

‘It’s like a spinning wheel – CO₂ goes to plankton, goes to CO₂, ‘Said Raven.

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

“It’s been a bit of a mystery,” Raven said.

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

“They put forward the idea that maybe microbes in these zones are still eating organic carbon, but breathing sulfate,” Raven said. The idea, dubbed “cryptic sulfur cycles,” was somewhat difficult to accept, mainly because the products of this microbial sulfate reduction (MSR) were difficult to detect and because other compounds in the area, such as nitrates, were more energetically favorable to metabolize.

However, according to the study, “there is emerging molecular and geochemical evidence to suggest that MSR may occur in (oxygen-deficient zones) despite abundant dissolved nitrate.”

The researchers tested whether this puzzling process could hide in large (> 1 mm), rapidly sinking organic particles by collecting particles from the oxygen-depleted zone in the eastern tropical north of the Pacific Ocean roughly off the northwest coast of Mexico.

“It’s really just this polymeric, sticky stuff,” Raven said of the accumulations of mostly dead phytoplankton, feces, other tiny organisms, and bits of sand and clay glued together in a “fluffy” matrix. Collecting these particles is an achievement in itself for researchers combing the vast oceans in search of relatively small, diffuse particles.

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

Pickled phytoplankton

Results of the analysis, such as evidence of organic sulfur production in the samples, show what Raven calls a “pickling” of the dead phytoplankton as they sink through the anoxic region.

“Phytoplankton grow in the ocean on the surface, but gravity makes them sink,” she said. As they pass through the anoxic region, these organic aggregates undergo sulfurization, which has the effect of shielding the carbon in their core from enzymes or other substances that would otherwise wear them away.

“Even if it gets into the sediment, bacteria cannot eat these organic particles there,” noted Raven. And like the pickles we know and love, the preservation process makes the organic particle resistant to bacteria, she said, which could explain why more organic carbon is found in the sediments beneath anoxic ocean zones.

Sulfurization of organic carbon particles in anoxic ocean zones, while recently confirmed in modern oceans, is actually an ancient process, Raven explained.

“It’s the same process that can make petroleum,” she said, pointing out that where oil beds are found, that’s sulfur too. This process may have been 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 resulted in the burial of massive amounts of carbon and anoxic water across the Atlantic.

“What we didn’t know is whether this happened in these less extremely modern environments,” said Raven.

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

“Potentially as these zones expand, there could be negative feedback – more CO₂ in the atmosphere creates higher temperatures, making these zones bigger, ”said Raven. These larger zones then collect more CO₂ and put it in the sediment and rocks. This feedback could help Earth balance its carbon cycle over time, she said, “ but we need to know how this relates to everything else. ‘