Oceans without oxygen

News – Without released oxygen to sustain animals or plants, marine anoxic zones are areas where only environmentally friendly microbes can survive.

“You don’t get a big fish,” said Morgan Raven, a UC Santa Barbara biologist. “You won’t even get a charismatic zooplankton.” But while anoxic oceans may look alien to oxygen-breathing organisms like ourselves, they are full of life, she said. .

These strange ecosystems are expanding, thanks to climate change – a development that is a cause for concern for fisheries and anyone who depends on oceans rich in oxygen. But what Raven is interested in is the changing chemistry of the oceans – the largest carbon sink on Earth – and how it could transfer carbon from the atmosphere to long – term reservoirs such as rocks.

“What will happen to our carbon cycle as we get those large areas of the ocean that are free of oxygen?” she said. This question was at the heart of research by Raven and his colleagues Rick Keil (University of Washington) and Samuel Webb (Stanford Linear Acceleration Laboratory) in a paper published in the journal Science.

‘Spinning Wheel’

In oceans full of oxygen, carbon is largely moved around by food web processes that begin with phytoplankton-fixing carbon dioxide that forms a photosynthesis at the surface of the water.

“Most of the time they just eat them with zooplankton,” Raven said. But if larger animals do not eat them, they go to the depths where they respect carbon dioxide and eliminate organic carbon.

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

Without zooplankton and fish, however, more of the organic carbon beneath can survive and deposit at depth, she said. In fact, sediments beneath these anoxic zones typically have more organic carbon deposits than their oxygen-rich counterparts. However, according to the researchers, we do not have a “full mechanical understanding” of how this happens.

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

The team shed light on a hypothesis form created about a decade ago by University of Southern Denmark physicist Don Canfield and his colleagues.

“They put out this idea that maybe within those zones, microbes are still eating organic carbon, but relieving sulfate,” Raven said. Called “cryptic sulfur cycling,” the idea was somewhat difficult to accept largely because the results of this microbial sulfate reduction (MSR) were difficult to find, and because other fertilizers in the area, such as nitrates, were more potent for metabolizing.

However, according to the study, “molecular and geochemical evidence is emerging that indicates that MSR may occur in (oxygen-deprived zones) despite the presence of high levels of dispersed nitrates.”

The researchers tested the feasibility of hiding this enigmatic process within large organic particles (> 1mm), which were rapidly sinking by collecting grains from the Tropical North Pacific oxygen-deficient zone, near on the northwest coast of Mexico.

“This is just the sticky polymeric material,” Raven said of the accumulations of dead phytoplankton, fecal matter, other small organisms and pieces of sand and clay that stick together in a “floral” matrix. .Collection of these grains is an achievement for researchers combing the oceans for small fragments.

“My colleagues from the University of Washington had this collection device that really made this possible,” she said, and the collected grains were sent to Stanford Synchotron Radiation Lightsource for analysis.

Pickled phytoplankton

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

“Phytoplankton grow in the surface ocean, but because of gravity, they sink,” she said. As they fall through the anoxic region, these organic accumulations undergo sulfurization, which affects the protection of the carbon at the heart of the enzymes or other substances that would wear them away.

“Even when it reaches the sediment, bacteria there can’t eat those organic particles,” Raven said. And just like the pickles we know and like, the preservation process makes the organic part 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 grains in anoxic ocean zones, although newly proven in modern oceans, is indeed an ancient process, Raven explained.

“It’s the only process that petrol can do as well,” she said, pointing out where oil beds are available, as well as sulfur. This process may have been widespread during the Cretaceous period (145.5 to 65.5 million years ago), when the Earth was constantly tropical and the ocean was subject to geological events and catastrophe that led to the burial of large amounts of carbon, and anoxic waters throughout the Atlantic.

“What we didn’t know was whether this was going on in the not-so-new environments,” Raven said.

What remains to be seen is how these reduced oxygen zones interact with climate change.

“As these zones expand, there could be negative feedback – more CO2 in the atmosphere produces higher temperatures, which makes these zones larger, “Raven said.” These large zones then capture more CO2 and put it in the sediment and rocks. “This feedback could help the Earth balance its carbon cycle over time, she said,” but we need to know how to this connects to everything else. “