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Photo credit: pixabay once we smell rotten eggs, I'm afraid we can't forget them. On Earth about 250 million years ago, a similar odor was probably everywhere. Even more frightening is that the odor comes from a toxic substance-hydrogen sulfide.

Have you ever smelled rotten eggs? When you pick up an egg without knowing anything-it looks good, but it's gone bad-with a light knock, the egg suddenly explodes, and in the "sizzling" sound of oil and egg white collision, a foul smell comes to your face. Simply put, there are only two words: "sour."

When eggs go bad, protein is decomposed by bacteria, which will produce sulfides, ammonia, amines and other substances, which all have strong irritating odors and together constitute the stench we smell. Although the smell of rotten eggs itself is so rich, if we say the word "rotten egg smell", we first think of only one substance: hydrogen sulfide.

If you search for "hydrogen sulfide" online, you will find warnings such as "toxic" and "flammable" in addition to descriptions such as "rotten egg smell at low concentrations." Even if you haven't smelled it, you can probably imagine how terrible this gas is.

If we were unlucky enough to hit a rotten egg while cooking, at least we could scrub it up quickly and ventilate the kitchen. But on Earth about 250 million years ago, it is likely that large swaths of the ocean smelled of rotten eggs, and there was no place for life to hide.

The biggest problem for all creatures living at this time was not the smell, but the deadly threat it posed-a mass extinction event.

Of the five mass extinctions in Earth's history, the worst wasn't the one that wiped out most dinosaurs, but the end-Permian mass extinction that occurred about 252 million years ago. This extinction event has caused a devastating blow to the entire Earth's ecosystem, directly wiping out 80% of marine species, and life on land is also doomed.

As with other mass extinctions, the cause of the end-Permian extinction has long been controversial. Many studies attribute the extinction event to the Siberian volcanic eruption. Of course, no volcano, no matter how large, can cover the entire earth with lava. The real danger is likely to be climate change caused by volcanic eruptions. But what exactly happened after the eruption? What climate change caused such a severe extinction?

In a recent study published in Nature Geoscience, researchers at the University of California, Riverside, suggest that hydrogen sulfide, which may smell like rotten eggs, poisons a large number of organisms.

The "dead sea" is deep in the ocean, where dissolved oxygen is limited. If excess organic matter (often from the corpses of photosynthetic organisms) settles from the surface at special times, the decomposition process of these "corpses" can easily consume the remaining oxygen. In such anoxic environments, a special class of bacteria may "step forward" and "breathe" sulfate and reduce sulfur to hydrogen sulfide.

Such environments are called euxinic, a beautiful name, but they are often deadly to most creatures-oxygen is scarce for survival, and toxic hydrogen sulfide is present. Fortunately, the conditions for the stable existence of such "dead seas" are very harsh. At present, less than 0.5% of the world's seabed is in a quiet sea environment, and it is often located in deep seas.

However, if such a water body covers a larger area, it can not only directly threaten life in the ocean, but also release toxic hydrogen sulfide gas into the atmosphere, thus affecting the ecological environment on land.

In fact, there is considerable evidence that large areas of the ocean were in a calm environment during the end-Permian mass extinction event. A 2008 study published in Science found evidence for this in a particular group of Chlorobiaceae bacteria.

Green sulfur bacteria are anaerobic photosynthetic bacteria that can utilize sulfur compounds such as hydrogen sulfide for photosynthesis. The anaerobic photosynthesis of green sulfur bacteria needs to meet two basic conditions: first, in the surface of the ocean, there is enough sunlight; second, there is sufficient sulfide. In surface water, sulfur compounds mostly come from hydrogen sulfide gas released from deep water. In other words, the presence of green sulfur bacteria means that this place was once in the "dead sea."

Photosynthetic photoreaction system of green sulfur bacteria (Photo source: Chen et al., 2020) Researchers analyzed cores from corresponding formations in the Perth Basin in Australia and found pigment substances unique to green sulfur bacteria. In addition, the existence forms of iron elements in the strata and sulfur isotope data also prove that many oceans were in a quiet environment at the end of the Permian. Hydrogen sulfide released by microorganisms not only turns the ocean into a true "sea of death," but also fills the terrestrial environment with unpleasant odors.

Volcano's "stink cycle" So what causes Tranquility to expand? In the new study, published in Nature Geoscience, scientists use models of the Earth system to make their predictions.

According to this model, volcanic-induced climate change has two effects: on the one hand, heavy rainfall and acid gases dissolved in rainwater cause more rock to weather into the ocean, providing nutrients for producers in the ocean; on the other hand, global warming will enhance the metabolism of producers in the surface ocean. These two effects lead to the same result-more organic particles sinking into the deep ocean and more oxygen consumption.

Dominik Hülse, a researcher at the University of California, Riverside, and one of the study's authors, said: "Our study shows that the entire ocean is not entirely oxygen-deficient. This often starts in deeper water. As volcanoes emit large amounts of greenhouse gases and temperatures rise, the oxygen-deficient zones become larger and more toxic, and invade shallower seas where most marine animals live. "In turn, the expansion of calm waters also causes more marine life to die, and their decay consumes more oxygen, thus maintaining this 'odor cycle.'

The smell in the air makes Dominik Hülse pinch his nose in pain (Credit: Dominik Hülse / UCR) The hydrogen sulfide threat around him is not something that will happen only in the history of the earth. It does not even require large-scale climate change. Some man-made pollution may also cause similar results in local areas.

In October last year, residents of Carson City suddenly noticed that the air in their neighborhood had become extremely smelly. After smelling it for a long time, they also experienced headaches, nausea, sore throats and burning eyes. According to the Los Angeles Times, an investigation revealed that a large amount of chemicals such as ethanol stored in a warehouse fire in Carson area flowed down the sewer into the nearby Dominguez Strait.

The investigation speculated that chemicals killed organisms in the strait, thus putting it into a "odor cycle"-biological decay leading to oxygen deprivation in the water, and bacteria producing large amounts of hydrogen sulfide in an oxygen-deficient environment. After the hydrogen sulfide was released into the atmosphere, local authorities received nearly 5000 complaints within half a month, forcing about 3200 residents in Carson and surrounding areas to temporarily move to hotels.

"We can't say for sure whether the ancient mass extinction event will be repeated today," Hülse said."However, this study does suggest that higher atmospheric CO2 concentrations may cause unexpected changes in the environment. "

Reference link:

https://www.science.org/doi/full/10.1126/science.abb6350#tab-citations

https://www.science.org/doi/10.1126/science.1104323

https://www.nature.com/articles/s41561-021-00829-7#MOESM1

This article comes from Weixin Official Accounts: Global Science (ID: huanqiukexue), author: Mao Mao, proofread: chestnut

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