A chemical reaction that gives flavour to cooked food may lock away millions of tonnes of carbon in the seabed each year. The process might even have helped create the conditions for complex life to evolve.
The Maillard reaction occurs between sugars and amino acids when temperatures rise above roughly 140°C. This chemical process produces a range of complex, carbon-rich compounds, giving colour and flavour to foods such as seared meat, roasted vegetables and toasted bread.
Minerals containing manganese can act as a catalyst, enabling the reaction to occur at temperatures as low as 25°C.
To explore whether it can happen at even lower temperatures, Caroline Peacock at the University of Leeds, UK, and her colleagues added either iron or manganese minerals to a solution containing the sugar glucose and the amino acid glycine.
When the mixtures were incubated at 10°C – roughly the temperature of the seabed at the edges of continents – the minerals sped up the Maillard reaction by around 100 times, compared with mixtures of sugar and amino acids without the catalysts.
Further analysis revealed that the process produced compounds that are found in marine sediment samples. This suggests the Maillard reaction occurs on the ocean floor, where iron and manganese minerals are commonly found, says Peacock.
On the seabed, dead plants and animals provide a source of sugars and amino acids that microbes ingest as a source of energy. During this process, the microbes convert the carbon in dead organisms into carbon dioxide, which can re-enter the atmosphere.
If the Maillard reaction is happening on the ocean floor, this could cause the carbon found in sugars and amino acids to be stored in large, complex polymers that microbes find harder to ingest, says Peacock.
Over thousands or millions of years, these polymers would be buried deeper beneath the sea floor as dead material accumulated on the seabed. “If you can get your carbon through the 1-metre danger zone [at the top of the sea floor], where carbon generally is attacked and degraded and turned back into carbon dioxide by microbes, that will lock it away from the atmosphere,” says Peacock.
The researchers estimate that iron and manganese minerals may lock away roughly 4 million tonnes of carbon each year. Without this process, Earth’s atmosphere may have warmed by a further 5°C over the past 400 million years.
They also estimate that the Maillard reaction in marine sediments may have increased atmospheric oxygen levels by up to 8 per cent over the past 400 million years because burying carbon allows more oxygen to reach Earth’s atmosphere, says Peacock.
“This process has such a profound impact on atmospheric oxygen,” she says. “Because complex life forms require higher levels of oxygen, as they’re more energetically demanding, we think it’s reasonable to surmise this process had a hand in creating conditions required for complex life.”
The team has also found that the reaction can occur in soil that contains iron and manganese minerals, which suggests boosting the minerals in soil could help capture carbon from the atmosphere, says Peacock.
“This is a superb study,” says Jan Amend at the University of Southern California. It highlights how iron and manganese chemistry, which has been largely overlooked in most climate and atmospheric studies, can play a huge role in atmospheric chemistry and Earth’s surface temperature, he says.