The oceans store around 700 billion tonnes (771 billion US tons) of carbon, much of which would otherwise be filling our atmosphere and making the planet uninhabitable. Yet not all marine carbon gets preserved this way, and we have a very poor understanding of why some carbon is stored and some is broken down and released. New evidence suggests pigments play a part in the process, raising the possibility that the colors of marine plants and algae may be important in saving the world.
The ocean's carbon is known as dissolved organic matter (DOM). Although some DOM gets released back into the atmosphere, it has a lower proportion of radioactive carbon-14 than living things, meaning that much of it remains in the oceans for a very long time. With a lifetime of more than 1,500 years, DOM effectively represents a way of storing carbon long enough to leave it out of climate change calculations.
The carbon in DOM helps form complex molecules filled with double bonds and oxygen-linked atoms. Yet the exact nature of the composition of these molecules has been poorly understood, and the precursor molecules are even more of a mystery.
A team led by Dr Neal Arakawa from the University of California, San Diego analyzed a sample of DOM collected from the Pacific coast of North America. The molecules were exposed to mild acids, breaking off sugars and amino acids, before being studied using comprehensive gas chromatography. The findings are reported in Science Advances and suggest that DOM is rich in molecules very similar to carotenoid degradation products. These, as their name suggests, are what you get when the carotenoids that give plants their red, yellow, and orange colors are broken down. Beta-carotene, the carotenoid from carrots and pumpkins, was found to degrade to a water-soluble substance similar to DOM.
The researchers confirmed that a minimum of 4 percent of DOM originates in carotenoids, but the proportion may actually be much larger. This may sound small, but not only is it 28 billion tonnes (30 billion US tons) throughout the oceans, it represents a remarkably large proportion considering that carotenoids are only a small component of most living things. The work also offers a starting point for identifying other sources of DOM.
The discovery raises the intriguing possibility that marine life with a higher concentration of carotenoids might be more likely to remain in the oceans after death, rather than transforming into simple molecules that can be released back into the atmosphere. The paper doesn't explore the possibility that we could do something to enhance this – perhaps breeding carotenoid-rich species – but simply understanding one part of the DOM equation could at least help us make better predictions about rates of carbon storage.