Finding out that a process that controls Earth’s climate has been going on in ways scientists didn’t fully understand has been happening in the ocean that covers most of the planet. This is a quietly humbling realization. Researchers came to this realization at the end of last year, when Alyson Santoro, a microbial oceanographer at UCSB, led a team that published results in Nature Geoscience that changed what scientists thought they knew about how carbon is fixed in the deep sea.
In short, the microbes that scientists thought were doing most of the carbon-fixing work in the deep ocean—a group of ammonia-oxidizing archaea—aren’t as responsible as they were thought to be. There has been something else taking up the slack. That something else turns out to be heterotrophs, which are living things that normally take in organic carbon instead of fixing inorganic carbon dioxide. It looks like they’ve been doing both in secret.
That little thing is more important than it seems at first. Every year, about a third of the carbon dioxide that people release into the air is taken in by the ocean. In a practical sense, it’s the reason why climate change hasn’t happened faster. When carbon falls from the air into deep water, it can stay there for hundreds or even thousands of years. It’s not just a school project to figure out which microbes cause that process and how they do it. It changes how well scientists can guess what the ocean can and can’t hold in the next few decades.
Santoro’s team spent almost ten years looking into a difference that kept bothering researchers. Scientists looked at how much carbon was being fixed in deep water and found that more carbon was being fixed than the energy sources that the archaea that break down ammonia could likely support. In the dark ocean, there just wasn’t enough energy from nitrogen to explain what was being seen. We failed to close the budget.
One theory had been around for a long time: maybe those archaea were just very efficient and needed less nitrogen than was thought to fix the same amount of carbon. In earlier work, Santoro had already tested that idea and mostly thrown it out. The team tried something different. Barbara Bayer, the lead author, planned an experiment to use phenylacetylene, a chemical inhibitor, to specifically stop the activity of archaea. This inhibitor was shown to not affect the activity of other microbes. It made sense: if these archaea were really fixing carbon, then blocking them should make rates drop.
They didn’t fall. In some places, they hardly moved at all. That told the researchers that something important was taking place that the current model didn’t take into account.
Take a moment to think about what heterotrophs are, because the finding only makes sense when you know more about them. These are living things that are designed to eat, not make things. They eat organic matter that falls through the water column and breaks down. This includes microscopic pieces of dead marine life that drift to the seafloor. Scientists have always known that heterotrophs could fix some inorganic carbon on the side, but they thought this contribution was too small to be significant. It turned out that guess was wrong.
Aside from the scientific details, what’s interesting about this is what it says about how knowledge grows and then stops growing. It’s easy to make a model when the measurements fit well enough and the gaps are explained away instead of being questioned. It took almost ten years of hard work and an intentionally designed inhibitor experiment to change the numbers. That shows how long it can take for science to change a working assumption, even when the numbers quietly show that something is wrong.
We are still figuring out what this means in real life. Now, Santoro’s group is looking into how the carbon fixed by these heterotrophs ends up being used by other parts of the food chain. What chemicals do these cells give off? More up the chain, how does that feed other living things? These inquiries are important because carbon in the deep ocean doesn’t just sit there; it moves, changes, and interacts in ways that affect the ocean’s ability to store carbon for a long time.
For now, it’s clear that the deep ocean is made up of more complicated parts than what textbooks say. Even now, the archaea are still there and getting things done. But they’ve been on stage with other things that no one was paying close attention to the whole time. That difference should be taken seriously in a system that covers most of the planet and may be one of the few ways we can still survive climate change.
