Imagine a world where our understanding of the ocean's vital role in absorbing carbon dioxide is turned upside down. That's exactly what's happening in the deep sea, where scientists are discovering that carbon is being 'fixed' in ways they never anticipated! Researchers at UC Santa Barbara, in collaboration with other institutions, are challenging long-held beliefs about how carbon dioxide is captured and stored in the dark, mysterious depths of our oceans. This groundbreaking work, led by microbial oceanographer Alyson Santoro, helps to bridge a significant gap between previous estimates of nitrogen availability and the actual measurements of dissolved inorganic carbon (DIC) fixation in these deep waters. This project was supported in part by the National Science Foundation, highlighting the importance of this research.
"Something that we've been trying to get a better handle on is how much of the carbon in the ocean is getting fixed," Santoro explained. "The numbers work out now, which is great." In essence, they're finally piecing together a more complete picture of the ocean's carbon cycle.
The ocean plays a crucial role as the Earth's largest carbon sink, absorbing approximately one-third of all human-generated carbon dioxide emissions. It acts as a planetary buffer, helping to regulate global temperatures. Because of this vital function, scientists are intensely focused on unraveling the complexities of how carbon enters, moves through, and is ultimately stored within the ocean. Think of it as understanding the ocean's 'carbon budget' to ensure it can continue to effectively regulate our climate.
"We want to know how carbon moves around the deep ocean, because in order for the ocean to impact the climate, carbon has to make it from the atmosphere to the deep ocean," Santoro emphasized. This makes the deep ocean a key player in the global carbon cycle.
So, who are the key players in this carbon fixation process? At the ocean's surface, phytoplankton, microscopic single-celled organisms, utilize photosynthesis to absorb inorganic carbon dioxide. Like land plants, these autotrophs create their own food using carbon dioxide and water, releasing oxygen as a byproduct. They're the foundation of the marine food web and critical for carbon capture.
But here's where it gets controversial... For years, scientists believed that while surface phytoplankton were significant, a substantial amount of DIC fixation also occurred in the deep, dark ocean, far beyond the reach of sunlight. It was largely assumed that this deep-sea carbon fixation was primarily driven by autotrophic archaea. These archaea obtain energy by oxidizing ammonia (a nitrogen-containing compound) rather than relying on sunlight, making them ideally suited for the sunless depths. Could it be that we've been giving them too much credit?
However, when researchers meticulously examined the nitrogen-based energy needs of these carbon-fixing microbes by carefully sampling the water column, they encountered a significant problem: the numbers simply didn't add up. There was a glaring discrepancy between the measured rates of carbon fixation and the available energy sources in the deep ocean.
"There was a discrepancy between what people would measure when they went out on a ship to measure carbon fixation and what was understood to be the energy sources for microbes," Santoro explained. "We basically couldn't get the budget to work out for the organisms that are fixing carbon." In other words, for these microbes to fix carbon, they need energy. But the amount of nitrogen-derived energy present in the deep ocean seemed insufficient to support the high carbon fixation rates that were being observed. Imagine trying to run a car on an empty gas tank – it just wouldn't work!
This puzzling mismatch has occupied Santoro and lead author Barbara Bayer for nearly a decade, as they've diligently worked to close a critical gap in our understanding of the ocean's carbon cycle. Previous studies explored the possibility that the carbon-fixing archaea were far more efficient than initially thought, requiring less nitrogen to fix the same amount of carbon. However, their research demonstrated that this hypothesis was not supported by the evidence. And this is the part most people miss: it wasn't just about finding the microbes, but understanding their energy requirements.
For this new study, the researchers took a different approach, focusing on a crucial question: How much do these ammonia oxidizers actually contribute to the overall dissolved inorganic carbon fixation in the dark ocean? To address this, Bayer designed a carefully targeted experiment. "She came up with a way to specifically inhibit their activity in the deep ocean," Santoro explained. By using a specialized chemical inhibitor to limit the activity of these ammonia oxidizers, the team anticipated a significant reduction in carbon fixation rates. The inhibitor, phenylacetylene, was rigorously tested to ensure it had no other measurable effects on other community processes, ensuring the results were directly linked to the targeted archaea.
The results were surprising! Despite effectively inhibiting the activity of these abundant ammonia-oxidizing archaea, the rate of carbon fixation in the study areas didn't decrease as much as expected. This suggests that other players must be involved!
If ammonia-oxidizing archaea are not responsible for as much carbon fixation as previously believed, other microbes must be stepping up to fill the void. The potential contributors now include a wider range of microorganisms within the surrounding community, particularly bacteria and other types of archaea.
"We think that what this means is that the heterotrophs -- microorganisms that feed on organic carbon from decomposing microbes and other marine life -- are taking up a lot of inorganic carbon in addition to the organic carbon that they usually consume," Santoro said, "meaning that they're also responsible for fixing some carbon dioxide."
"And that's really interesting because even though we know this to be a theoretical possibility, we didn't really have a quantitative number on what fraction of the carbon in the deep ocean was getting fixed by these heterotrophs versus autotrophs. And now we do." This suggests a more complex and interconnected food web than previously imagined.
These new findings go beyond simply identifying the carbon fixers at depth. They also offer valuable insights into the structure and sustainability of the deep ocean's food web. "There are basic aspects of how the food web works in the deep ocean that we don't understand," Santoro said, "and I think of this as figuring out how the very base of the food web in the deep ocean works." It's like discovering a hidden foundation upon which the entire ecosystem is built.
The journey doesn't end here. Santoro and her collaborators plan to delve deeper into the intricacies of carbon fixation in the ocean, exploring the complex interplay between the nitrogen cycle, the carbon cycle, and other elemental cycles, including those of iron and copper. "The other thing we're trying to figure out is once these organisms fix the carbon into their cells, how does it become available to the rest of the food web?" she noted. "What kinds of organic compounds might they be leaking out of their cells that could be feeding the rest of the food web with?" These are the questions that will drive future research and further refine our understanding of the ocean's carbon cycle.
This research raises some fascinating questions. Could it be that heterotrophic bacteria are playing a far more significant role in deep-sea carbon cycling than previously thought? How will these findings impact our models of the ocean's carbon sink capacity and its ability to mitigate climate change? Share your thoughts and perspectives in the comments below!
