WATCH: Secrets of the Seep, a WFSU Ecology Original Documentary
In August of 2023, I traveled off the coast of Oregon on the Research Vessel Atlantis. The Atlantis has a sterling reputation among ocean researchers, and on that voyage, it was carrying a celebrity submersible: the Alvin. Alvin is famous for having been used to locate the wreckage of the Titanic. That was a high-profile discovery, but far from its greatest contribution to science. I was on the Atlantis with an international team of researchers studying a globally important process; and yet, as I learned about their research, I kept thinking about Farmer Herman’s compost pile back in Tallahassee.
When I interviewed Herman Holley about compositing, he stressed the importance of turning the pile. Reshuffling your veggie scraps and lawn clippings keeps oxygen circulating through the mixture, facilitating the process of aerobic decomposition. When you don’t stir your compost, it breaks down more slowly, and it could create methane. This is what happens in a landfill. Anyone who has driven past trash mountains in south or central Florida knows the smell of anaerobic decomposition.
One reason to compost your table scraps rather than send them to the landfill is to reduce the amount of methane, a potent greenhouse gas, that goes into the atmosphere. Years before I boarded a boat full of geochemists, I learned about these processes at Turkey Hill Farm. It made it easier to understand what happens in sediments far beneath the surface of the ocean.
Creating the world’s largest methane source
The ocean bottom functions much like a landfill for our planet’s organic matter (and, sadly, for human waste as well). Much of what dies in the ocean, from whales down to plankton, falls to the bottom. River systems also flush a significant amount of organic matter from land into the oceans. There is no oxygen at the ocean bottom, and so this matter decomposes anaerobically.
More methane is generated in ocean sediments than anywhere else on earth. This research team is studying a natural process that keeps most of that methane out of the atmosphere.
They call their project SeepDOM. Seep is for methane seeps, also known as cold seeps. These are one of a handful of features that emit methane from ocean sediments into the water column. Of these features, hydrothermal vents are the best known, though cold seeps are more numerous. One paper I found estimates that, worldwide, there are hundreds of vents and thousands of seeps.
Fun fact: researchers using the Alvin discovered hydrothermal vents in 1977, and methane seeps in 1983, off Florida’s Gulf coast.
The DOM in SeepDOM stands for Dissolved Organic Matter, which, in Secrets of the Seep, the team uses almost interchangeably with DOC (Dissolved Organic Carbon). Dr. Laura Lapham explains DOC/ DOM this way:
“Tea’s a great example,” Laura says. She is the team’s Chief Scientist, leading a contingent from the University of Maryland Center for Environmental Studies. “You have these leaves for whatever kind of tea you like, and you put it in water. You have to heat it, and those compounds [that] come up, that’s dissolved organic carbon. That’s dissolved organic matter.”
Where does Dissolved Organic Carbon/ Matter come from?
When you drink tea, you’re drinking DOM. If you’re in the ocean, you’re swimming in DOM. There’s a lot researchers don’t know about DOM, but there is a lot of it in the ocean, and it is part of the food web.
“Dissolved organic carbon is the largest reservoir of organic matter in the ocean,” says Dr. John Pohlman. He’s a research chemist with the US Geological Survey, and a Principal Investigator for the SeepDOM project. “Methane in the seafloor is the most abundant form of methane on earth.”
There’s a lot of organic carbon in the ocean and its sediments. Understandably, scientists want to know how this carbon moves within the global carbon cycle. They want to understand what happens to methane – which is, again, a potent greenhouse gas – when it enters the water column.
One clue is the age of DOC near methane seeps. John was part of a research team that found DOC near methane seeps was older than DOC in the rest of the ocean. Part of the DOC carbon mixture is Carbon 14, the isotope used in radiocarbon dating. The age of DOC could tell us where it comes from, and the DOC in their study was near a source of methane, which forms slowly over geologic time.
The SeepDOM team’s mission? To unravel this connection. Their hypothesis? Microbes are converting methane to DOC/ DOM. This process could be what keeps this methane from entering the atmosphere.
Sampling the ocean bottom
I’ll leave the weather drama to those of you who watch the documentary (and please do! It’s right at the top of the page, if you missed it). The short version is that, on their first attempt to collect water from above a seep, waves pushed the Atlantis a kilometer from their site over Hydrate Ridge. Weather was also too rough there to launch the Alvin.
Our first look at a methane seep came from a piece of equipment called the multicore. Once in the water, the multicore remained tethered to the ship, sending images back to the ship’s computer lab. Using the images as a guide, the research team would direct the ship’s crew to position the Atlantis directly over the part of the seep they wanted to sample.
Sunlight does not reach 800 meters beneath the surface of the ocean. It’s not usually an active place, at least not in a way that we can see. But the seafloor was not as empty as the researchers thought it might be. Sea stars were the most numerous of the animals we could see. We could also see a few pink bottom-feeding fish: the shortspine thornyhead. There was the occasional king crab.
Those are the animals on the ocean mud around seeps.
The most visible signs of a methane seep are the organisms that, directly or indirectly, feed on methane and other chemicals in the seafloor. They do so through a process known as chemosynthesis. Up here on the surface, plants use sunlight to convert carbon dioxide and water into carbohydrates, which they use to build their bodies. Our food web is based on photosynthesis, whether we eat plants directly, or eat plant-eating animals.
Where there is no sunlight, microbes can take energy from chemicals such as methane or hydrogen sulfide. So, when choosing a sample site, the SeepDOM team is looking for… microbes?
What does a methane seep look like?
Actually, yes, they look for microbes. But how do they see them?
“The most important thing we look for are bacterial mats,” says Dr. Jeff Seewald. Jeff is a Senior Scientist in Marine Chemistry and Geochemistry at Woods Hole Oceanographic Institution. He’s also a PI on the SeepDOM team. “And that’s a telltale sign that there’s fluid or chemicals being released at the seafloor. The bacteria that form these mats, they live off the chemical energy that’s contained in those fluids. When it mixes with seawater, this creates what’s called a redox gradient. But it’s basically the chemical reactions, just like when you burn natural gas there’s a lot of energy released. They’re actually using the chemicals in the same way to get energy to live off of.”
We’ll go into that process in a second. For now, we just need to know that microbes such as Beggiatoa form white mats at the seep setting. If there’s iron in the sediment, microbes could make iron sulfide precipitate, which is black. So, patches of white and/ or black would indicate a methane seep.
Microbes color the seafloor with their activity, and they also form symbiotic relationships with animals. One such animal that came up in sediment cores was an Acharax, a genus of mussels whose gills house sulfur-oxidizing bacteria. The bacteria receive shelter within the shell of a bivalve, and the mussel is fed the byproduct of chemical reactions.
Closer to home, in the Gulf, tubeworms have a similar relationship with microbes at methane seeps.
The SeepDOM teams also looked for bubbles coming up from the seep, which they only saw at one site in Astoria Canyon.
Microbes dealt it, and we smelt it
This is the WFSU Ecology Blog, not the WFSU Chemistry Blog. But I will do my best to explain the hypothesized methane/ DOC connections as simply as possible. It starts with the smell of rotten eggs.
When that first multicore came back up, Dr. Karen Lloyd (currently at the University of Southern California) stuck her finger into the captured muck and smelled it. Never have I seen people so pleased about an odor that would cause most of us to roll down our car windows.
As Laura Lapham explains, “It’s really important because it is the byproduct of the microbial reaction that we’re looking for, that sulfate reduction, taking sulfate and potentially methane, with the organisms working together to take those compounds and eventually make sulfide, the the rotten egg smell. And so we use [smell] just as the first indicator.”
In this reduction, microbes break apart molecules and reassemble their atoms into new molecules.
Sulfate has one sulfur and four oxygen atoms. Methane has one carbon and four hydrogen atoms. Water has (you should all know this) two hydrogen and one oxygen atom. Microbes chemically burn these molecules, and essentially regroup the different hydrogen, sulfur, carbon, oxygen, and other atoms into new molecules. One byproduct of this process is hydrogen sulfide (two hydrogen, one sulfur). Another byproduct is dissolved organic carbon/ matter.
Dissolved organic matter contains carbon in different mixtures with nitrogen, phosphorous, sulfur, hydrogen, oxygen, and/ or a number of other elements. Depending on the composition of the different DOM molecules, they could feed different types of microbes, entering a larger ocean food web. Or they might not be in a form that any organism can consume.
Methane-derived carbon in the great big ocean
How far does methane-derived carbon travel through the ocean food web? Is there any in my grouper sandwich? We explore the food web of the ocean bottom, and its connectedness with the rest of the ocean, on our next episode of Coast to Canopy, WFSU’s Ecology podcast. That will drop on May 13. I talked to Dr. Karen Lloyd and Dr. Ellen Lalk from from the SeepDOM team, and Dr. Amy Baco-Taylor, Professor of Oceanography and Environmental Science at Florida State University.
Chemosynthetic deep-sea communities were discovered less than 50 years ago. In the grand scheme of science, that’s not a long time, and seeps and vents are not the most accessible places on Earth. There’s a lot we don’t know, but every expedition like this one expands our understanding.
One moment that hints at a larger connection came during the last Alvin dive of the cruise, when a school of sablefish harassed the submersible and prevented the team from getting their samples. Sablefish are a large, commercially fished species. They inhabit the brighter open ocean above the seeps, but are known to descend to these depths. Why do they do this? And what do they eat when they’re down there?
Science billions of years in the making
Since the beginning of life on Earth, microbes have helped regulate the chemistry of our oceans. But the ocean is acidifying and becoming warmer, altering the chemistry of our planet’s largest carbon sinks. Before we can know how these changes will affect the global carbon cycle, we need to understand the processes that drive it.
That work is accomplished one sample at a time. It is accomplished with every drop of water squeezed from between grains of sediment while jamming out to Guns’n’Roses, and with every syringe of methane extracted from a late night water sample. It is accomplished with a lot of planning and a little bit of luck, despite the weather and aggressive fish.
We went out to sea a year and a half ago, and the first papers are just starting to be published. The team will be publishing papers from this project for years. The initial work they did on the ship seemed to confirm their hypothesis, but the work needs to be done.
“Ultimately, what our responsibility as scientists is to write this down in publications so that other scientists and the public can be aware of what we’re doing out here,” says John Pohlman. “Why is it important for advancing science? Why is it important for understanding the health of the oceans, and how the carbon cycle on the Earth is operating, and its sensitivity to climate change and other aspects?”
Photo odds and ends
Cups for the Alvin
As we see in the documentary, the research team drew images on styrofoam cups to send down on the Alvin. They hung the cups on the outside of the submersible in a mesh bag. The pressure at 800 meters deep squeezes the air from the hollow spaces in the styrofoam, shrinking them “to the size of about a shot glass.” Below are a few that didn’t make it into the show.
Below is my cup, post-Alvin dive. The black cat on the sea floor is our family pet, Maurice, which I thought my children would enjoy.
This material is based upon work supported by the National Science Foundation under grant numbers #OCE-2049517, #OCE-2048831, #OCE-2048357. Any opinions, findings, conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
