Tiny Cows of the Sea

By Jaime Fuller

CowsOfTheSeaThere are many earthly phenomena that seem inexplicable. For instance, oceans are a source of methane to the atmosphere, and methane is a greenhouse gas that traps 20 times more heat than carbon dioxide. Surface waters of the oceans are supersaturated with methane relative to the Earth’s atmosphere. This phenomenon is termed the “marine methane paradox.” Certain bacteria that release methane during metabolism can partially explain it. The reason this is so strange is that methane production by microbes was thought to only take place under anaerobic conditions, i.e. when oxygen is lacking. Yet water in the ocean has plenty of oxygen—hence the conundrum.

Angelicque White, assistant professor at Oregon State in Ocean Ecology and Biogeochemistry, hypothesized that methane was produced when microorganisms in the ocean utilized methylphosphonic acid (MPn) as a source of phosphorus. “MPn is produced by certain organisms and used to build cell walls,” pointed out White. “When they die or are grazed, these bits of cell wall are then broken down by other classes of organisms looking for ‘food,’ elements such as carbon, nitrogen, or phosphorus.” All life forms require phosphorus, as phosphate is a necessary component in DNA, RNA, and ATP and is used in the structure of all cell membranes.

One possible source of the methane was the nitrogen-fixing bacteria known as Trichodesmium. They readily use MPn. The only problem was that Trichodesmium are rare in the marine environment. They are typically found in blue, open gyres of oceans that are low in phosphorus, because they can metabolize a wide variety of organic compounds. Yet methane is found all over the ocean, in distinct subsurface layers, year-round. There had to be another source that was cosmopolitan and abundant throughout the year.

That’s where Stephen Giovanni came in, a professor of microbiology at OSU. Early in his career he discovered the most prolific organism on the planet—a marine bacteria called SAR11. For every liter of seawater, there are 100 to 500 million of these comma-shaped organisms. In comparison, there are only one or two cells of Trichodesmium in that same liter. SAR11 exist everywhere in the ocean, at all depths, and they have the ability to metabolize a wide range of organic materials. Most organisms compete for organic matter produced by plants and algae, but SAR11 are the most successful heterotrophs in the ocean. They are also very small and simple. “Sometimes the best solution to the problem is the smallest, simplest, and most efficient,” said Giovanni.

Giovanni’s research includes genome sequencing, which provides valuable clues about organisms make a living. SAR11 has a gene that codes for a methyl phosphonate cleavage enzyme. This means SAR11 may have the ability to separate phosphorus from methane. Giovanni’s grad student, Paul Carini, went to White to give her this news, as he knew she was studying methane production. The question then was, do they actually make the enzyme? Just because a gene is present does not mean it is expressed.

White began measuring methane production from SAR11 under various conditions. She found that SAR11 does produce methane, but only when starved for phosphate. “We can think of SAR11 as the tiny cows of the sea,” jested White. Methane is simply a natural by-product of their metabolism when phosphate is in low supply. White and Giovanni have been studying the range of organic materials SAR11 can use. The bacteria are very difficult to culture and grow slower than most, making experiments challenging. Yet as White notes, “Controlled lab experiments give us a little peek into their daily lives and help us to understand how they respond to changes in their environment.”

Now that they have another piece in the puzzle, the next steps require going out in the field and sampling seawater. “It’s a tricky game to scale up from lab to ocean,” White explained. The goal is to understand if SAR11 is the only source of methane or if there are other sources out there. Does the production of methane vary depending on the season or location?

The stratified layers of open ocean that are low in phosphate, the central gyres (often called the deserts of the oceans), are expanding because of climate change. Since SAR11 thrive in these low-nutrient conditions, their range and perhaps methane production might be increasing. It’s not possible to say with any certainty whether more methane will be produced because of ocean desertification. “Even if you know a lot about the details, a very complicated system still displays unpredictable behavior,” clarified Giovanni.

Understanding methane production in the ocean helps sort out what processes are responsible for producing that methane, thus providing clues as to how that production is regulated. “We certainly haven’t solved this story in its entirety,” concluded Giovanni.