It sounds like a cryptic fortune cookie: He who adds carbon to the ocean will find that it has less.

Adding carbon compounds to ocean water can sometimes affect microbe communities in ways that result in less stored carbon dioxide than has been assumed, a new study published online August 20 in Nature suggests. The oceans’ carbon storage is an important factor in predicting the severity of climate change.

In designing computer simulations of carbon dioxide and its effects on global climate, scientists assume the ocean can absorb a certain amount of the greenhouse gas. These assumptions are based on the idea that other nutrients such as nitrogen determine how much CO₂ phytoplankton — the microscopic “plants” of the sea — will absorb from the atmosphere. The new research, while still preliminary, suggests that CO₂ absorption by the oceans is much more complex.

“The whole calculation of how much carbon you can absorb in the ocean is linked to this question of how microbes near the ocean’s surface respond to carbon and nutrient [ratios],” says study coauthor Frede Thingstad, a microbiologist at the University of Bergen in Norway.

Like plants on land, phytoplankton near the ocean’s surface convert sunlight into useable, chemical energy by absorbing CO₂ and producing carbon-containing compounds such as glucose. Some of this glucose is released into the water, where it serves as a dissolved source of food for bacteria and other microbes. When currents carry this carbon-rich water down into the deep ocean, the carbon can remain stored for centuries.

Thingstad’s group showed that when other nutrients are plentiful and carbon compounds are scarce, adding even small amounts of glucose causes bacteria populations to boom. These proliferating bacteria gobble up the other nutrients, leaving little behind for phytoplankton, the microscopic “plants” of the sea that convert carbon dioxide into forms that can be stored long-term in the deep ocean.

At the same time, instead of absorbing CO₂ as phytoplankton do, bacteria release it as they respire. This CO₂ escapes back to the atmosphere, lowering the total carbon content of the water.

So under these conditions, phytoplankton populations wither and absorb less CO₂, while surging bacteria populations convert more organic carbon back into CO₂.

“This predicts that you’d actually have a carbon release under some conditions,” comments Joe Vallino, a computational microbial biogeochemist at the Marine Biological Laboratory in Woods Hole, Mass. “Instead of burying more carbon, you’ll bury less than if you didn’t have the increased carbon inputs.”

Although the new interaction could influence how much CO2 is pulled from the atmosphere and “buried” in the ocean, these carbon inputs don’t come from atmospheric CO2. Instead, water flowing into the ocean from rivers can carry large amounts of dissolved forms of organic carbon, such as compounds from soil and from river organisms. Oil spills and other forms of pollution can also add carbon compounds to ocean water.

“This is a very important piece of evidence if we try to understand what will happen in the future,” comments Ulf Riebesell, a biological oceanographer at the Leibniz Institute of Marine Sciences at the University of Kiel in Germany. “Models just haven’t considered these kinds of interactions” between phytoplankton and bacteria.

Thingstad and his colleagues grew arctic microbe communities in tanks holding a cubic meter of seawater. For some tanks, the scientists limited the amount of carbon-containing glucose available, and in other tanks nitrogen and other nutrients were in short supply. In the nutrient-deprived tanks, adding glucose had little effect because the bacteria didn’t have enough nutrients to thrive. But in the glucose-limited tanks, adding more glucose led to the counterintuitive drop in overall carbon content.

Although the study was based on Arctic microbe communities, Thingstad says the results, which reveal a basic interaction between phytoplankton and bacteria, probably hold true for oceans worldwide.

Scientists are still uncertain what fraction of the world’s oceans are nutrient-limited and what fraction are carbon-limited, Thingstad says. “The whole system works very differently in these two situations.”

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