As the oceans warm, the microbes and plankton that live in them are set to be affected drastically - but scientists say they have no idea whether the changes will fuel climate change or work to counter it.
Writing in Science, Oregon State University scientists point out that nearly half the world's photosynthesis is carried out by microbial plankton - and that the process of marine carbon production and consumption is much faster than on land.
A turnover of terrestrial plant biomass takes 15 years, they say, while marine turnover takes just six days.
"We're just beginning to understand microbial diversity in the oceans and what that may mean to the environment," says Stephen Giovannoni, an OSU professor of microbiology.
"However, a large portion of the carbon emitted from human activities ends up in the oceans, which with both their mass of water and biological processes act as a huge buffer against climate change. These are extremely important issues."
As the ocean surface warms, it will become more stratified, with layers mixing less than in the past. While this should reduce overall ocean productivity, the implications for carbon sequestration and global warming are unclear.
"Some warming of surface waters may reduce carbon sequestration, which could cause a feedback loop to increase global warming," says Giovannoni.
"Other forces, what we call the microbial carbon pump, could cause carbon to sink into the deep ocean and be segregated from the atmosphere for thousands of years. We know both of these processes exist, but which one will become dominant is unpredictable, because we know so little about ocean microbes."
Giovannoni is calling for more aggressive development and implementation of marine microbial monitoring technology, and says more standardization is needed on the tools used to assess microbial diversity.