Ocean Acidification

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Plankton's shifting role in deep sea carbon storage

The tiny phytoplankton Emiliania huxleyi, invisible to the naked eye, plays an outsized role in drawing carbon from the atmosphere and sequestering it deep in the seas. But this role may change as ocean water becomes warmer and more acidic, according to a San Francisco State University research team.

In a study published in Global Change Biology, SF State Assistant Professor of Biology Jonathon Stillman and colleagues show how climate-driven changes in nitrogen sources and carbon dioxide levels in seawater could work together to make Emiliania huxleyi a less effective agent of carbon storage in the deep ocean, the world's largest carbon sink.

...coccoliths formed under conditions of high carbon dioxide and high ammonium levels were incomplete or hollow, and contained less than the usual amount of inorganic carbon, the researchers note.

"The ratio of inorganic to organic carbon is important," Stillman explains. "As inorganic carbon increases, it adds more ballast to the hard shell, which makes it sink and makes it more likely to be transported to the deep ocean. Without this, the carbon is more likely to be recycled into the Earth's atmosphere."

"Our results suggest in the future there will be overall lower amounts of calcification and overall lower amount of transport of carbon to the deep ocean," he adds.

Emiliania huxleyi typically use nitrates to make proteins, but this form of nitrogen may be in shorter supply for the phytoplankton as the world's oceans grow warmer and more acidic, Stillman and colleagues suggest. In the open ocean, nitrates are upwelled from deep waters, but a thickening layer of warmer surface water could inhibit this upwelling. At the same time, the warmer temperatures favor bacteria that turn recycled nitrogen from surface waters and the atmosphere into ammonium, and acidification inhibits the bacteria that turn ammonium into nitrate.

It is that warmer, thicker surface layer that is becoming more stabilized with the warming of the sea surface that Boyce etal., 2010 suggested is the prime reason for the decline of phytoplankton. This work confirms the effects of acidification on phytoplankton and the feedback loop formed when acidification reduces the effectiveness of the biological pump, which in turn helps to cause more sea surface warming due to increased atmospheric CO2, causing the shallow part of the thermocline to thicken and stabilize even more, which causes even more phytoplankton decline.

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Quite a bombshell really, one that has got very little coverage.