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Warming ocean puts carbon-capturing plankton at risk

Warming ocean puts carbon-capturing plankton at risk

The ocean is the largest carbon sink we have, and within it sits a microscopic phytoplankton called Emiliania huxleyi. Naked to the human eye, this phytoplankton plays a critical role in drawing carbon from the atmosphere and sequestering it deep in the seas. But with climate change causing ocean water to become warmer and more acidic, researchers have found this phytoplankton could be under threat. 

In a recent study published in the journal 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. 

According to Stillman, these changes could have a massive impact on the planet’s future climate, especially when you consider atmospheric carbon dioxide levels continue to rise sharply as a result of fossil fuel burning and other human activities. 

But how exactly does this phytoplankton work to capture carbon? While floating free in the sunny top layers of the oceans, the phytoplankton develop elaborate plates of calcified armor called coccoliths. The coccoliths form a hard and heavy shell that eventually sinks to the ocean depths. 

"About 80 percent of inorganic carbon trapped down there is from coccoliths like these," said Stillman. 

In a bid to discover how ocean acidification and changes in the ocean's nitrogen cycle -- both hallmarks of climate warming -- might effect coccolith development, Stillman and his team raised more than 200 generations of Emiliania huxleyi in the lab, adjusting carbon dioxide levels and the type of nitrogen in the phytoplankton's seawater bath. 

They found that high levels of carbon dioxide -- which make the water more acidic -- along with a shift in the prevailing nitrogen type from nitrates to ammonium -- "had a synergistic effect" on the phytoplankton's biology and growth. 

In particular, 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 noted. 

"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," Stillman said.