Global warming threatens key ocean bacteria

Loss of nitrogen-fixers would ripple through ocean ecosystems

Increasing CO2 concentrations may threaten fundamental ocean chemistry. @bberwyn photo.

Staff Report

In another sign that greenhouse gas emissions are going to fundamentally affect ecosystems in ways that we are just beginning to understand, a team of researchers has shown that bacteria in the ocean are losing their ability to fix nitrogen.

The study, published in the journal Science, took a close look at a type of cyanobacteria called Trichodesmium, which through nitrogen fixation, Trichodesmium converts nitrogen gas into ammonia and other molecules that organisms need for survival.

“This is one of the major sources of nitrogen for other organisms in the open ocean,” said Sven Kranz, assistant professor of Earth, Ocean and Atmospheric Science at Florida State University and a co-author of this study. “If Trichodesmium responds negatively to the environmental changes forced upon the ocean by fossil fuel burning, it could have a large effect on our food web.”

Trichodesmium is thought to be responsible for about 50 percent of marine nitrogen fixation, so a decline in its ability could have a major ripple effect on marine ecosystems.

The effects of climate change on Trichodesmium have been studied extensively by scientists in labs across the globe but with widely different results. Some scientists found that increased carbon dioxide in ocean waters caused a decline in nitrogen fixation, while others saw huge increases. Because of the large role these bacteria play in the health of the Earth’s oceans, Kranz and his colleagues sought to resolve the discrepancies.

Some of these discrepancies, they found, are based on the preparation of the water in which these organisms typically grow under laboratory conditions. For example, the researchers found contamination by elements such as ammonia or toxic elements like enhanced copper concentration.

“Any slight differences in the specific ingredients of the water — in this case artificial seawater that scientists prepare — can have a huge effect on the outcome,” Kranz said.

The authors also found that increased carbon dioxide could sometimes stimulate nitrogen fixation but this was offset by the negative effects of the increased ocean acidity.

For this study, Kranz focused on the preliminary data collections and how the cyanobacteria reacted to changing concentrations of iron and carbon dioxide. Shi’s group in China conducted further studies including protein analysis and replicated this work in the field, conducting experiments in the South China Sea in May 2016.

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