As oceans warm in a changing climate, bacteria might be much more able to assimilate typically inert atmospheric nitrogen into biologically useful compounds, in an unexpected boost to the food chain.
David A. Hutchins at the University of Southern California and his colleagues have discovered that in warmer water certain cyanobacteria, Trichodesmium, become dramatically more efficient at using sparse amounts of iron to accomplish this nitrogen transformation, known as nitrogen fixation. These cyanobacteria are the main source of this fixed nitrogen in the ocean. (Nat. Clim. Change 2018, DOI: 10.1038/s41558-018-0216-8).
The amount of nitrogen fixed by organisms including cyanobacteria limits the amount of food available to marine life—including the fish we eat. In turn, the amount of iron available to those cyanobacteria, which need the iron for enzymatic catalysis and to move electrons around, limits nitrogen fixation levels.
In Hutchins’s experiments, the team grew Trichodesmium erythraeum in cultures that either were rich in iron or had very little iron available. They grew both culture types at 22, 27, 32 and 35 °C. All the iron-rich cultures grew more than the iron-deficient cultures. For iron-rich cultures, the cyanobacteria’s optimal growing temperature was 27 °C, rising to 32 °C for the iron-limited cultures.
The team also measured the cyanobacteria’s ability to fix nitrogen across the different temperatures. As the temperature rose from 22 to 32 °C, the iron-deficient cultures became better at fixing nitrogen. Their efficiency rose by 308%, but even this improved number was fivefold below the efficiency of the iron-rich cultures.
When the researchers measured iron levels inside the iron-deficient Trichodesmium cultures, they realized the cyanobacteria had much less iron in their cells at the warmer temperature. Taken together with the 308% increase in nitrogen-fixation efficiency, this meant that the cellular iron use efficiencies (IUEs) in the laboratory experiment rose by up to 470%. (The IUE is the molar quantity of nitrogen fixed by Trichodesmium per unit time per mole of cellular iron.) “In other words, at higher temperatures they can provide the food chain with much more fixed nitrogen using the same amount of iron,” Hutchins says.
Hutchins’s team then used these results in computer models of changing climate. They predict that by 2100, there could be a 76% rise in IUEs. “This could translate into much more biological production in parts of the ocean like the central Pacific that are basically deserts today due to lack of iron,” says Hutchins.
Patricia L. Yager, an oceanographer at the University of Georgia who is an expert in the biogeochemistry of oceans, says that the work shows nicely that climate change is about more than just temperature changing.
“Many things change both physically and chemically when temperatures rise. Complex ecosystems respond in nonlinear ways to these multiple changes,” she says.