Globally important marine cyanobacteria evolve to become irreversibly adapted to a high carbon dioxide (CO2) environment, finds a study in Nature Communications. Under these conditions, the microbes display increases in growth rates and nitrogen fixation, which remain high even after they return to present-day levels of CO2.

Marine cyanobacteria play a key role in fertilising the world's ocean by fixing atmospheric nitrogen and turning it into a form that is more useable to other plankton species. Previous studies have shown that when certain cyanobacteria are grown under elevated levels of atmospheric CO2 they enhance their nitrogen fixation rates, although it is not clear whether this response can persist in the long-term.

David Hutchins and colleagues grow experimental cultures of the globally abundant cyanobacteria Trichodesmium for 850 generations (4.5 years) under conditions mimicking the high-levels of atmospheric CO2 predicted for the year 2100. They find immediate increases in growth rates and nitrogen fixation, which remain high throughout the experimental period, persisting at the same rate even after cultures are transplanted back into present day CO2 conditions.

As well as increasing their rate of nitrogen fixation, these irreversibly adapted cultures also double the time it takes to reach their peak fixation rate after daylight hours begin, from 3-5 hours in the present day to 5-9 hours under projected future conditions. Such permanent alterations to the physiology of marine microbes under realistic future CO2 levels are likely to have dramatic effects on biogeochemical nutrient cycling and food web ecology in the world's oceans.