Long-term experimental evolution of marine cyanobacteria under elevated CO2 conditions shows potential irreversible adaptations for nitrogen fixation.
Nitrogen is a limiting nutrient in many ecosystems because most life cannot directly use the dinitrogen gas (N2) that makes up 80% of our atmosphere. Therefore, many systems harbor “nitrogen-fixers”: organisms that are able to grab molecular N2 from the air and convert it into usable compounds such as ammonium and nitrates.
The marine system is no exception – Trichodesmium is an extremely abundant genus of marine cyanobacteria that is estimated to fix nearly half the nitrogen used by other organisms in the open ocean.
Scientists have been studying the effects of various conditions on Trichodesmium nitrogen fixation rates for decades. However, a recent study published in Nature Communications revealed the potentially harmful long-term effects of elevated carbon dioxide (CO2) levels on global ocean nitrogen fixation.
Researchers from University of Southern California and Woods Hole Oceanographic Institution performed a 4.5-year experimental evolution study with Trichodesmium erythraeum strain IMS. They cultured six replicates under “normal” CO2 (380ppm; current level) and six under “elevated” CO2 conditions (750ppm; levels projected for 2100).
After 4.5 years, the study found nitrogen fixation rates had nearly doubled in the “elevated” group compared to “normal” group, which stayed consistent with natural rates. Similarly, growth rate in the “elevated” group was markedly enhanced.
This phenomenon of increased nitrogen fixation under elevated CO2 had been shown in previous short-term studies. However, the researchers took their experimental evolution one step further.
The “elevated” group, after 4.5 years at 750ppm CO2, was moved to “normal” conditions and cultured for two more years – nothing changed.
The Trichodesmium group that was grown in elevated CO2 during 4.5 years of experimental evolution retained their increased nitrogen fixation and growth rates even two years after being switched to “normal” conditions.
This “elevated” group that was switched back to “normal” conditions also retained other interesting traits, such a resistance to growth declines during phosphorus limitation and longer peaks in nitrogen fixation during daylight, characteristics not present in the Trichodesmium grown for 4.5 years under “normal” conditions or those found in nature.
Interestingly, the researchers noted no significant difference in expression of the ~1,500 measured proteins between the “normal” and “elevated” strains – not even genes for nitrogen fixation or photosynthesis differed.
How could this be? The research team found that a protein called DNA methyltransferase, a regulatory protein, had an activity much higher in the “elevated” group after it was switched back to “normal” conditions. This shows that gene regulation, rather than the genes themselves, may have evolved over the 4.5-year experiment to confer the enhanced nitrogen fixation and growth rate.
Though the potential nitrogen fixation sounds advantageous, the researchers warn that increased activity of this nitrogen fixer could be bad news for future oceans.
For example, increased productivity of Trichodesmium could lead to trace nutrient depletion and larger periodic population die-offs. There are always trade-offs in a world with finite resources and it is difficult to predict how large systems such as the oceans will change, the researchers noted.
This study is a great reminder of the complexity of the natural environment. As humans continue to affect the planet in myriad ways, systems will adapt, perhaps unpredictably.