Irreversibly increased nitrogen fixation in Trichodesmium experimentally adapted to elevated carbon dioxide

Area of Science:

• Marine microbiology
• Biogeochemistry
• Evolutionary biology

Background:

• Marine cyanobacteria Trichodesmium are crucial for global nitrogen cycling.
• Short-term studies show Trichodesmium nitrogen fixation increases with elevated carbon dioxide (CO2) due to physiological plasticity.
• Long-term adaptive responses of Trichodesmium to rising CO2 remain largely unknown.

Purpose of the Study:

• To investigate the long-term adaptive evolutionary responses of Trichodesmium to projected future elevated CO2 levels.
• To determine if acquired fitness increases under high CO2 are maintained after returning to ancestral CO2 conditions.

Main Methods:

• Experimental evolution of Trichodesmium under sustained elevated CO2 conditions.
• Assessing nitrogen fixation and growth rates before and after selection.
• Evaluating changes in diel nitrogen fixation patterns and DNA methyltransferase activity.
• Testing growth rates under phosphorus limitation.

Main Results:

• Extended experimental evolution under elevated CO2 led to irreversible, substantial increases in Trichodesmium nitrogen fixation and growth rates.
• These enhanced rates persisted for hundreds of generations even after transfer back to ambient CO2 levels.
• Selected strains showed constitutive rate increases, altered diel nitrogen fixation, and increased DNA methyltransferase activity.
• High CO2-selected Trichodesmium exhibited improved phosphorus-limited growth rates.

Conclusions:

• Trichodesmium exhibits unprecedented microbial evolutionary adaptation to high CO2, with acquired fitness benefits being stable and irreversible.
• This adaptation suggests a significant advantage for Trichodesmium in a future acidified and potentially more nutrient-limited ocean.
• The findings highlight the remarkable evolutionary plasticity of key marine microorganisms in response to anthropogenic climate change.