Program

The dissolution of rising atmospheric carbon dioxide (CO₂) in the ocean leads to a shift in seawater carbonate chemistry, including an increase in pCO₂ and a decrease in pH, changes collectively referred to as ocean acidification. Many researches have been conducted to investigate the impact of ocean acidification on Trichodesmium, a colonial nitrogen (N₂)-fixing marine cyanobacterium that plays an important role in the structure and function of the ecosystem in oligotrophic oceans. To the best of our knowledge, however, how pCO₂ and pH individually may affect N₂ fixation and growth of Trichodesmium remains essentially unknown. Here, we manipulated the carbonate chemistry of an artificial seawater medium by varying the concentration of dissolved inorganic carbon (DIC) and meanwhile maintaining the pH with organic buffers. We then assessed the growth and N₂ fixation of Trichodesmium IMS 101 in response to different pH/pCO₂ matrix [i.e., constant pH (8.1 and 7.8) or pCO₂ (400 μatm and 800 μatm)]. Our study reveals that higher pCO₂ could slightly stimulate growth and N₂-fixation of Trichodesmium due to the down regulation of the CO₂-concentrating mechanism (CCM), while lower pH has negative impacts that overwhelm the positive effects of high CO₂. Overall, N₂ fixation and growth of Trichodesmium decline in an acidified ocean.