Impact of increasing carbon dioxide on dinitrogen and carbon fixation rates under oligotrophic conditions and simulated upwelling

Dinitrogen (N2) fixation is a major source of bioavailable nitrogen to oligotrophic ocean communities. Yet, we have limited understanding how ongoing climate change could alter N2 fixation. Most our based on short-term laboratory experiments conducted individual N2-fixing species whereas community-level approaches are rare. In this longer-term in situ mesocosm study, aimed improve the role rising atmospheric carbon dioxide (CO2) and simulated deep water upwelling (C) rates natural plankton community. We deployed nine mesocosms subtropical North Atlantic Ocean enriched seven these with CO2 yield range treatments (partial pressure CO2, pCO2 = 352–1025 μatm). measured C both light dark incubations over 55-day study period. High negatively impacted phase before upwelling, while effect reversed bloom decay after added nutrients were consumed. Dust deposition nutrient-rich increased nifH gene abundances selected clades including unicellular diazotrophic cyanobacterium clade UCYN-B. Elevated phase. conclude that elevated pulses pronounced effects diazotrophy primary producers, dust modify assemblages.