Abstract
AbstractDinitrogen (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 of our understanding is based on short‐term laboratory experiments conducted on individual N2‐fixing species whereas community‐level approaches are rare. In this longer‐term in situ mesocosm study, we aimed to improve our understanding on the role of rising atmospheric carbon dioxide (CO2) and simulated deep water upwelling on N2 and carbon (C) fixation rates in a natural oligotrophic plankton community. We deployed nine mesocosms in the subtropical North Atlantic Ocean and enriched seven of these with CO2 to yield a range of treatments (partial pressure of CO2, pCO2 = 352–1025 μatm). We measured rates of N2 and C fixation in both light and dark incubations over the 55‐day study period. High pCO2 negatively impacted light and dark N2 fixation rates in the oligotrophic phase before simulated upwelling, while the effect reversed in the light N2 fixation rates in the bloom decay phase after added nutrients were consumed. Dust deposition and simulated upwelling of nutrient‐rich deep water increased N2 fixation rates and nifH gene abundances of selected clades including the unicellular diazotrophic cyanobacterium clade UCYN‐B. Elevated pCO2 increased C fixation rates in the decay phase. We conclude that elevated pCO2 and pulses of upwelling have pronounced effects on diazotrophy and primary producers, and upwelling and dust deposition modify the pCO2 effect in natural assemblages.
Highlights
The nutrient-rich deep water and dust deposition had a marked impact on both N2 fixation and C fixation rates, magnitudes higher than the difference between CO2 treatments
Our analysis indicated that higher NOxÀ concentrations from addition of deep water in the middle of the study did not inhibit N2 fixation rates, providing more support for challenging the classical paradigm in our understanding of N2 fixation
Unicellular diazotrophic cyanobacterium clade UCYN-B seems to have benefited the most from the deep water addition, while gammaproteobacterial abundances might have been introduced to the mesocosms with the deep water addition
Summary
Previous studies on open ocean N2 fixers were carried out mainly on Trichodesmium and Crocosphaera (UCYN-B), two common diazotrophic species in the ocean, in monoclonal culture experiments (e.g., Hutchins et al, 2007; Fu et al, 2008; Wannicke et al, 2018). Culture studies are not fully representative as they overlook an important part of the diazotrophic community and their interactions in the environment To address these limitations and gain insights into a natural community’s response to high pCO2, we have sub-sampled an in situ mesocosm experiment with eight different pCO2 levels ranging from the present day atmospheric value to levels projected for the end of this century under selected CO2 emission scenarios (Bindoff et al 2019)
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