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

• Published in: Limnology and Oceanography
• Main Authors: Singh, Arvind, Bach, Lennart T., Löscher, Carolin R., Paul, Allanah J., Ojha, Narendra, Riebesell, Ulf
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2021
• Subjects: North Atlantic
• Online Access: https://portal.findresearcher.sdu.dk/da/publications/e70d84c8-e41e-4f90-a0be-18e74dad03cf; https://doi.org/10.1002/lno.11795; https://findresearcher.sdu.dk/ws/files/190557000/lno.11795.pdf

Dinitrogen (N 2 ) fixation is a major source of bioavailable nitrogen to oligotrophic ocean communities. Yet, we have limited understanding how ongoing climate change could alter N 2 fixation. Most of our understanding is based on short-term laboratory experiments conducted on individual N 2 -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 (CO 2 ) and simulated deep water upwelling on N 2 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 CO 2 to yield a range of treatments (partial pressure of CO 2 , pCO 2 = 352–1025 μatm). We measured rates of N 2 and C fixation in both light and dark incubations over the 55-day study period. High pCO 2 negatively impacted light and dark N 2 fixation rates in the oligotrophic phase before simulated upwelling, while the effect reversed in the light N 2 fixation rates in the bloom decay phase after added nutrients were consumed. Dust deposition and simulated upwelling of nutrient-rich deep water increased N 2 fixation rates and nifH gene abundances of selected clades including the unicellular diazotrophic cyanobacterium clade UCYN-B. Elevated pCO 2 increased C fixation rates in the decay phase. We conclude that elevated pCO 2 and pulses of upwelling have pronounced effects on diazotrophy and primary producers, and upwelling and dust deposition modify the pCO 2 effect in natural assemblages.