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 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.

中文翻译：

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在贫营养条件和模拟上升流下增加二氧化碳对二氮和碳固定率的影响

二氮 (N ₂ ) 固定是贫营养海洋群落生物可利用氮的主要来源。然而，我们对持续的气候变化如何改变 N ₂固定的了解有限。我们的大部分理解是基于对单个 N ₂固定物种进行的短期实验室实验，而社区层面的方法很少见。在这项长期原位中观研究中，我们旨在提高我们对上升的大气二氧化碳 (CO ₂ ) 和模拟深水上升流对天然贫营养浮游生物群落中N ₂和碳 (C) 固定率的作用的理解。我们在亚热带北大西洋部署了九个中宇宙，并用 CO 富集了其中的七个₂产生一系列处理（CO ₂分压，p CO ₂  = 352–1025 μatm）。在 55 天的研究期间，我们测量了明暗培养中 N ₂和 C 的固定率。在模拟上升流之前，高p CO ₂对贫营养阶段的光和暗 N ₂固定率产生负面影响，而在消耗添加的养分后，对光衰期的光 N ₂固定率产生负面影响。富含营养的深水的尘埃沉积和模拟上升流增加了 N ₂固定率和nifH选定进化枝的基因丰度，包括单细胞固氮蓝细菌进化枝 UCYN-B。升高的p CO ₂增加了衰变阶段的 C 固定率。我们得出结论，升高的p CO ₂和上升流脉冲对固氮和初级生产者有显着影响，上升流和灰尘沉积改变了自然组合中的p CO ₂效应。