Garcia Nathan S, Hutchins David A
Marine and Environmental Biology, Department of Biological Sciences, University of Southern California, Los Angeles, CA, United States of America.
PLoS One. 2014 Dec 11;9(12):e114465. doi: 10.1371/journal.pone.0114465. eCollection 2014.
Biological N2 fixation is the dominant supply of new nitrogen (N) to the oceans, but is often inhibited in the presence of fixed N sources such as nitrate (NO3-). Anthropogenic fixed N inputs to the ocean are increasing, but their effect on marine N2 fixation is uncertain. Thus, global estimates of new oceanic N depend on a fundamental understanding of factors that modulate N source preferences by N2-fixing cyanobacteria. We examined the unicellular diazotroph Crocosphaera watsonii (strain WH0003) to determine how the light-limited growth rate influences the inhibitory effects of fixed N on N2 fixation. When growth (µ) was limited by low light (µ = 0.23 d-1), short-term experiments indicated that 0.4 µM NH4+ reduced N2-fixation by ∼90% relative to controls without added NH4+. In fast-growing, high-light-acclimated cultures (µ = 0.68 d-1), 2.0 µM NH4+ was needed to achieve the same effect. In long-term exposures to NO3-, inhibition of N2 fixation also varied with growth rate. In high-light-acclimated, fast-growing cultures, NO3- did not inhibit N2-fixation rates in comparison with cultures growing on N2 alone. Instead NO3- supported even faster growth, indicating that the cellular assimilation rate of N2 alone (i.e. dinitrogen reduction) could not support the light-specific maximum growth rate of Crocosphaera. When growth was severely light-limited, NO3- did not support faster growth rates but instead inhibited N2-fixation rates by 55% relative to controls. These data rest on the basic tenet that light energy is the driver of photoautotrophic growth while various nutrient substrates serve as supports. Our findings provide a novel conceptual framework to examine interactions between N source preferences and predict degrees of inhibition of N2 fixation by fixed N sources based on the growth rate as controlled by light.
生物固氮是海洋新氮（N）的主要来源，但在存在固定氮源如硝酸盐（NO3-）的情况下通常会受到抑制。人为向海洋输入的固定氮正在增加，但其对海洋生物固氮的影响尚不确定。因此，全球对海洋新氮的估计取决于对调节固氮蓝细菌氮源偏好的因素的基本理解。我们研究了单细胞固氮菌沃森氏球石藻（菌株WH0003），以确定光限制生长速率如何影响固定氮对固氮的抑制作用。当生长（µ）受低光照限制时（µ = 0.23 d-1），短期实验表明，相对于未添加NH4+的对照，0.4 µM NH4+使固氮减少了约90%。在快速生长、高光适应的培养物中（µ = 0.68 d-1），需要2.0 µM NH4+才能达到相同效果。在长期暴露于NO3-的情况下，固氮的抑制也随生长速率而变化。在高光适应、快速生长的培养物中，与仅以N2生长的培养物相比，NO3-不会抑制固氮速率。相反，NO3-甚至支持更快的生长，这表明仅N2的细胞同化速率（即二氮还原）无法支持球石藻的光特异性最大生长速率。当生长受到严重光限制时，NO3-不支持更快的生长速率，反而相对于对照将固氮速率抑制了55%。这些数据基于光能是光合自养生长的驱动力而各种营养底物起支持作用这一基本原理。我们的研究结果提供了一个新的概念框架，用于研究氮源偏好之间的相互作用，并根据光照控制的生长速率预测固定氮源对固氮的抑制程度。
