Chemical and Biological Engineering, Colorado School of Mines, Golden, Colorado, USA.
National Renewable Energy Laboratory, Golden, Colorado, USA.
mSystems. 2023 Feb 23;8(1):e0060120. doi: 10.1128/msystems.00601-20. Epub 2023 Jan 4.
The open ocean is an extremely competitive environment, partially due to the dearth of nutrients. Trichodesmium erythraeum, a marine diazotrophic cyanobacterium, is a keystone species in the ocean due to its ability to fix nitrogen and leak 30 to 50% into the surrounding environment, providing a valuable source of a necessary macronutrient to other species. While there are other diazotrophic cyanobacteria that play an important role in the marine nitrogen cycle, is unique in its ability to fix both carbon and nitrogen simultaneously during the day without the use of specialized cells called heterocysts to protect nitrogenase from oxygen. Here, we use the advanced modeling framework called ultscale ultibjective ystems nalysis (MiMoSA) to investigate how Trichodesmium erythraeum can reduce dimolecular nitrogen to ammonium in the presence of oxygen. Our simulations indicate that nitrogenase inhibition is best modeled as Michealis-Menten competitive inhibition and that cells along the filament maintain microaerobia using high flux through Mehler reactions in order to protect nitrogenase from oxygen. We also examined the effect of location on metabolic flux and found that cells at the end of filaments operate in distinctly different metabolic modes than internal cells despite both operating in a photoautotrophic mode. These results give us important insight into how this species is able to operate photosynthesis and nitrogen fixation simultaneously, giving it a distinct advantage over other diazotrophic cyanobacteria because they can harvest light directly to fuel the energy demand of nitrogen fixation. Trichodesmium erythraeum is a marine cyanobacterium responsible for approximately half of all biologically fixed nitrogen, making it an integral part of the global nitrogen cycle. Interestingly, unlike other nitrogen-fixing cyanobacteria, does not use temporal or spatial separation to protect nitrogenase from oxygen poisoning; instead, it operates photosynthesis and nitrogen fixation reactions simultaneously during the day. Unfortunately, the exact mechanism the cells utilize to operate carbon and nitrogen fixation simultaneously is unknown. Here, we use an advanced metabolic modeling framework to investigate and identify the most likely mechanisms uses to protect nitrogenase from oxygen. The model predicts that cells operate in a microaerobic mode, using both respiratory and Mehler reactions to dramatically reduce intracellular oxygen concentrations.
开阔的海洋是一个竞争非常激烈的环境，部分原因是缺乏营养物质。海洋固氮蓝藻束毛藻是海洋中的关键物种，因为它能够固定氮并将 30%到 50%的氮渗漏到周围环境中，为其他物种提供了一种有价值的必需大量营养素来源。虽然还有其他固氮蓝藻在海洋氮循环中起着重要作用，但 独特之处在于它能够在白天同时固定碳和氮，而无需使用专门的细胞（称为异形胞）来保护氮酶免受氧气的影响。在这里，我们使用称为 ultscale ultibjective ystems nalysis (MiMoSA) 的先进建模框架来研究束毛藻如何在有氧气的情况下将二分子氮还原为铵。我们的模拟表明，氮酶抑制最好用米氏门控竞争抑制来建模，并且沿着丝状体的细胞通过 Mehler 反应保持微需氧状态，以保护氮酶免受氧气的影响。我们还研究了位置对代谢通量的影响，发现尽管都以光合自养模式运行，但丝状体末端的细胞与内部细胞的代谢模式明显不同。这些结果使我们对该物种如何能够同时进行光合作用和固氮有了重要的了解，使其与其他固氮蓝藻相比具有明显的优势，因为它们可以直接利用光能来满足固氮的能量需求。束毛藻是一种负责约一半生物固定氮的海洋蓝藻，是全球氮循环的重要组成部分。有趣的是，与其他固氮蓝藻不同， 不使用时间或空间分离来保护氮酶免受氧气中毒；相反，它在白天同时进行光合作用和固氮反应。不幸的是，细胞同时利用固氮和固碳的具体机制尚不清楚。在这里，我们使用先进的代谢建模框架来研究和确定细胞同时利用固氮和固碳的最可能机制。该模型预测细胞在微需氧模式下运行，同时使用呼吸和 Mehler 反应来显著降低细胞内的氧浓度。
