Horticultural Sciences Department, University of Florida, Gainesville, FL 32611, USA.
Department of Bacteriology, University of Wisconsin-Madison, Madison, WI 53706, USA.
Plant Sci. 2023 Oct;335:111815. doi: 10.1016/j.plantsci.2023.111815. Epub 2023 Aug 3.
Synthetic biology and metabolic engineering promise to deliver sustainable solutions to global problems such as phasing out fossil fuels and replacing industrial nitrogen fixation. While this promise is real, scale matters, and so do knock-on effects of implementing solutions. Both scale and knock-on effects can be estimated by 'Fermi calculations' (aka 'back-of-envelope calculations') that use uncontroversial input data plus simple arithmetic to reach rough but reliable conclusions. Here, we illustrate how this is done and how informative it can be using two cases: oilcane (sugarcane engineered to accumulate triglycerides instead of sugar) as a source of bio-jet fuel, and nitrogen fixation by bacteria in mucilage secreted by maize aerial roots. We estimate that oilcane could meet no more than about 1% of today's U.S. jet fuel demand if grown on all current U.S. sugarcane land and that, if cane land were expanded to meet two-thirds of this demand, the fertilizer and refinery requirements would create a large carbon footprint. Conversely, we estimate that nitrogen fixation in aerial-root mucilage could replace up to 10% of the fertilizer nitrogen applied to U.S. maize, that 2% of plant carbon income used for growth would suffice to fuel the fixation, and that this extra carbon consumption would likely reduce grain yield only slightly.
合成生物学和代谢工程有望为全球问题提供可持续的解决方案，例如逐步淘汰化石燃料和替代工业固氮。虽然这一承诺是真实的，但规模很重要，解决方案的连锁效应也是如此。规模和连锁效应都可以通过“费米计算”（也称为“信封背面计算”）来估计，这种计算方法使用无争议的输入数据和简单的算术来得出大致但可靠的结论。在这里，我们将说明如何进行此操作，以及使用两个案例：油甘蔗（经过工程改造以积累甘油三酯而不是糖的甘蔗）作为生物喷气燃料的来源，以及玉米气生根分泌的粘液中的细菌进行固氮，这将如何具有启发性。我们估计，如果在所有当前的美国甘蔗地上种植油甘蔗，它最多只能满足当今美国喷气燃料需求的 1%左右，如果将甘蔗地扩大到满足这一需求的三分之二，那么化肥和炼油厂的需求将产生巨大的碳足迹。相反，我们估计，气生根粘液中的固氮作用可以替代美国玉米所施化肥氮的 10%，用于生长的植物碳收入的 2%就足以满足固定作用，而这种额外的碳消耗可能只会略微降低谷物产量。
