论文标题
什么限制了光合作用？识别地球系统中生物圈的热力学约束
What limits photosynthesis? Identifying the thermodynamic constraints of the biosphere within the Earth system
论文作者
论文摘要
光合作用将阳光转化为供应地球生物圈的化学自由能，但在远低于热力学允许的水平上。我在这里建议光合作用在热力学上受到限制，但是这种极限间接地在水和二氧化碳的材料交换上。我使用基于全球观察的辐射，光合作用，降水和蒸发的数据集证实了这种解释。我首先计算陆地生态系统中光合作用的转化效率及其气候变化，中位效率为0.78％（n = 13445）。速率与蒸发密切相关（R2 = 0.89），这证明了光合作用与材料交换的耦合的重要性。然后，我会从使用太阳辐射和降水数据集之间从表面和大气之间的最大材料交换中推断出蒸发。该推断的速率与基于观测的蒸发数据集（R2 = 0.85）密切相关。当此速率转换回光合作用活动时，所得模式与基于观测的数据集高度相关（R2 = 0.56）。这支持了这样的解释，即限制陆地光合作用的不是直接的能量，而是由阳光驱动的物质交换。这种解释可以解释陆地生态系统中光合作用的非常低的转化效率以及其空间变化。更普遍地，这意味着人们需要考虑与生活相关的必要物质流和交流，以了解生活的热力学。最终，这需要一个将生物圈的活性与地球系统中传输过程的热力学约束联系起来的视角。
Photosynthesis converts sunlight into the chemical free energy that feeds the Earth's biosphere, yet at levels much lower than what thermodynamics would allow for. I propose here that photosynthesis is nevertheless thermodynamically limited, but this limit acts indirectly on the material exchange of water and carbon dioxide. I substantiate this interpretation using global observation-based datasets of radiation, photosynthesis, precipitation and evaporation. I first calculate the conversion efficiency of photosynthesis in terrestrial ecosystems and its climatological variation, with a median efficiency of 0.78% (n = 13445). The rates tightly correlate with evaporation (r2 = 0.89), which demonstrates the importance of the coupling of photosynthesis to material exchange. I then infer evaporation from the maximum material exchange between the surface and the atmosphere that is thermodynamically possible using datasets of solar radiation and precipitation. This inferred rate closely correlates with the observation-based evaporation dataset (r2 = 0.85). When this rate is converted back into photosynthetic activity, the resulting patterns correlate highly with the observation-based dataset (r2 = 0.56). This supports the interpretation that it is not energy directly that limits terrestrial photosynthesis, but rather the material exchange that is driven by sunlight. This interpretation can explain the very low, observed conversion efficiency of photosynthesis in terrestrial ecosystems as well as its spatial variations. More generally, this implies that one needs to take the necessary material flows and exchanges associated with life into account to understand the thermodynamics of life. This, ultimately, requires a perspective that links the activity of the biosphere to the thermodynamic constraints of transport processes in the Earth system.
