Kleidon Axel
Max-Planck-Institute for Biogeochemistry, Hans-Knöll-Str. 10, D-07745 Jena, Germany.
Biochim Biophys Acta Bioenerg. 2021 Jan 1;1862(1):148303. doi: 10.1016/j.bbabio.2020.148303. Epub 2020 Sep 11.
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. I substantiate this proposition for the photosynthetic activity of terrestrial ecosystems, which are notably more productive than the marine biosphere. The material exchange for terrestrial photosynthesis involves water and carbon dioxide, which I evaluate 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 0.77% (n = 13,274). The rates tightly correlate with evaporation on land (r = 0.87), 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 land evaporation dataset (r = 0.84). When this rate is converted back into photosynthetic activity, the resulting patterns correlate highly with the observation-based dataset (r = 0.66). 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.
光合作用将阳光转化为化学自由能，为地球生物圈提供能量，但其效率远低于热力学所允许的水平。我在此提出，光合作用在热力学上仍然受到限制，但这种限制是通过间接作用于物质交换来实现的。我以陆地生态系统的光合活性为例来证实这一观点，陆地生态系统的生产力明显高于海洋生物圈。陆地光合作用的物质交换涉及水和二氧化碳，我利用基于全球观测的辐射、光合作用、降水和蒸发数据集对其进行评估。我首先计算了陆地生态系统中光合作用的转换效率及其气候学变化，中位数效率为0.77%（n = 13274）。这些速率与陆地蒸发紧密相关（r = 0.87），这表明光合作用与物质交换耦合的重要性。然后，我利用太阳辐射和降水数据集，从地表与大气之间热力学上可能的最大物质交换中推断出蒸发量。这个推断出的速率与基于观测的陆地蒸发数据集密切相关（r = 0.84）。当这个速率转换回光合活性时，得到的模式与基于观测的数据集高度相关（r = 0.66）。这支持了一种解释，即限制陆地光合作用的不是能量本身，而是由阳光驱动的物质交换。这种解释可以说明陆地生态系统中观测到的极低的光合作用转换效率及其空间变化。更普遍地说，这意味着要理解生命的热力学，就需要考虑与生命相关的必要物质流动和交换。最终，这需要一个将生物圈活动与地球系统中传输过程的热力学约束联系起来的视角。
