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What limits photosynthesis? Identifying the thermodynamic constraints of the terrestrial biosphere within the Earth system.
Biochimica et Biophysica Acta (BBA) - Bioenergetics ( IF 4.3 ) Pub Date : 2020-09-11 , DOI: 10.1016/j.bbabio.2020.148303
Axel Kleidon 1
Affiliation  

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 (r2 = 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 (r2 = 0.84). When this rate is converted back into photosynthetic activity, the resulting patterns correlate highly with the observation-based dataset (r2 = 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.



中文翻译:

什么限制了光合作用?确定地球系统内陆地生物圈的热力学约束。

光合作用将太阳光转换成提供地球生物圈的化学自由能,但其含量远低于热力学所允许的水平。我在这里提出,光合作用在热力学上仍然受到限制,但是这种限制间接地影响了物质交换。我为陆地生态系统的光合作用证实了这一主张,该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,我使用基于全球观测的辐射,光合作用,降水和蒸发数据集进行评估。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变化,平均效率为0.77%(n = 13,274)。速率与土地上的蒸发紧密相关(r 2  = 0.87),这表明光合作用与物质交换的耦合非常重要。然后,我使用太阳辐射和降水数据集推断出表面和大气之间最大程度的物质交换所产生的蒸发,这在热力学上是可能的。该推断速率与基于观测的土地蒸发数据集密切相关(r 2  = 0.84)。当该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(r 2 = 0.66)。这支持了以下解释:不是直接限制地面光合作用的能量,而是由阳光驱动的物质交换。这种解释可以解释在陆地生态系统中观察到的光合作用转换效率极低以及其空间变化。更一般而言,这意味着人们需要考虑与生命相关的必要物质流动和交换,以理解生命的热力学。最终,这需要一个将生物圈活动与地球系统中运输过程的热力学约束联系起来的观点。

更新日期:2020-09-20
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