不依赖地球生物的光合宜居区模型，发现类地行星可在红外观测到新类型光合生命。
An Agnostic Machine Learning Model of Photosynthetic Habitability
- 基于热力学与氧化还原化学构建通用光合模型，无地球生物偏见
- 模拟显示光合生存力随轨道距离线性下降，远超传统平方衰减预测
- 揭示冷矮星系中红外光合氧生产可能，适合探测新型地外生命信号
系外行星生物标志物搜索通常以行星环境能否维持光合作用为指引。为此，近期提出了光合宜居区（PHZ），即经典宜居带与恒星辐射可驱动光合作用轨道范围的重叠区域。现有PHZ估算依赖地球浮游植物的光响应曲线，隐含地球中心偏见。本文提出一种无偏倚的PHZ模型，基于热力学与氧化还原化学构建的通用光合机制，不参照具体生物体。模型建立在通用光化学反应上：光子捕获耦合供体分子氧化与CO2还原。通过遗传算法优化光学特性与CO2还原速率，在主序星系外行星的辐照谱下进行模拟。结果表明，光合生物会进化出更大的光收集结构以应对低通量，使光合可行性仅随轨道距离线性下降，而非传统平方衰减。因此，无偏倚的PHZ显著扩展于以往地球基准估计。对于冷M型矮星，可见光氧光合在宜居带外缘受通量限制；但无氧光合及假设的近红外驱动氧光合在M、K、G型恒星整个宜居带均可行。这表明，M型矮星系外行星或可维持强效氧光合，其反射特征信号位于近红外波段而非可见光。
原文摘要 · Abstract (English)
The search for exoplanet biosignatures is guided by whether planetary environments can sustain photosynthesis. As such, the Photosynthetic Habitable Zone (PHZ) was recently proposed, as the overlap between the canonical habitable zone and the orbital range where stellar irradiance is sufficient to drive photosynthesis. Existing PHZ estimates rely on empirical light-response curves from Earth phytoplankton, and thus include implicit Earth-centric biases. We introduce an agnostic PHZ derived from a generalized model of photosynthesis grounded in thermodynamics and redox chemistry, without reference to model organisms. The model is built on a generic photochemical reaction in which photon capture couples oxidation of a donor molecule to the reduction of CO2. The optical properties and CO2 reduction rate are optimized against irradiance spectra for exoplanets orbiting main-sequence stars, using a genetic algorithm that mimics evolution by natural selection. Our simulations predict that photosynthetic organisms compensate for reduced flux by evolving larger light-harvesting structures. As a result, photosynthetic viability declines only linearly with orbital distance, despite stellar flux falling off quadratically. As such, the agnostic PHZ expands well beyond previous Earth-based estimates. Earth-like (visible light) oxygenic photosynthesis is flux-limited at the outer habitable zone for cool M-dwarf stars; however, both anoxygenic photosynthesis and a hypothetical, NIR-driven oxygenic photosynthesis are viable across the entire habitable zone for M, K, and G stars. This implies that M-dwarf exoplanets could sustain robust oxygenic photosynthesis, though it would be different to that found on Earth, presenting reflectance biosignatures in the NIR band rather than the visible.
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