Photosynthesis is an ancient metabolic process that began on early Earth and offers plentiful energy to organisms that can utilize it such that that they achieve global significance. The potential exists for similar processes to operate on habitable exoplanets and result in observable biosignatures. Before the advent of oxygenic photosynthesis, the most primitive phototrophs, anoxygenic phototrophs, dominated surface environments on the planet. Here, we characterize surface polarization biosignatures associated with a diverse sample of anoxygenic phototrophs and cyanobacteria, examining both pure cultures and microbial communities from the natural environment. Polarimetry is a tool that can be used to measure the chiral signature of biomolecules. Chirality is considered a universal, agnostic biosignature that is independent of a planet's biochemistry, receiving considerable interest as a target biosignature for life-detection missions. In contrast to preliminary indications from earlier work, we show that there is a diversity of distinctive circular polarization signatures, including the magnitude of the polarization, associated with the variety of chiral photosynthetic pigments and pigment complexes of anoxygenic and oxygenic phototrophs. We also show that the apparent death and release of pigments from one of the phototrophs is accompanied by an elevation of the reflectance polarization signal by an order of magnitude, which may be significant for remotely detectable environmental signatures. This work and others suggest that circular polarization signals up to ∼1% may occur, significantly stronger than previously anticipated circular polarization levels. We conclude that global surface polarization biosignatures may arise from anoxygenic and oxygenic phototrophs, which have dominated nearly 80% of the history of our rocky, inhabited planet.

中文翻译：

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作为全球表面生物特征的原始光养生物的分光偏振测定

光合作用是一种古老的代谢过程，它起源于地球早期，为生物体提供充足的能量，可以利用它来实现全球意义。类似的过程可能存在于可居住的系外行星上并产生可观察的生物特征。在有氧光合作用出现之前，最原始的光合生物，即无氧光合生物，主导着地球的表面环境。在这里，我们表征了与各种缺氧光养菌和蓝藻样本相关的表面极化生物特征，检查了自然环境中的纯培养物和微生物群落。旋光法是一种可用于测量生物分子手性特征的工具。手性被认为是一种普遍的、不可知的生物印记，它独立于行星” s 生物化学，作为生命探测任务的目标生物特征受到相当大的关注。与早期工作的初步迹象相反，我们表明存在多种独特的圆偏振特征，包括偏振的大小，与各种手性光合色素和无氧和有氧光养生物的色素复合物相关。我们还表明，其中一个光养生物的色素明显死亡和释放伴随着反射偏振信号升高一个数量级，这对于远程可检测的环境特征可能很重要。这项工作和其他工作表明，可能会出现高达 1% 的圆极化信号，明显强于先前预期的圆极化水平。