Oxygenic photoautotrophs are, paradoxically, subject to photoinhibition of their photosynthetic apparatus, in particular one of its major components, the Photosystem II (PSII). Photoinhibition is generalized across species, light conditions and habitats, imposing substantial metabolic costs that lower photosynthetic productivity and constrain the niches of photoautotrophy. As a process driven by light reaching PSII, light attenuation in optically thick samples influences both the actual extent, and the detection, of photoinhibition. Chlorophyll fluorescence is widely used to measure photoinhibition, but fluorescence-based parameters are affected by light attenuation of both downwelling incident radiation traversing the sample to reach PSII, and emitted fluorescence upwelling through the sample. We used modelling, experimental manipulation of within-sample light attenuation, and meta-analysis of published data, to show substantial, differential effects of light attenuation and depth-integration of emitted fluorescence upon measurements of photoinhibition. Numerical simulations and experimental manipulation of light attenuation indicated that PSII photoinactivation tracked using chlorophyll fluorescence can appear to be over three times lower than the inherent cellular susceptibility to photoinactivation, in optically-dense samples such as leaves or biofilms. The meta-analysis of published data showed that this general trend was unknowingly present in the literature, revealing an overall difference of more than five times between optically thick leaves and optically thin cell suspensions. Although fluorescence-based parameters may provide ecophysiologically relevant information for characterizing the sample as a whole, light attenuation and depth integration can vary between samples independently of their intrinsic physiology. They should be used with caution when aiming to quantify in absolute terms inherent photoinhibition-related parameters in optically thick samples.

自相矛盾的是，产氧光合自养生物受到其光合装置，特别是其主要成分之一，光系统II（PSII）的光抑制作用。光抑制作用普遍存在于物种，光照条件和栖息地之间，这会带来大量的代谢成本，从而降低光合作用的生产力并限制光合植物的生态位。作为受光到达PSII的驱动过程，光学上较厚的样品中的光衰减会影响光抑制的实际范围和检测。叶绿素荧光被广泛用于测量光抑制，但是基于荧光的参数受到穿过样品到达PSII的向下入射入射辐射以及穿过样品向上发射的荧光的光衰减的影响。我们使用建模 样品内光衰减的实验性操作，以及对公开数据的荟萃分析，显示了光衰减和光抑制测量时所发射荧光的深度积分的显着差异效果。数值模拟和光衰减的实验处理表明，在诸如叶子或生物膜等光学密集样品中，使用叶绿素荧光追踪的PSII光灭活似乎比固有的细胞对光灭活的敏感性低三倍以上。对已发表数据的荟萃分析表明，这种总体趋势在文献中不为人所知，揭示出光学上较厚的叶片和光学上较薄的细胞悬液之间的总体差异超过五倍。尽管基于荧光的参数可以提供整体上与样品生理生态相关的信息，但是样品之间的光衰减和深度积分可以独立于其固有生理而变化。当以绝对术语量化光学厚度样品中固有的与光抑制相关的参数时，应谨慎使用。