Stable, long-term interactions between fungi and algae or cyanobacteria, collectively known as lichens, have repeatedly evolved complex architectures with little resemblance to their component parts. Lacking any central scaffold, the shapes they assume are casts of secreted polymers that cement cells into place, determine the angle of phototropic exposure and regulate water relations. A growing body of evidence suggests that many lichen extracellular polymer matrices harbor unicellular, non-photosynthesizing organisms (UNPOs) not traditionally recognized as lichen symbionts. Understanding organismal input and uptake in this layer is key to interpreting the role UNPOs play in lichen biology. Here, we review both polysaccharide composition determined from whole, pulverized lichens and UNPOs reported from lichens to date. Most reported polysaccharides are thought to be structural cell wall components. The composition of the extracellular matrix is not definitively known. Several lines of evidence suggest some acidic polysaccharides have evaded detection in routine analysis of neutral sugars and may be involved in the extracellular matrix. UNPOs reported from lichens include diverse bacteria and yeasts for which secreted polysaccharides play important biological roles. We conclude by proposing testable hypotheses on the role that symbiont give-and-take in this layer could play in determining or modifying lichen symbiotic outcomes.

中文翻译：

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3D 生物膜：寻找将地衣共生体结合在一起的多糖。

真菌与藻类或蓝细菌（统称为地衣）之间稳定、长期的相互作用，反复进化出与其组成部分几乎没有相似之处的复杂结构。由于缺乏任何中央支架，他们假设的形状是分泌聚合物的铸模，将细胞粘合到位，确定向光照射的角度并调节水的关系。越来越多的证据表明，许多地衣细胞外聚合物基质含有传统上不被视为地衣共生体的单细胞、非光合生物 (UNPO)。了解这一层的有机体输入和吸收是解释 UNPO 在地衣生物学中所起的作用的关键。在这里，我们回顾了迄今为止从完整的粉碎地衣和从地衣报告的 UNPO 确定的多糖组成。大多数报道的多糖被认为是结构细胞壁成分。细胞外基质的组成尚不确定。几条证据表明，一些酸性多糖在中性糖的常规分析中逃避了检测，可能与细胞外基质有关。来自地衣的 UNPOs 包括多种细菌和酵母菌，分泌的多糖对其具有重要的生物学作用。最后，我们提出了关于共生体在这一层中的给予和接受在确定或改变地衣共生结果方面可能发挥的作用的可检验假设。几条证据表明，一些酸性多糖在中性糖的常规分析中逃避了检测，可能与细胞外基质有关。来自地衣的 UNPOs 包括多种细菌和酵母菌，分泌的多糖对其具有重要的生物学作用。最后，我们提出了关于共生体在这一层中的给予和接受在确定或改变地衣共生结果方面可能发挥的作用的可检验假设。几条证据表明，一些酸性多糖在中性糖的常规分析中逃避了检测，可能与细胞外基质有关。来自地衣的 UNPOs 包括多种细菌和酵母菌，分泌的多糖对其具有重要的生物学作用。最后，我们提出了关于共生体在这一层中的给予和接受在确定或改变地衣共生结果方面可能发挥的作用的可检验假设。