Table 1_Linking microbial ecology to the cycling of neutral and acidic polysaccharides in pustular mats from Shark Bay, Western Australia.xlsx

Cyanobacteria and other microbes in peritidal microbial mats have produced extracellular polymeric substances (EPS) for more than two billion years. The production and degradation of EPS contributes to the biogeochemical cycling of carbon and carbonate precipitation within modern microbial mats, but key microbes involved in the cycling of EPS remain unidentified. Here, we investigate the cycling of EPS in the peritidal pustular mats of Shark Bay, Western Australia. We characterize the chemical composition of EPS produced by cyanobacterial enrichment cultures under natural and UV-stress conditions and link these findings to the metabolic potential for EPS production and degradation encoded in 84 metagenome-assembled genomes (MAGs) from the mat community. We further identify the key microbial degraders of specific acidic and neutral polysaccharides in this community by cultivating enrichment cultures on seven commercially available polysaccharides representative of those present in the mats and assessing the dominant taxa. All sequenced Cyanobacteria MAGs have the potential to synthesize mannose, fucose, glucose, arabinose, rhamnose, galactose, xylose, N-acetylglucosamine, galacturonic acid and glucuronic acid. Biochemical analyses confirm the presence of nearly all these monosaccharides in the hydrolysates of EPS extracted from UV- and non-UV exposed cyanobacterial enrichments. Ultraviolet radiation influences the structure and composition of EPS by reducing the hydration, potentially due to cross-linking among polymers in EPS and increasing the relative abundances of uronic acids and xylose in polysaccharides. Analyses of carbohydrate-active enzymes (CAZymes) in the MAGs and of 16S rRNA sequences from experimental polysaccharide enrichments point to major roles for Bacteroidetes, Planctomycetes, and Verrucomicrobia in the cycling of acidic EPS. These experiments reveal a complex interplay among microbial community composition, CAZyme diversity, environmental stressors, and EPS cycling, which together shape carbon flow and biomineralization in pustular mats in Shark Bay.

潮间带微生物垫中的蓝细菌（Cyanobacteria）与其他微生物，已合成胞外聚合物（extracellular polymeric substances，EPS）长达20余亿年。EPS的合成与降解过程，支撑着现代微生物垫内的碳生物地球化学循环与碳酸盐沉积，但参与EPS循环的关键微生物仍未被明确鉴定。本研究针对澳大利亚西部鲨鱼湾的潮间带脓疱状微生物垫，探究其EPS循环机制。我们分别在自然与紫外胁迫条件下，对蓝细菌富集培养物所产生的EPS进行化学组成表征，并将该结果与该微生物垫群落中84个宏基因组组装基因组（metagenome-assembled genomes，MAGs）所编码的EPS合成与降解代谢潜能相关联。我们进一步以7种代表性商业化多糖（即该微生物垫中存在的多糖类型）为底物培养富集培养物，并通过分析优势类群，鉴定出该群落中负责降解特定酸性与中性多糖的关键微生物。所有已测序的蓝细菌MAG均具备合成甘露糖、岩藻糖、葡萄糖、阿拉伯糖、鼠李糖、半乳糖、木糖、N-乙酰葡糖胺、半乳糖醛酸与葡糖醛酸的潜能。生化分析证实，从经紫外胁迫与未胁迫的蓝细菌富集培养物中提取的EPS水解液中，几乎包含上述所有单糖。紫外辐射可通过降低EPS的持水性（可能源于EPS内聚合物间的交联作用），以及提升多糖中糖醛酸与木糖的相对丰度，从而改变EPS的结构与组成。对MAG中的碳水化合物活性酶（carbohydrate-active enzymes，CAZymes）以及实验多糖富集培养物的16S核糖体RNA（16S rRNA）序列进行分析后发现，拟杆菌门（Bacteroidetes）、浮霉菌门（Planctomycetes）与疣微菌门（Verrucomicrobia）在酸性EPS循环中发挥着核心作用。本研究揭示了微生物群落组成、CAZyme多样性、环境胁迫因子与EPS循环之间的复杂互作关系，这些因素共同塑造了鲨鱼湾脓疱状微生物垫内的碳流动与生物矿化过程。