Substratum-dependent responses of ciliate assemblages to temperature : a natural experiment in Icelandic streams - Dataset

Ciliate assemblages play a significant role in the microbial food web. The effects of environmental temperature on assemblage composition may be influenced by abiotic factors such as seasonality and disturbance, but the effects of temperature on ciliate assemblages found on different substrata have not been explored. Sandy bottoms and submerged rocks harbour dissimilar ciliate assemblages, and it might be expected that their ciliate assemblages will respond differently to temperature. We studied how alpha diversity, beta diversity and total biomass of ciliate protist assemblages found on sandy bottoms and submerged rocks differed in 13 geothermally heated streams in Iceland whose mean temperatures range from 5 to 20 °C. We recorded number of operational taxonomic units (OTUs) and measured the size of cells in ciliate assemblages from both substrata. Effects of temperature on natural ciliate assemblages were substratum dependent. On rock surfaces, both total ciliate biomass and alpha diversity declined with increasing temperature, and beta diversity increased with increasing temperature difference due to OTU nestedness (assemblages from warm streams being composed chiefly of subsets of the OTUs found in colder streams). In sandy substrata, however, ciliate assemblage composition was independent of temperature. Substratum‐specific responses may be due to differences in mechanical disturbance, nutrient availability or exposure to invertebrate grazers. Rock‐surface assemblages may be more exposed to the flow and retain less nutrient than those of sandy substratum; thus, they may be more strongly resource limited and more responsive to direct effects of temperature on metabolism. Alternatively, rock‐surface assemblages may be more exposed to grazing by invertebrates, which intensifies with temperature. Our study highlights the need to account for environmental context such as substratum type to fully understand the effect of temperature on microbial assemblages in streams. Future increases in global temperatures may affect fresh waters differently depending on their prevalent substratum. Those dominated by hard substrata may have their ciliate assemblages, and thus, food‐web structures and ecosystem functioning more strongly affected by warming relative to systems dominated by soft substrata.纤毛虫群落（ciliate assemblages）在微生物食物网中发挥着关键作用。环境温度对群落组成的影响可能受到季节变化、扰动等非生物因子的调控，但目前针对不同基质上栖息的纤毛虫群落的温度效应尚未得到探索。沙质底质与浸没岩石上栖息的纤毛虫群落组成存在显著差异，因此二者的群落对温度的响应模式或存在分化。本研究以冰岛13条地热加热溪流为研究对象，其水体平均温度区间为5~20℃，探究了沙质底质与浸没岩石上的纤毛虫原生生物群落的α多样性、β多样性及总生物量的变化差异。研究人员记录了两种基质上纤毛虫群落的操作分类单元（operational taxonomic units, OTUs）数量，并测定了单个细胞的尺寸。研究发现，温度对天然纤毛虫群落的影响具有基质依赖性：在岩石表面，纤毛虫总生物量与α多样性均随温度升高呈下降趋势；而β多样性则随温度梯度差增大而上升，这一现象源于OTU的嵌套结构（即暖溪流中的群落主要由冷溪流中OTU的子集构成）。但在沙质基质中，纤毛虫群落组成与温度并无关联。基质特异性的响应差异可能源于机械扰动、养分可用性或无脊椎动物捕食压力的不同。相较于沙质基质群落，岩石表面的群落更易受到水流冲击且养分留存能力更弱，因此可能受到更强的资源限制，同时对温度通过代谢途径产生的直接效应更为敏感。此外，岩石表面的群落可能更易遭受无脊椎动物的捕食，且该捕食压力随温度升高而加剧。本研究强调，若要全面理解温度对溪流微生物群落的影响，需将基质类型这类环境背景因子纳入考量范畴。未来全球气温升高可能对淡水生态系统产生差异化影响，具体取决于其优势基质类型：相较于以软质基质为主的淡水生态系统，以硬质基质为主的系统中，纤毛虫群落乃至食物网结构与生态系统功能可能会受到升温更强的冲击。