Data from: The Niphargus–Thiothrix association may be widespread in sulfidic groundwaters: evidence from southeastern Romania

Niphargus is a speciose amphipod genus found in groundwater habitats across Europe. Three Niphargus species living in the sulphidic Frasassi caves in Italy harbour sulphur-oxidizing Thiothrix bacterial ectosymbionts. These three species are distantly related, implying that the ability to form ectosymbioses with Thiothrix may be common among Niphargus. Therefore, Niphargus–Thiothrix associations may also be found in sulphidic aquifers other than Frasassi. In this study, we examined this possibility by analysing niphargids of the genera Niphargus and Pontoniphargus collected from the partly sulphidic aquifers of the Southern Dobrogea region of Romania, which are accessible through springs, wells and Movile Cave. Molecular and morphological analyses revealed seven niphargid species in this region. Five of these species occurred occasionally or exclusively in sulphidic locations, whereas the remaining two were restricted to nonsulphidic areas. Thiothrix were detected by PCR on all seven Dobrogean niphargid species and observed using microscopy to be predominantly attached to their hosts' appendages. 16S rRNA gene sequences of the Thiothrix epibionts fell into two main clades, one of which (herein named T4) occurred solely on niphargids collected in sulphidic locations. The other Thiothrix clade was present on niphargids from both sulphidic and nonsulphidic areas and indistinguishable from the T3 ectosymbiont clade previously identified on Frasassi-dwelling Niphargus. Although niphargids from Frasassi and Southern Dobrogea are not closely related, the patterns of their association with Thiothrix are remarkably alike. The finding of similar Niphargus–Thiothrix associations in aquifers located 1200 km apart suggests that they may be widespread in European groundwater ecosystems.泥钩虾属（Niphargus）是一类物种丰富的端足类甲壳动物属，广泛分布于欧洲全境的地下水生境中。栖息于意大利硫化物型弗拉沙西（Frasassi）洞穴的3种泥钩虾属物种，其体表定殖有氧化硫的发硫菌属（Thiothrix）细菌外共生体。这三个物种亲缘关系较远，提示与发硫菌属形成外共生关系的能力可能在泥钩虾属中普遍存在。因此，除弗拉沙西洞穴外，其他硫化物含水层中也可能存在泥钩虾属与发硫菌属的共生组合。本研究通过分析采自罗马尼亚南多布罗加地区部分硫化物含水层、可通过泉眼、水井及莫维勒洞穴（Movile Cave）获取的泥钩虾属与蓬突钩虾属（Pontoniphargus）样本，对该假说进行了验证。分子与形态学分析结果显示，该区域共存在7种钩虾类物种，其中5个物种偶尔或仅栖息于硫化环境中，剩余2个物种则仅分布于非硫化环境。通过PCR检测，在所有7种南多布罗加钩虾类物种中均检出了发硫菌属；显微镜观察显示，该菌主要附着于宿主的附肢之上。这些发硫菌附生菌的16S rRNA基因序列可分为两大演化支，其中一支（本文命名为T4）仅存在于硫化环境中采集的钩虾类体表。另一支发硫菌演化支则同时存在于硫化与非硫化环境来源的钩虾类体表，且与此前在弗拉沙西洞穴栖息的泥钩虾属物种体表发现的T3外共生体演化支无法区分。尽管弗拉沙西与南多布罗加地区的钩虾类亲缘关系并不密切，但二者与发硫菌属的共生模式却极为相似。在相距1200公里的两处含水层中发现相似的泥钩虾属与发硫菌属共生组合，表明这类共生关系可能广泛分布于欧洲地下水生态系统中。