Seagrasses and lucinid bivalves inhabit highly reduced sediments with elevated sulphide concentrations. Lucinids house symbiotic bacteria (Ca. Thiodiazotropha) capable of oxidising sediment sulphide, and their presence in sediments has been proposed to promote seagrass growth by decreasing otherwise phytotoxic sulphide levels. However, vast and productive seagrass meadows are present in ecosystems where lucinids do not occur. Hence, we hypothesised that seagrasses themselves host these sulphur-oxidising Ca. Thiodiazotropha that could aid their survival when lucinids are absent. We analysed newly generated and publicly available 16S rRNA gene sequences from seagrass roots and sediments across 14 seagrass species and 10 countries and found that persistent and colonising seagrasses across the world harbour sulphur-oxidising Ca. Thiodiazotropha, regardless of the presence of lucinids. We used fluorescence in situ hybridisation to visually confirm the presence of Ca. Thiodiazotropha on roots of Halophila ovalis, a colonising seagrass species with wide geographical, water depth range, and sedimentary sulphide concentrations. We provide the first evidence that Ca. Thiodiazotropha are commonly present on seagrass roots, providing another mechanism for seagrasses to alleviate sulphide stress globally.

海草和 lucinid 双壳类动物栖息在硫化物浓度升高的高度还原的沉积物中。Lucinids 拥有能够氧化沉积物硫化物的共生细菌（Ca . Thiodiazotropha），并且已经提出它们在沉积物中的存在通过降低否则具有植物毒性的硫化物水平来促进海草生长。然而，在没有 lucinids 的生态系统中，存在着广阔而多产的海草草甸。因此，我们假设海草本身含有这些硫氧化钙. 当 lucinids 不存在时，硫固氮菌可以帮助它们的生存。我们分析了来自 14 个海草物种和 10 个国家/地区的海草根和沉积物的新生成和公开可用的 16S rRNA 基因序列，发现世界各地的持久和定植海草含有硫氧化钙。硫固氮菌，无论是否存在 lucinids。我们使用荧光原位杂交来直观地确认Ca的存在。卵形嗜盐菌根上的硫代固氮菌，一种具有广泛地理、水深范围和沉积硫化物浓度的海草殖民物种。我们提供了第一个证据证明Ca. Thiiodiazotropha 通常存在于海草根部，这为海草在全球缓解硫化物压力提供了另一种机制。