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.
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We wish to thank Gary Kendrick, Jeremy Bougoure, Daniela Trojan, PWIS and PP for advice and fruitful discussions. MWF was supported by the Robson and Robertson postdoctoral fellowship awarded by the UWA Oceans Institute. This research was partly supported by the Integrated Coastal Analyses and Sensor Technology (ICoAST) project with funding from the Indian Ocean Marine Research Centre, a joint partnership between The University of Western Australia (UWA), the Australian Institute of Marine Science (AIMS), The Commonwealth Scientific and Industrial Research Organisation (CSIRO) and The Department of Primary Industries and Regional Development (DPIRD) WA. We also acknowledge the facilities, and the scientific and technical assistance of Microscopy Australia at the Centre for Microscopy, Characterisation & Analysis, The University of Western Australia, a facility funded by the University, State and Commonwealth Governments.
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Martin, B.C., Middleton, J.A., Fraser, M.W. et al. Cutting out the middle clam: lucinid endosymbiotic bacteria are also associated with seagrass roots worldwide. ISME J 14, 2901–2905 (2020). https://doi.org/10.1038/s41396-020-00771-3