Data for: Ocean warming indirectly affects seagrass performance through effects on sediment microbial communities

Ocean warming poses a growing threat to seagrasses and the valuable ecosystem functions they underpin. While belowground microbial communities can influence terrestrial plant health and stress responses, their role in seagrass performance under warming remains poorly understood, especially under field conditions. We conducted a common garden field experiment in Lake Macquarie (NSW, Australia), where some sites have experienced water temperatures 1–3 °C above ambient temperatures since 1984 due to a power station’s thermal plume. Zostera muelleri plants and sediments were sourced from long-term warmed and nearby ambient sites. We transplanted plants from both origins into the warmed site using sediments from either warm or ambient origins and with rhizosphere and/or bulk sediment microbial communities left intact or experimentally disrupted. This allowed us to test whether belowground microbes and warming history influenced[PG1] plant growth over one month. Warm-origin plants had lower aboveground biomass in intact compared to disrupted warm-origin sediments. For ambient-origin plants, aboveground growth did not differ between intact and disrupted warm-origin sediments but was lower in intact warm-origin sediments compared to intact ambient-origin sediments. Disruption of rhizosphere microbial communities did not affect plant growth across all treatments. These results suggest that bulk sediment microbial communities shaped by long-term warming can negatively impact seagrass growth and may play a stronger role than rhizosphere microbial communities under ocean warming. Bacterial community analyses showed higher relative abundances of potential sulphate-reducing taxa in the rhizosphere of plants in intact warm sediments, suggesting potential mechanisms negatively impacting seagrass growth. This study highlights the importance of sediment microbial communities in shaping seagrass responses to warming, underscoring the need to consider sediment microbial legacies in management. Incorporating belowground microbial associations into conservation and restoration strategies may help future-proof seagrass meadows under climate change.