The role of belowground microbes in seagrass responses to environmental stress

Abstract

Environmental change is altering environmental conditions, making it critical to understand the factors that contribute to the resilience of organisms under stress. Belowground microbial communities play a key role in plant health, but their impact on marine plants under stress is poorly understood. This thesis investigates the role of rhizosphere and bulk sediment microbial communities in the performance of the seagrass Zostera muelleri under elevated ocean temperatures and nutrient variability. Experimental manipulations of microbial communities revealed that while rhizosphere microbes supported seagrass growth across all conditions, intact bulk sediment microbial communities hindered plant performance under a marine heatwave scenario. This was linked to shifts in microbial communities, with fewer beneficial microbes and an increase in potential pathogens. A field experiment showed that sediment microbial communities with a history of elevated temperatures negatively impacted seagrass growth, regardless of the plants' origin. This was associated with increased sulphate-reducing bacteria and potential sulphide toxicity. Under nutrient stress, disrupting microbial communities in either the rhizosphere or bulk sediment reduced biomass under low and high nutrient levels. At medium nutrient levels, microbial disruption had little impact. These findings show that the influence of microbial communities on seagrass performance is context-dependent, with rhizosphere microbes generally benefiting plants, while bulk sediment microbes can have both positive and negative effects. This research highlights the importance of integrating microbial processes into seagrass restoration and conservation efforts.