Experimental evidence root-associated microbes mediate seagrass response to environmental stress

Below-ground microbiota play an important role in mediating environmental
conditions with important consequences for plant
performance. Microorganisms involved in
plant-soil interactions may be associated with roots or
bulk-soil; however, the relative influence of these below-ground microbial
assemblages on plant performance is poorly known, particularly for marine
plants.  We separately manipulated the root and sediment
microbial assemblages of the seagrass Zostera
muelleri in a fully factorial experiment to determine how these
assemblages determined plant response (e.g., growth) to nutrient
enrichment, a major stressor in marine systems.  Under ambient
nutrient conditions, seagrass growth was maintained regardless of root
microbial assemblage disruption. Under high nutrient stress, however,
seagrasses with disrupted root microbiota had reduced growth, whereas
growth was maintained in seagrasses with an intact root microbiota.
Disruption of bulk-sediment microbiota did not affect seagrass
growth. Nutrient elevation was correlated to enhanced abundances
of several putatively beneficial microbial taxa (e.g.
sulfide-oxidizing
Beggiatoaceae and denitrifying Geofilum
rubicundum) associated with roots.  Synthesis: Our
results suggest that under ambient nutrient conditions, microorganisms
play a reduced role in influencing plant performance, but under more
stressful conditions positive plant-root microorganism interactions
strengthened. These results are among the first to experimentally
determine that interactions between marine plants and the root-associated
microbiota are key drivers of seagrass performance under human-induced
environmental changes. This suggests that as in terrestrial systems,
marine plant resilience depends on the stress-mitigating functions of
their root-associated microbiota and disturbance to those plant-microbiota
interactions can be deleterious for plant performance. Improving our
understanding of these plant-microorganism interactions may be critical
for understanding the functioning and resilience of threatened marine
plants and developing more effective restoration strategies for them.