Plant−fungal symbiosis responds to experimental addition of resources and physical stressor in a salt marsh

Plant−fungal symbioses can have strong consequences for ecological communities and are sensitive to variation in abiotic factors. While the functions of mycorrhizal fungi are well established, the role of other root-colonizing fungi such as dark septate endophytes (DSE), which lack specialized structures for nutrient transfer, are less clear. DSE are ubiquitous in extreme and stressful ecosystems, including marine environments, and some studies suggest a potential role in plant nutrition. However, the response of DSE to nutrient availability and physical stress has rarely been tested in the field. We conducted a 10 mo field experiment to investigate how a symbiosis between the salt marsh plant Spartina alterniflora and DSE fungi responded to increased resources (nutrient addition) and physical stress (salt addition). Plant stem density and height increased in response to nutrient enrichment, consistent with past experiments in nutrient-limited marsh systems. Nutrient additions also increased S. alterniflora percent cover, but this effect was negated with elevated salinity. Nutrient addition decreased colonization by DSE hyphae by nearly half (8.8%, vs. 15.7% at ambient levels). Nutrients did not decrease DSE microsclerotia, which were marginally increased with the combination of added nutrients and salinity. These results are consistent with the view that plant−DSE interactions are based in part on enhanced nutritional condition of plants by fungi. In addition, there was a positive relationship between plant shoot growth and root colonization by DSE, suggesting a benefit of the association for the plants. Our results suggest that the poorly understood plant−DSE symbioses may be important in intertidal environments.