(Georgia Institute of Technology, 2023-04-25)
Rolando Betancourt, Jose Luis
Salt marshes are highly productive intertidal wetland ecosystems located along wave-protected coastlines. Although salt marshes provide numerous ecosystem services, they are vulnerable to degradation by climate change, sudden vegetation dieback events, and unsustainable coastal development. To inform the adaptive management of marsh ecosystems, this dissertation addresses gaps in fundamental science related to the resilience and adaptability of salt marshes to environmental stressors, as well as their potential for restoration. In the first section of this thesis, I leveraged in situ biomass gradients of Spartina alterniflora -a foundational plant species that predominates over primary production on US Atlantic and Gulf of Mexico coastlines- as a natural laboratory to investigate the relationship between plant primary productivity, and plant-microbe interactions in the root zone. Multi-omics and biogeochemistry approaches were closely coupled to interrogate plant-microbe interactions and their implications for ecosystem function. My results show that enhanced microbial activity in the rhizosphere replenishes nutrients and terminal electron acceptors in higher biomass stands. Specifically, my research uncovered novel sulfur-oxidizing endosymbionts that benefit S. alterniflora by detoxifying the root-zone, and by fixing C and N that may be transferred to the host plant. I conclude that marine coastal plants rely on the fast recycling of sulfur in their root zone for organic matter breakdown and nutrient cycling, and I show for the first time that the rapid cycling of sulfur supports nitrogen fixation in the roots of a coastal marine plant. A second investigation of my dissertation focused on an impaired salt marsh ecosystem in Charleston, SC that had experienced sudden vegetation dieback (SVD). I investigated the causal effects of dieback, the potential for restoration through grass planting, and the ecosystem's resilience to sea level rise. Dieback was associated with extreme weather events such as droughts and flooding influenced by hurricane Joaquin. I conclude that failure to consider the increasing frequency and intensity of extreme climatic events in ecosystem models underestimates salt marsh vulnerability to climate change and rapid restoration of marsh dieback is crucial to avoid further degradation by marsh erosion. Coastal restoration is a multibillion-dollar global industry and a common restoration strategy in the U.S. involves S. alterniflora grass planting. My dissertation has revealed that the root microbiome of this foundational plant is closely linked to its productivity and plays a key role in the functioning of salt marshes. Further, climate change mitigation actions are urgently needed to preserve coastal marsh ecosystems, and future research is warranted to harness the root microbiome to improve the resilience and restoration of coastal marshes.