MANIPULATING MICROBIOME-SMOOTH CORDGRASS ASSOCIATIONS TO PROMOTE COASTAL WETLAND RESTORATION

Abstract

Wetlands are among the most valuable ecosystems on Earth due to the multitude of ecosystem services they provide, such as water filtration, flood and groundwater storage, erosion control, and carbon storage. Wetlands also serve as important reservoirs of biodiversity. However, there has been as much as a 71% loss of wetlands globally since the beginning of the 20th century. One way to address wetland loss is through the re-planting of wetland species to support wetland functions. We can leverage community ecology principles to enhance the predictability of wetland restoration projects. Specifically, manipulating plant-microbe mutualisms could enhance the relative abundance of plant growth promoting rhizobacteria (PGPR), which are known to promote traits that buffer the conditions of stressful environments for their plant partners. For example, Sporobolus alterniflorus (smooth cordgrass) is a pioneer species with several important ecological functions in marsh habitats, making it a common planting for coastal marsh restorations. The survivorship of S. alterniflorus plantings is inconsistent, despite the species having high tolerances of salinity and wave energy. S. alterniflorus also has a relatively high tolerance to sulfide, a common component of sea salts and a phytotoxin important in the distribution of marsh plants. Specific PGPRs include sulfur-oxidizing bacteria, which are common in the S. alterniflorus root microbiome. These bacteria could detoxify the root zone by converting sulfide into sulfate. Therefore, a potential way to improve marsh restoration success is to understand the relationships between sediment microbial communities and the productivity of marsh grasses in the presence of abiotic stressors. To address this gap in knowledge, I addressed the following questions: 1) How does inoculation of established marsh sediment microbiomes influence S. alterniflorus biomass in conditions of abiotic stress? 2) How do the joint effects of the sediment environment and inundation influence S. alterniflorus biomass? I conducted a factorial mesocosm experiment with two levels of inundation (none and 8 hours/week), two levels of sediment treatment (high organic matter and sandy), and four levels of microbial inoculation (no addition, synthetic bacterial community, marsh sediment inocula, and combined synthetic bacterial community and marsh sediment inocula consortium). I measured bi-weekly plant height and stem counts; and after seven weeks, I measured seedling aboveground and root biomass. The results of the experiment showed that inundation had a greater influence on plant biomass than live marsh community inoculation in sandy sediment. Culture-based inoculation treatments had a significant effect on above and belowground biomass in organic sediment, although this may have been due to a fertilization effect of sulfur-oxidizing broth media. Interactions between inundation and inoculation had significant effects in sandy sediment. Contrary to expectations, inoculation with live marsh communities resulted in decreased belowground biomass in saltwater inundation treatments. Inoculation with live marsh communities and inundation with freshwater resulted in increased belowground biomass, but this difference was not significant. Inoculated and uninoculated plants did not have significantly different biomass in inundated or saline conditions. Plants in organic sediment or inundated with saltwater did not exhibit a stress response. The outcomes of this research will help inform improved methods of marsh restoration plantings, as well as clarify important stressors which limit marsh plant growth.