[Eoas-seminar] Oceanography Thesis Defense - Rachel Petet - March 28, 10am - CSL1005

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Mon Mar 18 09:15:14 EDT 2019

Oceanography Seminar

Rachel Petet

M.S. Biological Oceanography Candidate

Title:  Impacts of microbial community structure on denitrification rates in the rhizosphere of J. roemerianus and S. alterniflora collocated in a Gulf of Mexico saltwater marsh

Major Professor:  Dr. Olivia U. Mason

Date: March 28rd, 2019                                 Time: 10:00 AM

Location: Chemistry 1005 (CSL 1005)

Marshes are particularly important ecosystems, providing long-term soil carbon storage, flood protection and nutrient filtration. Nutrient filtering, specifically nitrate removal, is largely the result of belowground microbially mediated denitrification. Denitrification rates differ in Spartina alterniflora and Juncus roemerianus patches but determining how the associated microbial communities contribute to these differences is challenged by the inherent physicochemical variability in the belowground of plants at different elevations in the marsh. Here we had a unique opportunity to evaluate denitrification rates and the belowground microbial community in J. roemerianus and S. alterniflora collocated at the same elevation, thus experiencing the same inundation cycles, in a saltwater marsh. To determine denitrification rates sediment slurry incubations (15N-nitrate) were used. The microbial community structure was determined using "iTag" sequencing of 16S rRNA gene amplicons. Slurry experiments revealed that denitrification rates were consistently higher in J. roemerianus. Analysis of 16S rRNA exact amplicon sequence variants (ASVs) showed that the microbial communities were similar in both plant types, although significant oscillations in abundance of some ASVs was observed. To link the rate and microbial community data, Random Forest Modeling (RFM) was used to determine if specific microbes could be accurate predictors of higher or lower denitrification rates. RFM identified ASVs classified as Deltaproteobacteria; Desulfobacteraceae and Chloroflexi; Anaerolineaceae as the most important predictors of denitrification rates. These microbial predictors were also identified as core members of the rhizosphere of both plants. Desulfobacteraceae, indicates higher denitrification rates, while Anaerolineaceae points towards lower rates of nitrate removal. Desulfobacteraceae are known sulfate reducers, however some have been shown to utilize both nitrate and sulfate to grow chemolithoautotrophically by coupling sulfide oxidation to dissimilatory nitrate reduction. In fact, this pathway was identified in a complimentary metagenomic dataset. Collectively, our data revealed that J. roemerianus promoted greater belowground nitrate removal compared to S. alterniflora which may result from different plant characteristics that lead to oscillations in the abundance of core members of the microbial community that can serve as predictors of denitrification rates and that this reaction may be mediated by previously unsuspected sulfate reducing bacteria in our saltmarsh ecosystem

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