Masters Theses

Date of Award


Degree Type


Degree Name

Master of Science



Major Professor

Karen G. Lloyd

Committee Members

Allison Buchan, Frank Loeffler, Ania Sszynkiewicz


Biogeochemical processes drive the cycling of nutrients on Earth, both in surface and subsurface environments, with subduction representing a critical link between shallow and deep geochemical cycles. In subduction zones, sediments and subseafloor basement basalts are either transported into the mantle or are “recycled” back to the surface through fluid fluxes, volatile degassing, or magmatic events. Transformations of these subducted sediments occur in response to a variety of abiotic, thermogenic, or biological processes on both geochemical and biological time scales. However, the biological component to biogeochemical cycling of volatiles in subduction zones has largely been overlooked in the past. Recent advancements in high throughput sequencing have opened the door for the systematic study of the diversity of these communities. When integrated with available geochemical data, one can start to gain a better understanding of the complex interactions between the biotic and abiotic processes driving these cycles. To investigate potential interactions between abundant taxa and their environment, ordination analysis was applied to a large, biogeochemical dataset from 24 geochemically diverse hydrogeological sites in the volcanic region of Costa Rica, including 16S rRNA gene libraries containing >56,000 and >27,000 unique bacterial and archaeal amplicon sequence variants (ASV) sequences, respectively. Fluids show low input of photosynthesis-related genes or carbon with photosynthetic isotope signals, indicating that fluid microbial communities largely reflect shallow subsurface geochemical processes. A pH gradient is the primary driver of across-arc variation between the Outer Forearc and Forearc/Arc, while changes in temperature corresponding to changes in offshore bathymetry define along-arc variation. Based on these two geochemical gradients, we propose a 4 region model of microbial composition: 1) Northern Forearc/Arc – acidic and thermophilic; 2) Central Forearc/Arc – acidic and thermophilic, but less so than its northern counterpart; 3) Northern Outer Forearc – alkaline; and 4) Central Outer Forearc – alkaline. Regional niche separation of primarily chemolithotrophic microbial taxa reflect local subduction geochemistry, such as the acidophilic Sulfurihydrogenibium sp. dominating high temperature acidic springs in the Forearc/Arc. This research establishes the microbial responses to regional-scale geochemistry in a geothermal system, and shows that the effects of tectonic-scale processes can be observed in microbial community compositions.

Files over 3MB may be slow to open. For best results, right-click and select "save as..."