Microbialites of Lake Clifton, Western Australia : groundwater dependent ecosystems in a threatened environment
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Microbialites are organosedimentary deposits formed by interactions between microbial communities, sediments, and water. Modern microbialites, though rare, provide a window into the functioning of Earth’s first ecosystems. This dissertation focuses on understanding how lake water salinity and groundwater discharge influence modern thrombolitic microbialite development in Lake Clifton, Western Australia. Using porewater chemistry data complemented with thermal infrared and temperature profile measurements, I show that groundwater preferentially discharges through Lake Clifton thrombolites, which are highly permeable, and document springs emerging from the thrombolites. Groundwater discharge is the key environmental factor controlling nutrient availability in microbial mats growing on the thrombolites, which form in a phosphorous limiting and oligotrophic environment. I propose that thrombolites are self-organizing systems driven by feedbacks between groundwater discharge, microbial mat growth, and lithification of a permeable internal fabric. Using shotgun metagenomic sequencing and analysis, I show that microbial mats growing on thrombolites harbor distinct microbiomes that are enriched in photoautotrophic microorganisms and functional genes associated with photosynthesis compared to other microbial communities in the lake. Although thrombolites in Lake Clifton are threatened by a large increase in lake water salinity since the 1990’s, microbial mats growing on thrombolites still harbor the key metabolisms required for biologically inducing mineral precipitation and lithification may still be occurring. Finally, with a goal towards understanding the processes driving the drastic environmental changes occurring in Lake Clifton and managing its unique ecosystem, I use hydrochemical data and geochemical modeling to delineate the major factors contributing to the lake water salinity increase. Evaporation is an important process controlling the chemical evolution of lake water from the 1990’s to present but other contributing factors include changes in source groundwater composition, mixing, and reductions in photosynthesis and carbonate precipitation. Lake Clifton is one of the few known thrombolite-forming ecosystems and continued monitoring of this threatened site is critical not only for preservation but also for understanding how microbialite development is influenced by changing environmental conditions.