Date of Completion


Embargo Period



Pieter Visscher, Penny Vlahos, Andrew Bush, Gary Wikfors

Field of Study



Master of Science

Open Access

Open Access


Microbial mats are self-contained organosedimentary ecosystems that played an important role in the evolution of early life on Earth. Understanding the conditions of their formation and preservation in the fossil record could provide valuable insights into microbe-mineral interactions and paleoenvironmental reconstruction. Diatoms are found at the surface of extant microbial mats, but their frustules are not observed at depth and rarely in fossil microbialites, which suggest their frustules dissolve. The objective of this study is to investigate the role of cyanobacterial photosynthesis, which locally elevates pH, in the dissolution of diatoms frustules. Field data and laboratory studies determined the effect of pH and other environmental factors (e.g. temperature and salinity) on the dynamics of silica dissolution. Dissolved silica was measured over time in chemical and cyanobacterial incubations to which diatom frustules were added. Intact microbial mats were augmented with frustules to investigate the disappearance of diatoms with depth. Field-based and experimental observations were compared to a chemical equilibrium model. In chemical experiments, dissolution increases with pH, temperature, fracturing of frustules and the degree of undersaturation of silica in solution and was confirmed by the model. T. pseudonana dissolved at a greater rate than N. closterium across treatments. In cyanobacterial enrichments, the pH remained above 9 for most of the daylight period and dissolved silica concentrations increased. There were no clear trends observed with increased salinity. The preservation potential of diatom frustules within the mat was evaluated in the context of what was observed in the field, in lab experiments, and the model. Increased understanding of the fate of diatoms in extant systems facilitates a more accurately interpretation of fossil mats and ancient environments.

Major Advisor

Pieter Visscher