UC Berkeley

Iodine plays a largely unrecognized yet essential function in the history of life globally. It is the 53rd most common element in Earth's crust and one of the heaviest biologically relevant elements for animals and marine plants. While some iodine is present in terrestrial environments, it is generally enriched in marine ecosystems. In these environments, it is engaged in a prolific biogeochemical cycle, oscillating through multiple distinct oxidation states depending on the location and chemistry of the environment. The most common forms found are its most oxidized form (IO3-) and reduced forms like iodide (I-) and volatile organic iodines (VOI). Emerging evidence demonstrates that algae produce VOIs, which play an essential role in regulating climate at a local level. These VOIs impact climate by destroying tropospheric ozone, resulting in greater albedo (from cloud formation) and reduced radiative forcing. Numerous studies examining the iodine geochemical cycle exist; however, many stop short of describing the biological contribution to the observed geochemical phenomena. This dissertation opens by expanding on the current knowledge surrounding the iodine geochemical cycle. It describes the genetic toolkit certain microorganisms use to interact with iodine at different redox states and proposes a role for these microorganisms based on their presence globally. The story then shifts focus to a hitherto poorly characterized group of bacteria known as dissimilatory iodate reducing microorganisms (DIRM). I examine the physiology of these microorganisms by demonstrating their ability to grow on IO3- as the sole terminal electron acceptor. I show that this phenotype is enabled by a mobile genetic element known as the iodate reduction island (IRI) and demonstrate its distribution among numerous bacterial phyla. I conclude this dissertation by examining the geochemical variables associated with DIRM habitats and exploring various marine and non-marine environments where DIRM are expected to live. In its totality, this work seeks to demonstrate that DIRM are a ubiquitous class of microorganisms that derive energy from IO3- reduction and contribute to global iodine cycling in the process.