UC San Diego

As a scarce but essential micronutrient for microbial growth in the marine environment, iron plays a critical role in supporting marine primary productivity and is tightly coupled to the cycling of carbon and additional macronutrients. While significant progress in recent years has been made in understanding the distribution of iron and iron-binding ligands in the marine

environment, many questions remain regarding the mechanistic processes underlying these distributions.

Heterotrophic bacteria serve as the primary drivers of the turnover of organic matter in the marine environment and have a significant cellular iron requirement, making them an important link between iron and carbon biogeochemical cycles. The work presented in this dissertation aims to improve our understanding of the molecular mechanisms by which heterotrophic marine bacteria acquire iron from their surroundings and the subsequent effects of their metabolic activities on both iron and carbon biogeochemical cycling.

We have first developed Alteromonas macleodii ATCC 27126 as a model marine organism for the study of iron acquisition by heterotrophic bacteria. This has allowed us to characterize outer membrane TonB-dependent transporters for organically complexed forms of iron across the genus Alteromonas. This work revealed that a high diversity of iron-ligand compounds in the marine environment are potentially bioavailable. We have also utilized Alteromonas macleodii ATCC 27126 as a model organism for studying the functional role of siderophore production in iron acquisition in the marine environment. Through the insertional inactivation of the petrobactin biosynthetic pathway, we have determined that siderophore production increases the bioavailability of non-labile iron sources such as particulate minerals. Finally, in microcosm field experiments, we examined the transcriptional response of natural heterotrophic bacterial communities in the southern California Current System to iron additions demonstrating direct iron limitation of this community with consequences for downstream carbon processing.

Together, this work sheds light on the role of heterotrophic bacteria at multiple points within the marine iron cycle, from the incorporation of iron into the marine environment to controlling the balance between the export and recycling of iron and macronutrients. It is our hope that this work results in an improved mechanistic understanding of marine iron biogeochemical cycling.