The cycling of iron in the ocean is intimately tied to whole ecosystem processes and other biogeochemical cycles. While ocean basin surveys of iron concentration have revealed important sources to the ocean, questions remain regarding the controls on supply of this critical micronutrient to productive planktonic ecosystems. Using a combination of oceanographic-scale observations and mechanistic experiments, work in this thesis seeks to advance our understanding of controls of new and regenerated iron supply to coastal polar and temperate ecosystems and microbially-mediated cycling in such marine environments.

In Chapter 2 we comprehensively characterized the cryospheric inputs of iron to Andvord Bay, a cold glaciomarine fjord located on the west Antarctic Peninsula. Iron content and speciation were measured to characterize inter-seasonal changes in the water column, a subglacial meltwater plume and glacial ice, fjord sediments, including a first assessment of iron-binding organic ligands in a highly productive fjord environment. We utilized multi-elemental analyses to understand the cycling of iron sources. Results from a high-resolution numerical model revealed episodic atmospheric forcing events are important for supplying subglacial meltwater to the surface, and for the export of surface meltwater and associated iron away from the coast. In Chapter 3 we characterize iron supply and cycling in a coastal upwelling filament, a mesoscale feature which commonly occurs during times of intensified along-shore winds in eastern boundary current systems, such as the southern California Current. We used a Lagrangian framework to capture the biogeochemical evolution of a filament, including: the development of iron-limitation of the phytoplankton community, assessment of in situ geochemical proxies of iron stress, and impacts of diminishing iron flux on phytoplankton allometric ratios, namely the biogenic silica-to-organic carbon content of diatoms. We have determined filaments are not likely significant sources of dissolved iron to the open ocean, yet their importance for advecting particulate iron remains to be tested. Coincident with the development of iron-limitation of the phytoplankton community we observed greater biogenic silica-to-organic carbon ratios and enhanced export efficiency to depth. In Chapter 4, within microcosm a field experiment, we examined the production of organic iron-binding ligands during remineralization – an important process supplying iron regenerated from phytoplankton blooms. As a part of this work we developed an improved data processing methods for multiple analytical windows electrochemical iron ligand titration data, enabling more robust detection of distinct pools of iron-binding ligands produced by heterotrophic bacteria and in diverse oceanic waters.

Together, this work examines the processes involved in the flux of new iron to the ocean, and internal transformations affecting its fate within the water column. We hope this work results in an improved understanding of the controls on modern iron supply and ties to the marine biogeochemical cycles of macronutrients.

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.

This study of an upwelling cross-shore filament in the California Current System attempts to follow a parcel of water from upwelling source to offshore in order to characterize changes in silica cycling and associated impacts on carbon cycling under the influence of iron limitation. Water column samples were collected throughout the euphotic zone in conjunction with sediment traps following the filament in Lagrangian fashion (cycles); additional samples were taken across the filament (transects). The filament progressed from iron-replete to iron-limited throughout the study, identified by low silicic acid to nitrate ratios (Si:N) and elevated nitrate to iron (N:Fe) ratios in situ. Increases in biogenic silica to POC ratios (bSi:POC) in the water column were apparent with increasing iron limitation of the diatom-dominated community. The Lagrangian nature of the study improves overall understanding of the dynamics of biogenic silica as an upwelling filament develops to iron-limited, low Si:N water conditions. This work builds upon and adds context to previous studies which have shown that iron limitation affects silicic acid to carbon (Si:C) and silicic acid to nitrate (Si:N) uptake ratios in marine diatoms. These ratios have potential implications for both carbon and silica cycling in the ocean, impacting uptake and export of these elements. The filament was shown to transport biogenic silica laterally away from the coast, and enhanced silica ballasting was shown to be associated with increased export efficiency offshore.

This dissertation explores the role that organic ligands play in controlling copper speciation in a range of oceanic and coastal environments, using voltammetric methods. Chapter 1 details the analysis of samples collected on the 2013 U.S. GEOTRACES cruise in the Equatorial Pacific from Peru to Tahiti (GP16). This represents one of the largest copper speciation datasets to date, covering 22 full ocean depth profiles. Results demonstrate that copper ligands follow a similar pattern to dissolved copper of increasing concentrations with increasing depth. Free copper ion concentrations ([Cu2+]) also increase with depth. This dataset provides insight into the impact of large-scale ocean circulation and water mass aging with respect to copper speciation, with increasing Cu2+ and decreasing excess ligand concentrations in the oldest waters of the deep Pacific.

In Chapter 2, copper speciation from select samples in the GP16 data set is analyzed via a novel processing protocol that enables competitive ligand exchange titrations at multiple competition strengths to be analyzed as a unified dataset. This method utilizes publicly available processing tools to determine a unified sensitivity for the data set as a whole. In this way the full spectrum of the copper-binding ligand pool can be most accurately characterized at a range of analytical windows, revealing both weak and strong ligands. For GP16 samples, this analysis reveals two classes of copper-binding ligands present throughout the oceanic water column, with the weaker L2 ligands present at higher concentrations. Strong L1 ligands are present at or above dissolved copper concentrations, dominating copper speciation especially in surface waters where L1 concentrations exceed that of dissolved copper.

Copper binding ligands play a vital role in maintaining low concentrations of the Cu2+ ion in coastal environments, which helps buffer such systems from copper toxicity. In Chapter 3, the processing method developed in Chapter 2 is applied to an assessment of the ligand pool and free copper ion levels in San Diego Bay, an impacted urban environment with high concentrations of recreational watercraft coated with copper-based antifouling paint. Sampling campaigns at 7 locations within the Bay took place in Summer 2015, Spring 2016 and Fall 2016. Samples for total dissolved copper and copper speciation were collected, as well as samples for Synechococcus concentrations as a biological indicator of copper toxicity. Results from the three samplings show elevated levels (30 –120 nM) of dissolved copper throughout the Bay and a ligand pool capable of buffering most of the locations within the Bay from reaching toxic levels of the free copper ion. Shelter Island Yacht Basin, however, was consistently found to contain toxic levels of Cu2+, results that show a relationship with the lowest Synechococcus concentrations in two out of the three samplings. Findings of this study are relevant for ongoing remediation and regulation efforts in San Diego Bay.

A detailed picture of the chemical speciation of iron in seawater is required to comprehensively understand the marine iron cycle and its downstream effects on the cycles of other nutrient elements. Although much progress has been made in our understanding of iron-binding ligands in the marine environment, numerous questions remain as to their identity, structure, and biogeochemical sources and sinks. The recent proliferation of “omic” data from cultured organisms and environmental samples now allows us to address some of these chemical questions from a biologically informed perspective.

The research presented in this thesis examines some of the specific molecular mechanisms and underlying genetics that marine heterotrophic bacteria use to acquire iron from the unique chemical conditions of the marine environment. We first examined and reviewed the sources, transformations, and ultimate fate of heme and hemoproteins, iron containing molecules, in seawater. We used comparative genomics to characterize trace metal acquisition strategies in the marine Roseobacter and SAR11 clades, and we examined these uptake pathways in light of marine trace metal chemistry, microbial ecology, and genome evolution. We developed a genetic system for targeted mutagenesis and insertional inactivation of a predicted heme uptake TonB dependent outer membrane transporter in a model marine bacterial strain Ruegeria sp. TM1040. In a series of iron amended incubations from the California current ecosystem, we examined the heterotrophic bacterial assemblage composition using high throughput sequencing of bacterial marker genes in conjunction with chemical speciation and algal physiology. Finally, we examined putative iron uptake pathways in a metatranscriptome from natural marine phyto- and bacterioplankton communities in the southern California current and integrated patterns in the metatranscriptomic data with biogeochemical field data.

Our results have revealed genes responsible for trace metal metabolisms in marine heterotrophic bacteria and highlight heme uptake as a model heterotrophic iron assimilation pathway that is potentially important in particulate iron remineralization. In turn, we have provided additional insight to the chemical speciation of iron in seawater and have revealed potential molecular mechanisms by which heterotrophic bacteria shape the marine iron cycle.

The western Antarctic Peninsula is rapidly warming and its high-latitude fjord ecosystems are expected to be highly sensitive to climate warming (Weslawski et al. 2011; Cook et al. 2016). As the region continues to change, understanding the current nutrient budget will allow for better predictions of ecosystem response at all levels of the food web to variable future conditions. Analysis from two synoptic transects along Andvord Bay, bracketing the 2015-2016 austral summer, allows for primary production to be assessed through the depletion of dissolved inorganic nutrients, nitrate and silicate. Andvord Bay is a quiescent system that can experience surface nutrient replenishment during katabatic wind events otherwise sustaining favorable growth conditions throughout an extended growth season. The high concentration of surface nitrate and minimal presence of reduced nitrogen species in spring suggests new production dominates the early season while an observed four-fold increase in ammonium would support late season recycled production. Silicate depletion indicates 25-68% of the primary production in Andvord Bay was from diatom growth from December 2015 to April 2016. Modifications were made to the nutrient drawdown method for primary production estimation to adapt to conditions unique to the Andvord Bay region, replacing the baseline winter water requirement and adapting the growth periods. Applied, this method yields reasonable, conservative estimates for net community new production indicating greater production inside the fjord than the outside waters of the Gerlache Strait and supporting the hypotheses that fjords in the WAP are hot-spots of productivity.