Iron and copper are essential bioactive elements in the marine environment, but they have a complex chemical speciation dominated by a heterogeneous mixture of organic metal-ligand complexes. Numerous analytical constraints complicate the direct chemical characterization of these species, thus this work seeks to expand upon existing indirect electrochemical methods for examining copper and iron organic complexes in seawater. A multiple analytical window (MAW) electrochemical approach, which enables the detection of a broad spectrum of ligands, is applied in new regions of the ocean for copper and, for the first time, in studies of iron speciation. Chapter 2 describes the first application of the MAW electrochemical technique for copper speciation in the open ocean. Copper-binding ligands were measured in four surface water masses of the Antarctic Peninsula region, and each water mass was shown to contain distinct pools of ligands. Chapters 3 and 4 focus on applying the MAW electrochemical method to iron- binding ligands. In Chapter 3, iron-binding ligands were measured in central California coastal waters in the surface and benthic boundary layer (BBL), in order to validate the MAW approach for iron speciation in contrasting chemical regimes. Iron-binding ligands in surface waters were found to be chemically distinct from the BBL ligand pool. Chapter 4 explores San Francisco Bay as a source of iron-binding ligands to coastal California waters. Scavenging in the estuary caused the concentration of weaker ligands to decrease with salinity, while the strongest ligands remained largely resistant to flocculation. Chapter 5 applies the MAW electrochemical technique in experimental studies and water column profiles to interpret mechanisms of in-situ iron-binding ligand cycling in the southern California Current. Photochemical processes were found to dominate in near surface waters, while biological processes controlled ligand distributions in deeper waters. Overall, the simultaneous detection of multiple ligand classes has contributed significantly to our existing knowledge of metal ligand sources and sinks, and the unique chemical environments in which phytoplankton are utilizing trace nutrients

Nutrient dynamics, phytoplankton rate processes, and export were examined in a frontal region between an anticyclone and a pair of cyclones 120 km off the coast in the southern California Current System (sCCS). Low silicic acid: nitrate ratios (Si:N) and high nitrate to iron ratios (N:Fe) characteristic of Fe-limiting conditions in the sCCS were associated with the northern cyclone and with the transition zone between the cyclones and the anticyclone. Phytoplankton growth in low-Si:N, high-N:Fe waters responded strongly to added Fe, confirming growth limitation by Fe of the diatom-dominated phytoplankton community. Low Si:N waters had low biogenic silica content, intermediate productivity, but high export compared to intermediate Si:N waters indicating increased export efficiency under Fe stress. Biogenic silica and particulate organic carbon (POC) export were both high beneath low Si:N waters with biogenic silica export being especially enhanced. This suggests that relatively high POC export from low Si:N waters was supported by silica ballasting from Fe-limited diatoms. Higher POC export efficiency in low Si:N waters may have been further enhanced by lower rates of organic carbon remineralization due to reduced grazing of more heavily armored diatoms growing under Fe stress. The results imply that Fe stress can enhance carbon export, despite lowering productivity, by driving higher export efficiency.

In October 2008, we investigated pelagic community composition and biomass, from bacteria to fish, across a sharp frontal gradient overlying deep waters south of Point Conception, California. This northsouth gradient, which we called A-Front, was formed by the eastward flow of the California Current and separated cooler mesotrophic waters of coastal upwelling origin to the north, from warm oligotrophic waters of likely mixed subarcticsubtropical origin to the south. Plankton biomass and phytoplankton growth rates were two to three times greater on the northern side, and primary production rates were elevated 5-fold to the north. Compared with either of the adjacent waters, the frontal interface was strongly enriched and uniquely defined by a subsurface bloom of large diatoms, elevated concentrations of suspension-feeding zooplankton, high bioacoustical estimates of pelagic fish and enhanced bacterial production and phytoplankton biomass and photosynthetic potential. Such habitats, though small in areal extent, may contribute disproportionately and importantly to regional productivity, nutrient cycling, carbon fluxes and trophic ecology. As a general introduction to the A-Front study, we provide an overview of its design and implementation, a brief summary of major findings and a discussion of potential mechanisms of plankton enrichment at the front. © 2012 The Author.

Despite very low concentrations of cobalt in marine waters, cyanobacteria in the genus Prochlorococcus retain the genetic machinery for the synthesis and use of cobalt-bearing cofactors (cobalamins) in their genomes. We explore cobalt metabolism in a Prochlorococcus isolate from the equatorial Pacific Ocean (strain MIT9215) through a series of growth experiments under iron- and cobalt-limiting conditions. Metal uptake rates, quantitative proteomic measurements of cobalamin-dependent enzymes, and theoretical calculations all indicate that Prochlorococcus MIT9215 can sustain growth with less than 50 cobalt atoms per cell, ∼100-fold lower than minimum iron requirements for these cells (∼5,100 atoms per cell). Quantitative descriptions of Prochlorococcus cobalt limitation are used to interpret the cobalt distribution in the equatorial Pacific Ocean, where surface concentrations are among the lowest measured globally but Prochlorococcus biomass is high. A low minimum cobalt quota ensures that other nutrients, notably iron, will be exhausted before cobalt can be fully depleted, helping to explain the persistence of cobalt-dependent metabolism in marine cyanobacteria.