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An increased understanding of heterotrophic bacterial strategies for acquiring nutrients and trace elements is critical for elucidating their impact on biogeochemical cycling in the ocean. It is estimated that iron is a limiting nutrient for phytoplankton growth in over 30% of the open ocean, but still little is known about bacterial strategies for iron acquisition. Siderophore (Fe ligand) production by bacteria may play a major role in influencing the bioavailability of iron in the ocean. Despite the importance of siderophores in the environment, only limited information from a select group of bacteria is available. On a cruise through the Costa Rica Dome (CRD) upwelling region in July 2005, a library of 867 isolates from five depth profiles inside and outside of the dome was obtained and screened for siderophore production using the Chrome Azurol-S (CAS) assay. Phylogenetic affiliation of 134 isolates was determined by sequencing the 16s rDNA gene, and determined that gamma proteobacteria such as Alteromonas, Pseudoalteromonas, Halomonas, and Marinobacter dominated the collection, while alpha-proteobacteria such as Roseobacter were also represented. The isolates obtained from stations in the CRD showed greater siderophore-producing capabilities between 55m and 100m while strains isolated from outside the CRD had shallower peak (-8-35m) production. Functional group determination showed that hydroxamate production dominated from 50-150m, while hydroxamate and catechol production is roughly equal in shallower waters. By characterizing the siderophores produced by these isolates and determining the genetic make-up of the population, these findings further our understanding of how heterotrophic microbes affect biogeochemical processes and the competitive nature of nutrient acquisition. History and Background: Microbes and their role in the environment Microbes are ubiquitous and significant regulators of many of earth's fundamental processes. Bacteria are found nearly everywhere on Earth, growing in soil, acidic hot springs, radioactive waste, seawater, and deep in the Earth's crust (Fredrickson et al. 2004). Forty million bacterial cells can be found in a gram of soil, and more than a million bacterial cells can be found in a milliliter of fresh water. Approximately 5 x 1030 bacteria can be found on Earth (Whitman et al. 1998). In addition to their notorious role of causing disease, bacteria are vital in the recycling of nutrients. Despite their importance, most of these bacteria have not been characterized, and less than half of the phyla of bacteria have species that can be cultured in the laboratory (Whitman et al. 1998). Advances in genomics capabilities are providing a more refined view of the capabilities and complexity of the previously underestimated oceanic microbial community, particularly with respect to diversity and functionality. The challenge is integrating these discoveries at the systems level to elucidate specific microbial functional roles and overall system resilience. Understanding the role of microbes in structuring healthy and stressed marine ecosystems will provide the mechanistic basis for extrapolative models. Genome sequencing has confronted scientists with the reality that many functional capabilities and molecular mechanisms remain unknown. Evidence suggests that important discoveries remain. The relatively recent discovery of proteorhodopsin genes in bacteria is an example. This light-driven pigment converts light energy into an electrical gradient across the cell membrane, and is thought to be an additional, but not primary, source of energy (Beja et al. 2001). Previously thought to only exist in Archaea, proteorhodopsin has now been found in divergent marine bacterial taxa in diverse environments. Another example of an important metabolic discovery is that of the oceanwide distribution of anoxygenic phototrophic bacteria (Madigan 2003). Their contribution to oceanic energy and carbon budgets is much larger than previously thought. Other biogeochemical budgets, such as nitrogen, must now be reconsidered due to fresh evidence for nitrogen fixation in common oceanic systems. It is possible that these, as well as undiscovered metabolic pathways, contribute to oceanic biogeochemical budgets in ways that change our current estimates. Biological importance of iron These notable discoveries highlight our current knowledge limitations, as well as the importance of a thorough understanding of microbial metabolism and environmental interactions. A key component needed to link the contribution of bacteria to oceanic nutrient cycles and higher trophic levels is knowledge of the uptake strategies for nutrients that limit growth (Martinez et al. 2000). One such nutrient is iron. Along with other necessary nutrients such as carbon, nitrogen, and phosphorous, microbes need to take up iron. On a broad scale, iron's importance is illustrated by its involvement with regulating coastal and open ocean microbial diversity, controlling primary productivity levels in large areas of the open ocean, and controlling phytoplankton populations in upwelling regions (Ryther and Kramer 1961, Kirchman et al. 2000, Barber and Ryther 1969). On the cellular level, iron is involved in important biological processes such as photosynthesis, nitrogen fixation, methanogenesis, H2 production, the trichloroacetic acid (TCA) cycle, oxygen transport, gene regulation, DNA biosynthesis/repair, and detoxification of free radicals (Andrews et al 2003, Crichton and Ward 1992, 1998). Oceanic algae and bacteria have a high rate of division and turnover relative to land plants, creating a high turnover of nutrients. Biologically available iron is almost completely depleted in surface waters due to this high turnover and cellular quota. Surface iron concentrations are approximately one millionth that of the concentration inside of the microbial cells (Morel and Price 2003). While most nutrients are effectively recycled and regenerated in their respective part of the water column, depletion due to export is a problem; abiotic scavenging and sinking particles are constantly depleting the iron pool. Because of these export factors, water column iron concentrations increase slightly with depth due to remineralization by bacteria below the photic zone. Iron has two stable oxidation states (Fe (II) and Fe (III)) that can be manipulated to catalyze a wide number of biochemical reactions. For these biochemical reactions, iron is used by incorporation into proteins, or in complexes forming part of iron-sulfur clusters or heme groups. Incorporating iron into proteins allows for control of the local redox potential (ranging from -300 to +700mV), geometry and spin state of the iron atoms, such that they can fulfill their necessary biological function (Andrews 2003). While Fe (III) is the most widely available form of iron in the ocean, Fe (II) is ultimately the form that biology utilizes. While certain transporters and iron reductases can draw Fe (II) into the cell, known siderophores only bind Fe (III). Internal mechanisms then reduce the Fe (III) to Fe (II), allowing it to be used within the cell. The versatility of iron makes it biologically useful, but there are downsides to using this element. Although iron is the 4 h most abundant element in earth's crust, it is widely unavailable to biological systems in the ocean due to its insolubility in aerobic environments at neutral pH, as well as biological factors such as competitive uptake (Sunda and Huntsman 1995). In all, it is estimated that the growth of 30% of the primary production in the open ocean is limited by iron (Martin et al. 1994, Coale et al. 1996, Moore et al. 2004). Iron in surface seawater is present in extremely low concentrations [typically from 20 pM to 1 nM], which limits primary production by phytoplankton in regions characterized by high concentrations of nitrate and other nutrients but lower than expected concentrations of chlorophyll (HNLC, high nitrate low chlorophyll) (Gordon et al 1982, Landing and Bruland 1987, Martin at al. 1989, 1991, Bruland et al. 1994). HNLC regions include the subarctic Pacific, eastern equatorial Pacific, the Southern Ocean, and upwelling regions (de Baar et al. 1990, Martin and Fitzwater 1990, Coale et al. 1996). Heterotrophic bacteria are also limited by low iron levels in HNLC regions, though the complexity of their iron requirements and uptake strategies are poorly understood. In regions where phytoplankton are clearly iron limited, conflicting reports exist on iron limitation in heterotrophic bacteria, further highlighting our limited understanding of the complex cycling of this essential nutrient (Kirchman et al. 2000). It is estimated that >99% of the dissolved iron in seawater is complexed by organic ligands in the North Sea, Western Mediterranean, North Pacific, Northwest Atlantic, and equatorial Pacific (Gledhill and van den Berg 1994, van den Berg 1995, Rue and Bruland 1995, Wu and Luther 1995, Rue and Bruland 19971). The structural composition of this organic ligand pool is largely unknown. One major stumbling block in the characterization of the natural organic ligand pool is the large-scale sampling that is required. A milligram or more of ligand may be required for characterization, limiting the resolution of the samples that can be analyzed (Macrellis et al. 2001). While no siderophore structures have been characterized from the field, lab and field experiments show that siderophores are a component of this pool (Haygood et al. 1993, Rue and Bruland 1995, Lewis et al. 1995, Wilhelm et al. 1998, Hudson 1998, Hutchins et al. 1999b). Organic ligands isolated in the California upwelling region have been shown to possess iron-binding functional groups typical of siderophores (Macrellis et al. 2001). Most other thorough siderophore characterization has taken place on marine bacterial isolates in the laboratory, allowing for intense s

Siderophore production by heterotrophic bacterial isolates from the Costa Rica Upwelling Dome