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The potential for ocean iron (Fe) limitation has gone from a challenged hypothesis worthy of extensive field studies to established dogma for certain ocean regions, such as the Southern Ocean. The vast majority of both field and laboratory studies have focused on photosynthetic organisms such as diatoms and cyanobacteria. These have established iron starvation can induce compensatory responses including Fe saving metabolic changes and the induction of very specific Fe transport mechanisms. Despite the substantial advancements in our understanding of Fe biogeochemical cycling and the organismal response to Fe starvation, heterotrophic bacteria have been largely disregarded at both the molecular and global levels. Here we utilized the ubiquitous marine bacterium Altermonas macleodii for control laboratory studies examining the response to Fe limitation and the role of siderophore production on Fe acquisition. Controlled and replicated growth experiments with subsequent RNA-sequencing examined the organismal response to either carbon (C) or Fe limitation on a whole genome scale. An examination of the entire transcriptome response shows unique global responses to Fe and C-limitation, which is also reflected in the expression of the suite of TonB-dependent outer membrane receptors (TBDRs, Fig. 1). TBDRs are solely found in Prokaryotic organisms and facilitate the transport of large molecules, such as siderophore bound Fe and polysaccharides, across the outer membrane. While Fe starved, the transcriptome also suggests that A. macleodii also changes Fe requirements, with the induction of nickel containing superoxide dismutase and a Fe-free form of fumarase. A subsequent comparative genomic analysis shows that the Fe and C responsive TBDRs are conserved across nearly all Altermonas genomes sequenced to date (Fig. 2), which indicates that their expression may serve as transcriptional biomarkers for Fe or C limitation of Altermonas population in field studies. One major benefit of using A. macleodii as a model organism is the availability of genetic manipulations, which facilitate an explicit examination of gene function, rather than one just implied by annotation and gene expression. Utilizing targeted homologous recombination, we inactiviated one of the primary genes in a putative siderophore biosynthetic pathway found in the A. macleodii genome with a subsequent examination of the growth rate relative to the wild type on a variety of Fe sources. Not surprisingly, the siderophore was necessary for maximal growth for organically bound Fe (Fig. 3A). However, it was also apparent that the siderophore facilitated on growth of colloidal and particulate Fe that represent terrestrial or hydrothermal vent Fe inputs to the ocean (Fig. 3B). It was also shown that this gene cluster does synthesize a specific siderophore, petrobactin. This petrobactin biosynthetic cluster was shown to be globally distributed, according to a comparative analysis of marine metagenomes (Fig. 4). Based on synthesis of these results, we are starting to paint a broad conceptual pictute of the role of some Altermonas in global Fe cycling (Fig. 5). Namely, the organism responds differently to Fe and C starvation, which should allow for us to examine natural populations and assess their nutritional state. However, when Fe starved, A. macleodii will produce petrobactin, which not only facilitates the acquisition of Fe for itself, also liberates Fe from lithogenic particles for the greater microbial community. Potentially this represents a large scale symbiotic system. This work formed the primary dissertation for one PhD student as well as secondary chapters for another PhD student. 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