Iron limits phytoplankton productivity and biomass in significant portions of the global ocean. A number of studies over the past decade have shown that iron's primary role in phytoplankton is as a cofactor in photosynthetic light processing and electron transport proteins, linking its availability directly to primary productivity. Multiple consequences of this localization were explored in this thesis. Because photosynthetic iron demands dominate phytoplankton iron requirements, it has been suggested that primary productivity may be co-limited by iron and light when the availability of both factors is reduced. Support for this hypothesis had been previously obtained in iron limited regions with deep mixed layers, but the possibility that iron-light co-limitation may occur outside areas conventionally thought to be iron limited was explored in Chapter II of this thesis. Phytoplankton, particularly larger diatoms, displayed responses indicative of iron-light co-limited at subsurface chlorophyll maxima in the Southern California Bight and the eastern tropical North Pacific. In these water columns, lack of iron availability limits the growth of larger organisms with consequences for carbon export efficiency and ecosystem structure. The distribution and speciation (redox and organic) of iron in one of these regions, the eastern tropical North Pacific, is discussed in Chapter IV. Photosynthetic physiology is strongly impacted by iron limitation, due to the high iron content of the photosynthetic apparatus. Assessment of variability in photosynthetic physiological states has proven useful as a diagnostic of iron stress in laboratory cultures and mesoscale iron addition experiments. In Chapter III, changes in photosynthetic physiology across a gradient in iron stress are examined in relation to experimental responses to iron addition. It was determined that in a region of natural iron fertilization in the southern Drake Passage, photosystem II characteristics were correlated with responses to iron addition, and could serve as good indicators of the degree of iron stress in a system. microbial ecosystems. The cycling of heme in the ocean is not well understood, but because it is hydrophobic and photosensitive, direct uptake by bacteria from particulates may be an important pathway of recycling. In Chapter V, the ability of a particle associated bacterium, Microscilla marina, to take up heme is discussed and a potential heme transport system is identified in its genome. The putative heme transport system is expressed and upregulated under iron stress and when growing on heme as an iron source. The genomes of many diverse marine bacteria were searched for similar uptake systems and genes with strong similarity to known heme transporters were identified in alpha- and gamma- proteobacteria. However, no putative heme transporters were identified in cyanobacteria or oligotrophic bacteria such as Pelagibacter ubique, suggesting heme is primarily available on particles or in rich environments, in agreement with its chemistry. Further investigations into the uptake capabilities of marine microorganisms may provide additional insights into trace metal cycling
