Date of Award


Degree Type


Degree Name

Doctor of Philosophy


As primary producers, marine cyanobacteria regulate the biological and biogeochemical cycles of aquatic ecosystems and influence ocean-atmosphere gas exchange. The purposes of this project were to determine if a Synechococcus sp. was capable of utilizing a high-affinity iron transport system during periods of iron-limited growth and if physiological changes due to iron limitation could be identified. Resolution of these factors was achieved by maintaining the cyanobacterium Synechococcus sp. PCC 7002 in continuous culture chemostats over a range of iron availabilities.;Changes in physiology were detected over a range of iron concentrations. Reductions in the levels of photosynthetic pigments were accompanied by a deterioration of thylakoid integrity and changes in cellular carboxysome and polyphosphate bodies. Polypeptide profiles of thylakoid, cytoplasmic, and outer membrane fractions demonstrated the enhanced production of specific proteins during iron-limited growth. The photosynthetic efficacy (measured as oxygen evolution and carbon fixation) is significantly reduced during iron-limitation, with carbon incorporation being reduced from luxury levels to the minimum requirements for cellular carbon turnover.;Steady state cell densities within chemostats, combined with growth data obtained from batch cultures, demonstrate a non-linear response between iron concentration and cyanobacterial proliferation. Results suggest that this is brought about by changes in the cellular iron quotient, coupled with the activation of an energy dependant high-affinity iron transport system. The activation of this system involves the release of four novel iron-regulated, iron-binding cell products (siderophores) by Synechococcus sp. PCC 7002.;To determine the prevalence of siderophore production, ten species of cyanobacteria were examined for the ability to produce siderophores under iron-limiting conditions. In all cases these cyanobacteria were found to produce siderophores, and, in many species, the production of multiple siderophores was detected. Analysis of the chemical moieties associated with these compounds demonstrated that hydroxamate-type, catechol-type, and atypical-type iron chelators are produced by these cyanobacteria.;The presence of siderophores enhances the ability of cyanobacteria, grown under iron-limiting conditions, to assimilate iron from the environment via membrane-associated receptor proteins. Ferrisiderophore receptors are not expressed by cyanobacteria grown under iron-replete conditions.;An amalgamation of these results infers that some cyanobacteria utilize high-affinity iron transport systems, involving the serial transport of iron via soluble and membrane-associated ferrisiderophore complexes, in a process which requires the presence of membrane-specific receptors and ATP. This work demonstrates that the definition of affinity in iron transport must consider the biologically available and unavailable pools of iron in the environment. This work suggests that high-affinity iron transport in cyanobacteria involves the conversion of biologically unavailable iron to an available form.



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