Visible light was observed to induce reductive dissociation of organically complexed Fe and dramatically increase the short-term uptake rate of radiolabeled Fe by Microcystis aeruginosa PCC7806 in Fraquil* medium buffered by a single metal chelator, ethylenediaminetetraacetic acid (EDTA). Only wavelengths <500 nm activated Fe uptake indicating that Fe photochemistry rather than biological factors is responsible for the facilitated uptake. The measured rate of photochemical Fe(II) production combined with a significant decrease in (55)Fe uptake rate in the presence of ferrozine (a strong ferrous iron chelator) confirmed that photogenerated unchelated Fe(II) was the major form of Fe taken up by M. aeruginosa under the conditions examined. Mathematical modeling based on unchelated Fe(II) uptake by concentration gradient dependent passive diffusion of Fe(II) through nonspecific transmembrane channels (porins) could account for the magnitude of Fe uptake and a variety of other observations such as the effect of competing ligands on Fe uptake. Steady-state uptake rates indicated that M. aeruginosa acquires Fe predominantly during the light cycle. This study confirms that Fe photochemistry has a dominant impact on Fe acquisition and growth by M. aeruginosa in EDTA-buffered culture medium.

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The mode of Fe uptake by the cyanobacterium Microcystis aeruginosa cultured in Fraquil* (pH 8) containing Suwannee River fulvic acid (SRFA) was examined using short-term radiolabeled (55)Fe uptake assays and a kinetic model that describes extracellular Fe transformations. Both Fe(II) and Fe(III) uptake rates decreased substantially with increasing SRFA concentration as the availability of unchelated Fe decreased due to complexation by SRFA. Fe uptake rates under illuminated conditions were comparable to or slightly higher than those observed in the dark at the same Fe:SRFA concentration ratio, in contrast to results for systems containing ethylenediaminetetraacetic acid where Fe uptake rates were much greater under illumination than in the dark. The limited effect of light principally resulted from the relatively high rates of thermal dissociation and dark reduction of Fe(III) bound to SRFA and complexation of photogenerated Fe(II) by SRFA. Our findings imply that Fe uptake by M. aeruginosa at a fixed total Fe concentration of 200 nM is close to saturation when fulvic acid is present at concentrations near those typically found in natural waters (< ∼5 mg·L(-1)), with cellular growth likely to be limited by Fe availability only when natural organic matter is present at very high concentrations (>25 mg·L(-1)).

Cellular responses of Fe‐limited Microcystis aeruginosa were investigated under nutrient‐depleted and ‐replete conditions. Cellular growth, Fe quota and Fe uptake kinetics were examined in chemostat systems using nutrient‐replete Fraquil* (where all nutrients except for Fe are present at sufficient level to achieve optimal growth) and nutrient‐deplete Fraquil* (where some nutrients in addition to Fe are potentially growth‐limiting factors). For both nutrient conditions, cellular Fe quota increased with increasing dilution rate in a manner consistent with Droop theory. However, the Fe quota in nutrient‐deplete Fraquil* was determined to be lower, indicating lower cellular Fe requirement in the nutrient‐depleted condition. Short‐term Fe uptake assays indicated that cells acclimated in nutrient‐replete conditions adjust to various degrees of Fe stress by solely increasing maximum Fe uptake rate, consistent with expected negative feedback regulation. In contrast, the maximum Fe uptake rate decreased with increasing degree of Fe limitation in the nutrient‐depleted chemostat (particularly nitrate and molybdenum in this study). This non‐negative feedback regulation is likely associated with lower Fe requirement for specific functions (e.g., intracellular nitrate reduction). Cellular affinity for Fe uptake and cellular size were independent of degree of Fe stress for both nutrient conditions.