The importance and bioavailability of iron to Trichodesmium

The availability of iron (Fe) may be an important factor affecting the productivity of cyanobacteria. Understanding the importance of Fe to ecologically significant cyanobacteria and the determination of bioavailable Fe sources are critical for estimating the role that Fe may have in regulating the productivity of the ocean.; The importance of Fe for the N2 fixation and biomass of Trichodesmium colonies was investigated along a transect between Bermuda and Puerto Rico. Neither the Fe content of the colonies nor the total dissolvable Fe (TDFe) concentration of the seawater had a significant impact on either the productivity or abundance of Trichodesmium along the transect. This suggests that other factors besides Fe, such as water column stability or higher rates of atmospheric dust deposition in the Caribbean, may be responsible for the observed increase in Trichodesmium abundance and N2 fixation at sites south of Bermuda. However, luxury Fe uptake may explain the lack of correlation with the Fe content of the colonies and the TDFe concentration may not necessarily reflect the bioavailable sources of Fe.; Iron uptake experiments were conducted using model ligands labeled with 55Fe to compare the bioavailability of inorganic Fe(III), porphyrin-bound Fe(III), and siderophore-bound Fe(III) to Trichodesmium colonies in the Sargasso Sea and cyanobacterial cultures of Trichodesmium and Synechococcus. The bioavailability of Fe was dependent on both the ligand type and ligand functional group. In general, inorganic Fe(III) and siderophore-bound Fe(III) were more bioavailable to Trichodesmium colonies in the field compared to porphyrin-bound Fe(III). In contrast, the porphyrin-bound Fe(III) was equally or more available than inorganic Fe(III) for the cultures. Dark incubations of Trichodesmium colonies generally resulted in a lower Fe uptake relative to concurrent lighted incubations. Finally, Fe-limited cultures had a higher Fe uptake compared to the corresponding Fe-replete cultures. Overall, it appears that the chemical form of chelated Fe may play an important role in regulating the bioavailability of Fe to cyanobacteria.