FERRIOXAMINE-B AND FERRIOXAMINE-E AS IRON SOURCES FOR THE MARINE DIATOM PHAEODACTYLUM-TRICORNUTUM

The utilization of iron from the trihydroxamate-type siderophores ferrioxamine B (FOE) and ferrioxamine E (FOE) by the marine diatom Phaeodactylum tricornutum was studied. Cells grown under iron-limiting conditions took up iron faster from FOE and FOE than from Fe-EDTA and FeCl3. Minimal iron uptake from the siderophores was observed for cells grown in media with sufficient iron. The uptake of iron from FOE was inhibited by the ferrous ion chelator, BPDS, evidence that a reductive pathway is involved in the uptake mechanism. On the other hand, iron uptake from FOE was not affected by BPDS, indicating that FOE may be taken up as an intact complex by the cell. Uptake kinetics show that FOE is transported faster into the tell than the iron from FOE. Growth experiments, however, indicated that FOE is more efficient in supplying utilizable iron to the cell. The uptake mechanisms exhibited typical saturation kinetics, corresponding to the saturation of the transport ligand for FOE and of the reductive system for FOE. FOE transport yielded a lower K-m (half-saturation concentration) compared to FOE, indicating a more efficient 'true-siderophore' transport system. The results of our experiments suggest the existence of multiple pathway transport mechanisms for iron uptake from exogenous siderophores in P. tricornutum. The utilization of microbial siderophores by phytoplankton presents new aspects in bacteria-phytoplankton interaction.