Effect of hydroxamate and catecholate siderophores on iron availability in the diatom Skeletonema costatum: Implications of siderophore degradation by associated bacteria
The bioavailability of iron (Fe) across marine ecosystems, mainly determined by Fe speciation and species-specific requirements of phytoplankton, remains largely unresolved. Siderophores are relevant within the pool of organic ligands that control organic Fe speciation. The effect on growth and physiology of the diatom Skeletonema costatum following addition over time of the uncomplexed siderophores (apo-form) desferrioxamine B and enterobactin were studied in the laboratory. The diatom was grown in batch culture in concentration gradients up to 50 and 10,000 nM for enterobactin and desferrioxamine B respectively. The potential effect of siderophore degradation was analyzed by electrospray ionization mass spectroscopy (HPLC-ESI-MS). Growth of S. costatum was negatively correlated to desferrioxamine concentration. In treatments where >500 nM was added, growth was negligible until day 9 after which significant growth started. Fe uptake at day 9 was highest at 10,000 nM, while the Fe quota was the lowest. The addition of enterobactin had a negative effect on the abundance, the in-vivo fluorescence and the Fe quota in S. costatum only at the highest concentration of 50 nM, while the in-vivo fluorescence was enhanced at the lowest concentration. The bacterial abundance over time was also negatively correlated to the concentration for both siderophores, but at day 9 the bacterial uptake showed an increase proportional to the siderophore concentration. HPLC-ESI-MS analysis revealed the presence of tentative metabolites of desferrioxamine in 500 and 10,000 nM indicating changes in concentration of the apo-siderophore. In the presence of cathecolate and hydroxamate siderophores, S. costatum exhibited the capacity for different Fe uptake strategies. The late growth exhibited and the high Fe uptake after prolonged Fe-limited growth, suggests that Fe reduction at cell's membrane may be facilitated by possible degradation of desferrioxamine by the associated bacteria. The results emphasize the need for studying Fe bioavailability of algae together with the interacting bacterial community
