Siderophores in marine coastal waters and their relevance for iron uptake by phytoplankton: experiments with the diatomPhaeodactylum tricornutum

Natural marine bacteria populations collected from nearshore waters produce different types of siderophores depending on the degree of iron limitation. These siderophores can facilitate iron uptake in the marine diatomPhaeodactylum tricornutum. Water samples from 15 stations along the Italian coast of the northwest Adriatic Sea were collected and filter fractionated (3.0, 0.8 and 0.2 μm). Siderophore production in the fractions was determined using cross-feeding experiments with siderophore-auxotrophic bacteria. At most stations sampled, bacteria collected in the 3.0 and 0.8 μm filters produced siderophores which stimulated growth inMorganella morganii, the indicator strain for α-keto/α-hydroxy acids. The results suggest that MGF (‘Morganella-Growth Factor’) production is common among filamentous and appendaged bacteria or strains associated with particles. Natural bacteria populations grown in iron-deficient media stimulated growth of all the indicator strains in the cross-feeding tests. Examples of known MGF which supply iron toM. morganiiwere tested for their ability to act as iron source for the marine diatomP. tricornutum. Iron uptake from55Fe-MGFs was measured inP. tricornutumcells grown in Fe-sufficient and Fe-deficient media. Unchelated iron (55FeCl₃) and55FeEDTA were used as controls. The uptake of iron from the55Fe-MGF and55FeCl₃ by Fe-deficient cells was higher (109 to 150 pgFe mg–1) than from55FeEDTA (34 pgFe mg–1). Similarly, Fe-sufficientP. tricornutumtook up iron from the55Fe-MGF and55FeCl₃ to the same extent (~50 pgFe mg–1) while minimal uptake (8 pgFe mg–1) was measured from FeEDTA. In growth experiments where iron-deficient diatom cells were incubated in media containing different sources of iron, e.g. FeCl₃, Fe-MGF and FeEDTA, a greater increase in number was observed in cells supplied with Fe-MGF. Further experiments also show that the uptake of Fe from MGF was enhanced by light and that a reduction step was involved in the uptake process. MGF also promoted the uptake of colloidal ferrihydrites. This study gives further evidence that siderophores produced by bacteria can be utilized by phytoplankton as an iron source. We therefore suggest that these substances play an important role in increasing the availability of iron to phytoplankton in coastal waters and thus are major factors defining the chemistry of iron in the marine environment.