The utihzalon of iron from the trihydroxamate-type siderophores fernoxamine B (FOB) and fernoxamine E (FOE) by the manne diatom Phaeodactylum trlcornutum was studied Cells grown under iron-hmiting conditions took u p iron faster from FOE and FOB than from Fe-EDTA and FeCI, Minimal iron uptake from the siderophores was observed for cells grown in media with sufficient iron The uptake of iron from FOB 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 cell than the iron from FOB Growth expenments, however indicated that FOB 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 FOB FOE transport yielded a lower K, (half-saturation concentratlon) compared to FOB indicating a more efficient true-siderophore transport system The results of our expenments suggest the existence of mult~ple pathway transport mechanisms for iron uptake from exogenous siderophores in P trlcornutum The utilization of microbial siderophores by phytoplankton presents new aspects in bacteria-phytoplankton interaction

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 diatom Phaeodactylum 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 in Morganella 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 to M. morganii were tested for their ability to act as iron source for the marine diatom P. tricornutum.

The dinoflagellate Peridinium cf. quinquecorne Abé forms red tide‐like blooms in eutrophic shallow waters in the Philippines. The organism moves into a distinct near‐surface layer when intensive solar radiation occurs, but only during the incoming tide. Shortly before high tide, regardless of light levels, the dinoflagellates seem to disappear. Simple experiments show that once intensive radiation has been reduced Peridinium quinquecorne moves out of the water column and attaches itself to solid objects away from the light. The morphology of the organism, especially as related to attachment, was studied through SEM. Its high swimming velocity and the reaction to radiation and tidal changes are described. The possibility that, superimposed on its reaction to light, this dinoflagellate may follow intrinsic tide‐dependent oscillations is discussed.

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