UPTAKE SPECIFICITY FOR ORGANIC SUBSTRATES BY THE MARINE DIATOM MELOSIRA NUMMULOIDES(1)

Melosira nummuloides, clone Mel-3, shows a very high specificity with regard to its ability to take up organic substrates. Amino acids supplied in the medium at 1 X 10(-4) M are taken up at initial rates of the same order of magnitude as that of photoassirnilation of COj. However, sugars, sugar alcohols, or organic acids supplied at the same concentration are not taken up. The mechanism for uptake of amino acids appears to require energy, since tlie uptake of the amino acid analog α-aminoisobutyric acid is strongly inhibited by 2 f-dinitrophenul. The uptake mechanism does not appear to be inducible. The ability of M. numinuloides to utilize amino acids as a nitrogen source is quite restricted. Arginine, ghttamine, asparagine, proline, and glutamic acid were good nitrogen sources. Seventeen other amino acids, including α-aminoisobutyric acid, were unsatisfactory for growth, although they were rapidly taken up from the medium.