EFFECTS OF BROMIDE AND IODIDE ON STALK SECRETION IN THE BIOFOULING DIATOM ACHNANTHES LONGIPES (BACILLARIOPHYCEAE)

Extracellular polymeric substance (EPS) secretion was examined in the stalked marine diatom Achnanthes longipes Ag. in defined medium. This common biofouling diatom exhibited an absolute requirement for bromide for stalk production and substratum attachment, whereas elevated iodi de concentrations in the growth medium inhibited stalk formation and a dhesion. Varying EPS morphologies resulted from altering bromide and i odide levels: pads, stalk-pads, stalks, and no EPS. Cells showed no di fferences in growth with bromide or iodide concentrations, indicating that they were not physiologically stressed under conditions that impa ired EPS secretion. Cells grown in elevated iodide secreted significan tly more soluble extracellular carbohydrate into the medium, suggestin g that the EPS was soluble and unable to be polymerized into a morphol ogically distinct gel. By replacing sulfate with methionine, the diato m lost its ability to form stalks even in the presence of bromide indi cating that free sulphate may be required for proper cross-linking of stalk polymers. Lotus-FITC, a fluorescent-tagged lectin, preferentiall y labeled the EPS and, thus, was used to visualize and quantify EPS se cretion along a bromide gradient in conjunction with an image analysis system. This technique demonstrated a direct correlation between the amount of bromide present in the medium and the specific EPS morpholog y formed.