Extracellular Release of Organic Matter from Two Freshwater Diatoms

The 14C-labelled extracellular materials released from cultures of Synedra acus and Stephanodiscus hantzschii were separated by paper chromatography. The major components were identified and their rates of release measured. Stephanodiscus in axenic culture released the same extracellular materials under optimum light conditions as did the natural population from which the diatom was isolated. The major extracellular product of Stephanodiscus was glycollate; polysaccharide and amino acids were also released. The major extracellular product of Synedra was polysaccharide; amino acids were also released, but no glycollate. Under light intensities so high as to inhibit photosynthesis the arrays of extracellular substances were unchanged, but some rates of release were reduced. The array of products and the rates of release from Stephanodiscus in darkness are quite different from extracellular release during photosynthesis. Glycollate release from Stephanodiscus is greatest when the cultures are young and growing rapidly, older cultures release less.