Excretion of Some Organic Compounds by Marine Phytoplankton

The excretion of photoassimilated carbon was determined for 22 species of unicellular marine algae in culture during periods of log-phase growth and for some natural marine phytoplankton populations from Vineyard Sound and the Gulf of Maine. Carbon 14 tracers gave some information about the composition of the excreted material. Most of the algae excreted 3-6s of their photoassimilated carbon during logarithmic growth. A few species excreted as much as 10-25s under the same conditions. Algae subjected to two different light intensities, 3,000 lux and 25,000 Iux, varied less than 30% in their relative rates of excretion. However, algae exposed to direct sunlight (lOO,OOO120,000 lux) had very high excretion rates, possibly resulting from damage to the cells by photooxidation. Glycolic acid formed 938% of the total carbon excretion in Olisthodiscus sp., Chaetoceros pelagicus, Chlorococcum sp. and Skeletonema costatum. Lower relative amounts were excreted by most of the species studied. Skeletonema costatum excreted considerably more glycolic acid at light intensities below 15,000 lux than above. This is contrary to the findings of other investigators using the freshwater alga Chlorella pyrenoidosa. The amount of carbon excreted as protein ranged from 0.2 to 5.9%, and that excreted as chloroform-soluble material ranged from 2.8 to 10.3% of the total. After electrodialysis, 12-340/O of the compounds were recovered in the anion fraction. Considerable quantities of amino acids and peptides were also detected as excretory products of many of the algae. A few species excreted a single substance almost exclusively. Thus, Chlorella sp. excreted mainly proline, Dunaliellu tertiolectu mainly glycerol, and Olisthodiscus sp. mainly mannitol. Apparently healthy populations of natural phytoplankton excreted 4-16s of their photoassimilated carbon. However, 17-38s was excreted by a sample taken at the end of a diatom bloom, when a large number of empty frustules was present.