RELEASE OF ULTRAVIOLET-ABSORBING COMPOUNDS BY THE RED-TIDE DINOFLAGELLATE-LINGULODINIUM POLYEDRA

We tested the hypothesis that ultraviolet-absorbing compounds known as mycosporine-like amino acids (MAAs) are not only synthesized but also excreted by marine phytoplankton. An experiment was performed with cu ltures of the marine dinoflagellate Lingulodinium polyedra (previously known as Gonyaulax polyedra) exposed to visible (photosynthetically a vailable, PAR, 400 to 700 nm) and ultraviolet (UV, 290 to 400 nm) radi ation. Absorption properties of both particulate and dissolved organic matter pools (POM and DOM, respectively) showed maxima in ultraviolet absorption at 360 nm. Chromatographic analysis confirmed the presence of MAAs in both pools. Release of organic matter by L. polyedra, as m easured spectrophotometrically by changes in UV absorption in the surr ounding medium, showed a differential increase at 360 nm in cultures e xposed to UV-B + PAR radiation. The changes in absorption in the DOM f raction were inversely proportional to intracellular UV absorption. Ph otodegradation experiments in which the DOM fraction was exposed to vi sible and UV-B radiation showed a decrease in absorption with dose. Fi rst-order photooxidation decay rates varied between -0.005 and -0.26 m (2) (mol quanta)(-1) and were also a function of the initial optical d ensity (OD). These results indicate that UV-absorbing compounds synthe sized by phytoplankton, such as certain dinoflagellates, may be a comp onent of the DOM pool in surface waters of the ocean and contribute to the attenuation of UV radiation in the water column. Photooxidation c onsumes only 3 to 10% of the daily production of the DOM absorbing bet ween 280 and 390 nm (including MAAs). This suggests that MAAs dissolve d in seawater may contribute to the decrease of UV transmission throug h the water column on a time scale representative of phytoplankton gro wth (days) and bloom development (weeks).