Ultraviolet sunscreens in Gymnodinium sanguineum (Dinophyceae): Mycosporine-like amino acids protect against inhibition of photosynthesis

Marine phytoplankton are sensitive to inhibition of photosynthesis by solar ultraviolet (UV) radiation, although sensitivity varies, depending on the growth environment. A mechanism suggested to increase resistance to UV inhi bition is the accumulation of UV-absorbing compounds, such as the mycospori ne-like amino acids (MAAs) found in many marine organisms. However, the eff ectiveness of these compounds as direct optical screens in microorganisms h as remained unclear. The red-tide dinoflagellate Gymnodinium sanguineum Hir asaka accumulates about 14-fold more MAAs (per unit of chlorophyll) in high (76 W.m(-2)) than in low (15 W.m(-2)) growth irradiance. Biological weight ing functions were estimated for UV inhibition of photosynthesis and showed that the high-light-grown cultures have lower sensitivity to UV radiation at wavelengths strongly absorbed by the MAAs. The time course of photosynth esis during exposure to UV radiation was measured using pulsed amplitude mo dulated (PAM) fluorometry and displayed a steady-state level after 15 min o f exposure, indicating active repair of damage to the photosynthetic appara tus. Repair was blocked in the presence of the antibiotic streptomycin, yet high-light G. sanguineum remained less sensitive to UV radiation than did low-light cultures. These experiments show that MAAs act as spectrally spec ific UV/sunscreens in phytoplankton.