Photoinduction of UV-absorbing compounds in Antarctic diatoms and Phaeocystis antarctica

Abstract

The presence of UV-absorbing compounds, thelr In vivo absorption and photoinducibility were investigated in the colonial form of the prymnesiophyte Phaeocystis antarctica and 11 species of Antarctic diatoms. High in vivo phytoplankton absorption peaks centered at 315-320 nm wavelengths indicated the presence of UV-absorbing compounds in all cultures of P. antarctica examined. UVabsorbing compounds, detected as in vivo absorption peaks at slightly longer wavelengths (330-333 nm), were present in most diatom species, but were absent or present in minute amounts in smaller species. UV-absorbing compounds were identified in the diatoms as the mycosporine-like amino acids (MAAs) porphyra-334, mycosporine-glycine, and shinorine but could not be positively identified in P. antarctica. Accumulation of MAAs was induced by llght in the blue/UV portion of the spectrum (305-460 nm). For diatoms, induction was most effective at wavelengths between 370 and 460 nm with very little response observed at ultraviolet-B (UVB, 280-320 nm) wavelengths. In contrast, induction was maximal at 340 nrn and significant at wavelengths down to 305 nrn in colonies of P. antarctica. Induction requlred continuous illurn~nation and exhibited an Initial lag phase (10 to 24 h in length), an exponential phase (60 to 120 h in length), and a steady-state phase. The cellular content of MAAs was stable in viable cells kept for over 2 mo In prolonged darkness and became d~luted through successive cell divisions in cells grown in non-inducing ~llumination (red and yellow light). The potential sunscreening effect (S) of MAAs, calculated as the fractional reduction of UV exposure to a centrally-located cellular target, ranged from 0.03 to 0.50 for diatom cells and from 0.65 and 0.72 for P, antarctica colonies. S exhibited an increasing trend with cell and colony size. Whereas MAAs are thought to be intracellularly located, the large values of S determined for P. antarctica suggests that UV-absorbing compounds associated with the colonies are located within the extracellular colonial matrix. The results indicate that most phytoplankton species examined have the potential to respond to an increase in UVR resulting from seasonal changes in solar zenith angle by increasing their MAA content. A similar response to elevated UVB levels resulting from depleted ozone concentrations may be limited to P. antarctica.