PHOTOPHYSIOLOGY AND PHOTOACCLIMATION IN SURFACE SEA-ICE ALGAE FROM MCMURDO SOUND, ANTARCTICA

Microalgal absorption, pigment concentrations, photophysiology and the efficiency for energy conversion at photosystem II (Fv/Fm) were measu red for surface ice algal communities freshly collected from saline po nds overlying fast ice in McMurdo Sound, Antarctica, during austral sp ring and summer 1989-90. These parameters also were measured for surfa ce ice algae exposed in the laboratory to irradiances from 4 to 600 mu mol photons m(-2) s(-1). Freshly collected algae exhibited a pigment composition consistent with acclimation to high irradiance, which incl uded low intracellular chlorophyll (chl) a concentrations (0.19 to 0.5 0 kg m(-3)), low accessory photosynthetic pigments relative to chl a ( chl c: chl a = 0.16 to 0.25 mol mol(-1); fucoxanthin: chl a = 0.53 to 0.77 mol mol(-1)), and high photoprotective pigments relative to chl a (diatoxanthin + diadinoxanthin: chl a = 0.19 to 0.50 mol mol(-1)). In contrast, the photoadaptive index for freshly collected algae (E(k) = 37 to 45 mu mol photons m(-2) s(-1)) was less than the daily average photosynthetically active radiation reaching the algal communities dur ing the study (110 to 720 mu mol photons m(-2) s(-1)), indicating that the algae were not acclimated to their high irradiance environment. N o depression of photosynthesis was observed in the photosynthesis-irra diance curve at irradiances less than or equal to 250 mu mol photons m (-2) s(-1) (4- to 8-fold greater than E(k)) However, Fv/Fm (0.24 to 0. 43) and the quantum yield of photosynthesis (phi(C), 0.018 to 0.037 mo l C mol(-1) absorbed photons) were low in freshly collected algae, whi ch suggests that the algae were photoinhibited under natural illuminat ion conditions. Within 32 h after shifting algae to low irradiance, a relaxation from high-light stress was observed. Photosynthetic efficie ncy (alpha(b)), phi(C) and Fv/Fm increased by 165, 170 and 67%, respec tively, and E(k) decreased by 60%. In addition, whereas total cellular concentrations of photosynthetic pigments were unchanged, diatoxanthi n:chl a decreased by >75% due to the conversion of diatoxanthin to dia dinoxanthin. The presence of xanthophyll cycling and an observed depre ssion of relative maximum and minimum quantum yields of fluorescence i n response to high irradiance indicate that algae employed the dissipa tion of absorbed energy from the pigment bed of photosystem II as a pr otection mechanism from high irradiance. Indications of additional pho toprotection mechanisms and photoinhibitory damage were also observed. These results indicate that surface ice algae successfully inhabit th e surface ice habitat by employing a strategy of low-light harvesting, absorbed energy dissipation, and tolerance to photoinhibitory damage.