Photophysiology and photoacclimation in surface sea ice algae from McMurdo Sound, Antarctica

Abstract

Microalgal absorption, pigment concentrations, photophysiology and the efficiency for energy conversion at photosystem II (Fv/Fm) were measured for surface ice algal communities freshly collected from saline ponds overlying fast ice in McMurdo Sound, Antarctica, during austral spring and summer 1989-90. These parameters also were measured for surface ice algae exposed in the laboratory to irradiances from 4 to 600 μmol photons m-2 s-1. Freshly collected algae exhibited a pigment composition consistent with acclimation to high irradiance, which included low intracellular chlorophyll (chl) a concentrations (0.19 to 0.50 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 μmol photons m-2 s-1) was less than the daily average photosynthetically active radiation reaching the algal communities during the study (110 to 720 μmol photons m-2 s-1), indicating that the algae were not acclimated to their high irradiance environment. No depression of photosynthesis was observed in the photosynthesis-irradiance curve at irradiances ≤250 μmol 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 (φ(C), 0.018 to 0.037 mol C mol-1 absorbed photons) were low in freshly collected algae, which suggests that the algae were photoinhibited under natural illumination conditions. Within 32 h after shifting algae to low irradiance, a relaxation from high-light stress was observed. Photosynthetic efficiency (αb), φ(C) and Fv/Fm increased by 165, 170 and 67%, respectively, and E(k) decreased by 60%. In addition, whereas total cellular concentrations of photosynthetic pigments were unchanged, diatoxanthin: chl a decreased by >75% due to the conversion of diatoxanthin to diadinoxanthin. The presence of xanthophyll cycling and an observed depression of relative maximum and minimum quantum yields of fluorescence in response to high irradiance indicate that algae employed the dissipation of absorbed energy from the pigment bed of photosystem II as a protection mechanism from high irradiance. Indications of additional photoprotection mechanisms and photoinhibitory damage were also observed. These results indicate that surface ice algae successfully inhabit the surface ice habitat by employing a strategy of low-light harvesting, absorbed energy dissipation, and tolerance to photoinhibitory damage.