Photosynthesis of marine macroalgae in ice-covered and ice-free environments in East Antarctica

Abstract

Antarctic macroalgae survive extended periods of darkness followed by rapid and extreme increases in irradiance when sea-ice breaks out in summer. Algae from dark, ice-covered locations had low values of saturating irradiance (E k: 4-19 μmol photons m -2 s -1 ), low relative rates of electron transport (rETR max: 2.0-3.6 μmol electrons m -2 s -1 ) and high values of effective quantum yield (ΔF/: 0.52-0.70). Algae from brighter open-water locations had high E k (13-67), high rETR max (3.1-22.9) and lower ΔF/ (0.28-0.50). Overall values of ΔF/ and maximum quantum yield (F v /F m ) were generally higher for ice-covered algae. Photosynthetic pigment, C:N and δ 13 C varied little with irradiance. Diel photosynthetic changes were measured in situ using a custom-built, multi-channel fluorometer. Under sea-ice, ΔF/ of Iridaea mawsonii increased significantly from 0.54 at midday (∼4 μmol photons m -2 s -1 ) to 0.64 at midnight, despite the relatively low midday irradiance. ΔF/ of ice-free macroalgae also varied significantly over 24 h and between-sample variation exceeded that of under-ice algae. In vivo comparisons of ETR and oxygen evolution of I. mawsonii , Desmarestia menziesii and Himantothallus grandifolius suggested that over 50% of excitation energy was diverted from linear photosynthetic electron flow at E k ; quenching analysis indicated at least partial diversion to non-photochemical quenching (NPQ). During the increasing irradiance of a rapid light curve, values of photochemical quenching (qP) and NPQ of ice-covered algae changed more rapidly than those of open-water algae. Only ice-free H. grandifolius maintained high qP values when exposed to high actinic irradiances (a capacity apparently induced on sea-ice break out), while both ice-free and ice-covered I. mawsonii maintained high NPQ at low actinic irradiances (apparently a constitutive capacity). These attributes help explain the local distribution of macroalgae in the Antarctic, and show strong linkages with the rapid changes in irradiance during sea-ice break-out events. © 2006 British Phycological Society.