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

Summary

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 (Ek: 4–19 mmol photons m
2 s
1), low relative rates of electron transport (rETRmax: 2.0–3.6 mmol electrons m
2 s
1) and high values of effective quantum yield (
F/F0m: 0.52–0.70). Algae from brighter open-water locations had high Ek (13–67), high rETRmax (3.1–22.9) and lower
F/F0m (0.28–0.50). Overall values of
F/F0m and maximum quantum yield (Fv/Fm) were generally higher for ice-covered algae. Photosynthetic pigment, C:N and
13C varied little with irradiance. Diel photosynthetic changes were measured in situ using a custom-built, multi-channel fluorometer. Under sea-ice,
F/F0 m of Iridaea mawsonii increased significantly from 0.54 at midday (4 mmol photons m
2 s
1) to 0.64 at midnight, despite the relatively low midday irradiance.
F/F0m 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 Ek; 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.

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