MEPS 294:131-140 (2005)  -  doi:10.3354/meps294131

ABSTRACT: The abundance and photosynthetic characteristics of non-geniculate coralline algae were determined at Cape Evans, McMurdo Sound (Ross Sea), Antarctica, during spring 2003. At the time, the site was covered by 2.5 m of sea-ice through which only 0.48% of incident irradiance was able to penetrate. Coralline algae covered 4 to 60% of the seafloor from water depths of 13 to 26 m. Coralline algal crusts showed light-saturated rates of photosynthesis from 9.4 to 20 mmol O₂ m^–2 thallus d^–1 and dark respiration rates from 0.15 to 0.96 mmol O₂ m^–2 thallus d^–1. The average irradiance at which the onset of light saturated photosynthesis occurred (E_k) was 3.2 µmol photons m^–2 s^–1, measured using oxygen micro-electrodes, and 2.9 µmol photons m^–2 s^–1, measured using pulse amplitude modulated (PAM) fluorometry techniques. Irradiance at depths of 15 to 20 m did not exceed 2 µmol photons m^–2 s^–1 during the study, but frequently exceeded the light compensation point for photosynthesis (E_c), estimated as 0.10 µmol photons m^–2 s^–1. The effective quantum yield (ΔF/F_m’) of Photosystem II, measured in situ over diel periods at depths of 16, 18 and 20 m, averaged 0.691 (SD = 0.003), 0.639 (SD = 0.005) and 0.632 (SD = 0.007), respectively. These values were indicative of minimal down-regulation of photosynthesis, and were consistent with the observation that irradiance never exceeded E_k. Laboratory investigations, showed that on transfer from ambient irradiance to 23.5 µmol photons m^–2 s^–1, a change of 2 orders of magnitude, the algae showed minimal photoinhibition and a small, but statistically significant increase in E_k and maximum electron transport rate. Coralline algae at Cape Evans are able to persist with net carbon accrual under ice, while being able to tolerate periods of higher irradiance during ice-free conditions. This contrasts to Phyllophora antarctica which is dependent on ice-free periods to achieve net production.

KEY WORDS: Photosynthesis · Oxygen electrodes · Macroalgae · Phyllophora antarctica · Photoacclimation · Mesophyllum engelhartii · Synarthrophyton patena