THE PHOTOSYNTHETIC APPARATUS OF PHYTOPLANKTON FROM A PERENNIALLY ICE-COVERED ANTARCTIC LAKE - ACCLIMATION TO AN EXTREME SHADE ENVIRONMENT

Phytoplankton in perennially ice-covered Lake Bonney (Antarctica) are exposed to a limited range of light variation both in terms of intensity (<1-3% of incident) and spectral distribution (blue-green) during the austral spring and summer. This relative constancy is due to continuous sunlight, optical filtering through the 4.2 m ice cap and an absence of vertical mixing. The effects of this unique light environment on the structure and function of the photosynthetic apparatus were studied using measurements of P-700 reaction center content and spectral variation in photosystem II (PSII) fluorescence kinetics. Light-induced absorbance change at both 700 nm and 810 nm was used to measure P-700 concentration. The average ratio of total Chl/P-700 was 743 (mel mol(-1)), with a range of 480 to 1,039. These ratios were low in comparison to previous studies of phytoplankton growing in low-light cultures or algae growing beneath Arctic sea ice. A sample from the deep (17 m) layer dominated by Chlamydomonas subcaudata was grown in enriched culture media. PSII fluorescence kinetics were measured on thylakoid preparations in the presence of DCMU under blue-green (481 nm) and red (660 nm) light. C. subcaudata utilized blue-green light for photosynthesis more efficiently than the photobiologically well characterized C. reinhardtii (strain CC-124). These results, together with pigment analyses, suggest that carotenoids in Lake Bonney phytoplankton are more important in light harvesting as opposed to photoprotection.