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 intensi ty (<1-3% of incident) and spectral distribution (blue-green) during t he austral spring and summer. This relative constancy is due to contin uous sunlight, optical filtering through the 4.2 m ice cap and an abse nce of vertical mixing. The effects of this unique light environment o n the structure and function of the photosynthetic apparatus were stud ied using measurements of P-700 reaction center content and spectral v ariation 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 compariso n to previous studies of phytoplankton growing in low-light cultures o r algae growing beneath Arctic sea ice. A sample from the deep (17 m) layer dominated by Chlamydomonas subcaudata was grown in enriched cult ure media. PSII fluorescence kinetics were measured on thylakoid prepa rations 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. rei nhardtii (strain CC-124). These results, together with pigment analyse s, suggest that carotenoids in Lake Bonney phytoplankton are more impo rtant in light harvesting as opposed to photoprotection.