Photoacclimation of open Baltic algal associations

Abstract

The vertical distribution of photosynthetic efficiency of phytoplankton associations over the course of a high light day was investigated on two cruises into the Baltic proper in July 1998 and 1999. Measurements were made of chlorophyll a concentration, maximal photosynthetically active radiation, maximal photosynthetic efficiency (F v /F m ), effective photosystem II quantum yield and relative electron transport rate under simulated ambient irradiation using a Xenon pulse amplitude modulation fluorometer. Over the day, the potential photosynthetic performance changed mainly in the surface mixed layer (0–5 m). Despite high surface solar irradiation, even in the morning, maximal photosynthetic efficiency was kept stable over most of the day by physiological compensation mechanisms until indications of photoinhibition became evident in the afternoon. The algal composition in the drifting water body revealed a shift in the phytoplankton community through the middle part of the day. Phytoplankton composition at the surface shifted to a higher percentage of Bacillariophyceae towards noon, and towards a higher percentage of cyanobacteria from noon to early afternoon, a finding that has implications for the interpretation of the fluorescence signal. To study photoprotection at the pigment level, algae from the surface were illuminated for up to several hours with artificial light of ambient noon intensity and the changes in pigment content were monitored. As only minor amounts of violaxanthin were detectable, the observed high level of zeaxanthin must be attributed to cyanobacteria, which lack a xanthophyll cycle. After two hours of high irradiance, the ratio of zeaxanthin to chlorophyll a did not change, whereas over one hour the ratio of diadinoxanthin to diatoxanthin decreased, indicating the protective function of this type of xanthophyll cycle in the diatoms present in the samples. However, after a prolonged exposure to high irradiance of two hours, the pigment pool of diadinoxanthin and diatoxanthin decreased considerably and was lost after three hours of high irradiance treatment, whereas zeaxanthin remained stable.