Strategies of phytoplankton to perform effective photosynthesis in the North Water

Abstract

Abstract During the 1999 expedition of the International North Water Polynya Study in northern Baffin Bay, the photosynthetic characteristics of phytoplankton were measured using pulse-amplitude-modulated (PAM) fluorescence methods. We focused mainly on the functioning of the diadinoxanthin cycle, which was expected to be an effective protection system of photosystems from high irradiance under low temperature. In spite of the low temperature (ca. 0°C), the diadinoxanthin cycle was very active, as monitored by the change of non-photochemical quenching of fluorescence (NPQ). The extent of NPQ became larger when phytoplankton were exposed to higher irradiance, and seemed to be in good correlation with the total amount of diadinoxanthin plus diatoxanthin, which are responsible for the diadinoxanthin cycle. The NPQ was more active in the surface phytoplankton community than in the phytoplankton living at the bottom of the euphotic zone, which had less xanthophyll pigments. These results indicate that the diadinoxanthin cycle actively drains out excess light energy as non-radiative heat and protects photosystems under high irradiance. The active NPQ reduced the effective irradiance; the effective quantum yield of photosystem II became smaller under higher irradiance. Reflecting these features, the resulting rate of photosynthesis in surface phytoplankton tended to be smaller than that of phytoplankton at the bottom of the euphotic zone. The irradiance at the onset of light saturation (Ik) was generally higher than the reported values for polar phytoplankton and ice algae, probably because of the presence of a water–water cycle and/or other metabolic pathways. Our work also shows the usefulness of PAM measurements for ecological and biochemical studies of psychrophilic phytoplankton in polar regions.