Photoprotective acclimation of micro- and nano-size phytoplankton assemblages in the Indian sector of the Southern Ocean

Phytoplankton in the mixed layer is exposed to increasing levels of light when transported to the surface layer of the ocean. The photoprotective response of natural assemblages of phytoplankton can differ among community structures. We investigated photoprotective acclimation and xanthophyll cycle pigments in size-fractionated natural phytoplankton assemblages during the austral summer in the Indian sector of the Southern Ocean. We estimated concentrations of phytoplankton pigments in the micro-size fractions (>20 μm) and nano-size fractions (2–20 μm) by subtracting concentrations in the <20 μm fractions from concentrations in the bulk samples, and by subtracting concentrations in the <2 μm fractions from concentrations in the <20 μm fractions, respectively. Changes in the ratios of the xanthophyll cycle pigments diadinoxanthin (DD) and diatoxanthin (DT) were determined at three optical depths in the mixed layer and during 48 h deck incubations under solar photosynthetically available radiation and ultraviolet radiation. Large variations in (DD + DT)/Chl a in the mixed layer (percent coefficient of variation >67 %) and in deck incubation bottles under variable light conditions (>75 % of the temporal variation) for the micro-size fractions suggest a higher potential for photoprotective acclimation than for the nano-size fractions. Decreases in DT/(DD + DT) with increases in the optical depth of the mixed layer (ζ _MLD) suggest that larger variations in light availability in the mixed layer might predict lower values of DT/(DD + DT) at the surface, regardless of cell size.