Taxon-specific differences in photoacclimation to fluctuating irradiance in an Antarctic diatom and a green flagellate

Photoacclimation towards rapid changes in irradiance was studied in 2 Antarctic microalgae, the diatom Chaetoceros brevis (Bacillariophyceae) and the flagellate Pyramimonas sp. (Prasinophyceae). Both species were subjected to 3 different light regimes. Two regimes of vertical mixing (1:1 h and 3:3 h high light:low light) were superimposed on a diurnal light cycle mimicking the solar sine. For both species, maximum growth was recorded at the fastest rates of light fluctuation (1: 1 h). The light regimes had a minor effect on growth rates of the flagellate but growth in the diatom was reduced significantly at moderate rates of mixing (3:3 h). While xanthophyll cycling was active in both species, fluorescence analysis with a dual-modulated fluorometer showed that down-regulation of photosynthetic efficiency was more pronounced in Pyramimonas sp. (ca. 50%) than in C. brevis (ca. 30%). The ratio of photoprotectors to chlorophyll a was not significantly affected by the light regime, neither in the flagellate nor in the diatom. In the flagellate, state transition offered an additional means of energy dissipation during the periods of high irradiance. The diatom apparently acclimated to the average light intensity received and not to the maximum irradiance. It can be reasoned that this strategy led to photodamage during the 3:3 h cycling. It is argued that enhanced maintenance costs associated with necessary repair processes can explain the reduced growth rate that was observed. The ecological consequences for large bloom-forming diatom species are briefly discussed.