Pigment composition and photoacclimation as keys to the ecological success of <scp>G</scp> onyostomum semen ( <scp>R</scp> aphidophyceae, <scp>S</scp> tramenopiles)

Aquatic habitats are usually structured by light attenuation with depth resulting in different microalgal communities, each one adapted to a certain light regime by their specific pigment composition. Several taxa contain pigments restricted to one phylogenetic group, making them useful as marker pigments in phytoplankton community studies. The nuisance and invasive freshwater microalga Gonyostomum semen (Raphidophyceae) is mainly found in brown water lakes with sharp vertical gradients in light intensity and color. However, its pigment composition and potential photoadaptations have not been comprehensively studied. We analyzed the photopigment composition of 12 genetically different strains of G. semen by high performance liquid chromatography after acclimation to different light conditions. We confirmed the pigments chl a, chl c1c2, diadinoxanthin, trans-neoxanthin, cis-neoxanthin, α and β carotene, which have already been reported for G. semen. In addition, we identified, for the first time, the pigments violaxan-thin, zeaxanthin, and alloxanthin in this species. Alloxanthin has never been observed in raphidophytes before, suggesting differences in evolutionary plastid acquisition between freshwater lineages and the well-described marine species. The amount of total chl a per cell generally decreased with increasing light intensity. In contrast, the increasing ratios of the prominent pigments diadinoxanthin and alloxanthin per chl a with light intensity suggest photoprotective functions. In addition, we found significant variation in cell-specific pigment concentration among strains, grouped by lake of origin, which might correspond to genetic differences between strains and populations.