Photoacclimation in phytoplankton: implications for biomass estimates, pigment functionality and chemotaxonomy

Chl a and C-normalized pigment ratios were studied in two dinophytes (Prorocentrum minimum and Karlodinium micrum), three haptophytes (Chrysochromulina leadbeateri, Prymnesium parvum cf. patelliferum, Phaeocystis globosa), two prasinophytes (Pseudoscourfieldia marina, Bathycoccus prasinos) and the raphidophyte Heterosigma akashiwo, in low (LL, 35 μmol photons m⁻² s⁻¹) and high light (HL, 500 μmol photons m⁻² s⁻¹). Pigment ratios in LL and HL were compared against a general rule of photoacclimation: LL versus HL ratios ≥1 are typical for light-harvesting pigments (LHP) and <1 for photoprotective carotenoids. Peridinin, prasinoxanthin, gyroxanthin-diester and 19′-butanoyloxy-fucoxanthin were stable chemotaxonomic markers with less than 25% variation between LL versus HL Chl a–normalized ratios. As expected, Chls exhibited LL/HL to Chl a ratios >1 with some exceptions such as Chl c ₃ in P. globosa and MV Chl c ₃ in C. leadbeateri. LL/HL to Chl a ratios of photosynthetic carotenoids were close to 1, except Hex-fuco in P. globosa (four-fold higher Chl a ratio in HL vs LL). Although pigment ratios in P. globosa clearly responded to the light conditions the diadinoxanthin-diatoxanthin cycle remained almost unaltered at HL. Total averaged pigment and LHP to C ratios were significantly higher in LL versus HL, reflecting the photoacclimation status of the studied species. By contrast, the same Chl a-normalized ratios were weakly affected by the light intensity due to co-variation with Chl a. Based on our data, we suggest that the interpretation of PPC and LHP are highly dependent on biomass normalization (Chl a vs. C).