Physiological steady state of phytoplankton in the field? An example based on pigment profile of Emiliania huxleyi (Haptophyta) during a light shift

Abstract

A calcifying strain of Emiliania huxleyi was used to study the photoacclimation process during a shift from low (LL) to high (HL) photon flux density (PFD) under nutrient‐replete and pH‐ and [CO2]‐controlled continuous cultures. Physiological steady states were obtained after culturing the alga in each PFD for more than a month, and pigment profiles and cell volume changes were monitored for 25 d after the light shift. Fucoxanthin was the major carotenoid in LL, while under HL this role was assumed by 19'hexanoyloxyfucoxanthin (19Hex). The photoprotective pigments diadinoxanthin and diatoxanthin (Dd+Dt), normalized to chlorophyll a (Chl a), increased with increasing PFD, while Chl a content per cell and Chl c's and fucoxanthin, normalized to Chl a, decreased with increasing PFD. The sum of all carotenoids normalized to Chl a and the 19Hex+Fuco : Chl c ratio were remarkably constant from LL to HL conditions. The results confirm that the total amount of carotenoids was synthesized/catabolized in tandem with Chl a to a genetically predefined level independent of PFD. When normalized to a per cell basis, Chl a content reached the long‐term HL steady state after 17‐20 d, while Chl c, Fuco, and Dd+Dt, normalized to Chl a, reached the long‐term HL steady state after 5‐7 d. Growth rate adjustment was completed within 3 d after the transition to HL. When Chl a was normalized to cellular volume, the transition to a fully acclimated HL state was complete within 3 d. The results highlight the need for the critical evaluation of the normalization "currency" (i.e., cell number, volume, Chl a) to which mass is expressed during the photoacclimation process and suggest that natural phytoplankton populations are unlikely to ever be in true physiological steady state.