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 [CO 2 ]-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 199hexanoyloxyfucoxanthin (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.

Mechanisms of phytoplankton adaptation to environmental variability in a shelf ecosystem

“…Controlled culture experiments on the diatom Skeletonema costatum demonstrated that cellular chlorophyll a and fucoxanthin were elevated under low light but decreased at high irradiance as the levels of the photoprotective pigments diadinoxanthin and diatoxanthin increased (Anning et al, 2000). Similarly, the flagellate Emiliania huxleyi also demonstrated a decrease in cellular chlorophyll a and chlorophyll c in a shift from low to high light, while diadinoxanthin increased, but the content of 19 0 -hexanoylfucoxanthin did not differ significantly between the two light regimes (Leonardos, 2008). Natural populations adapt to local environmental conditions and in February 2010 the euphotic zone was generally deeper than the mixed layer, meaning that the diatom-flagellate communities were most likely subjected to slower mixing and had time to acclimate accordingly by increasing absorption by PSC and TChlc under decreasing and changing spectral irradiance through the euphotic zone.…”

Dimethyl sulfoniopropionate and dimethyl sulfide production in response to photoinhibition in Emiliania huxleyi

“…Cool-white fluorescent lamps were used to create PFDs of 30 mmol photons m 22 s 21 (low light, LL) and 300 mmol photons m 22 s 21 (high light, HL). Cells were maintained in balanced exponential growth at a target cell density of 8 3 10 5 6 30% cells mL 21 (Leonardos 2008). Bacterial abundance, quantified by flow cytometry of samples stained with SYBR-Green I, varied between 8 and 11 3 10 6 cells mL 21 in the experimental water, equivalent to a bacterial : algal cell ratio of approximately 10.…”

Using chlorophyll fluorescence to monitor yields of microalgal production

“…This shift can take several generations to reach steady state (Leonardos 2008); during that time, there will not be a good correlation between growth rate changes and pigment per unit culture volume or in vivo fluorescence (Wood et al 2005). In the present study, the change in light intensity was only a 30% reduction of the EXP 1 value, so cells might have responded to the new light regime without any significant changes in light-harvesting pigments.…”

“…Conventional methods for estimating biomass include measurements of cell density, dry weight of cells, and cellular organic carbon, which provide a good estimate of biomass as well as some indication of the physiological changes in cells. Cellular organic carbon, however, depends only on the final product of photosynthesis, and significant changes in cellular properties take at least 3 days to become apparent (Leonardos 2008). Alternative measurements are necessary to achieve prompt monitoring over shorter time scales such as daily monitoring.…”