Different strategies of photoacclimation by two strains of Emiliania huxleyi (Haptophyta)¹

Abstract

Photoacclimation involves the modification of components of the light and dark reactions to optimize photosynthesis following changes in available light. All of the energy required for photosynthesis comes from linear electron transport through PSII and PSI and is dependent upon the amount of light harvested by PSII relative to PSI (a* PSII and a* PSI ). The amount of light harvested is determined by the effective absorption cross-sections (r PSII , r PSI ) and cellular contents of the PSII and PSI reaction center complexes (RCII, RCI). Here, we examine the effective absorption cross-sections and reaction center contents for calcifying (B11) and noncalcifying (B92) strains of the globally important coccolithophorid Emiliania huxleyi (Lohmann) W. H. Hay et H. Mohler when grown under various photon flux densities (PFDs). The two strains displayed different ''strategies'' of acclimation. As growth PFD increased, B11 preferentially changed r and the cellular content of chl a per cell over PSU ''size'' (the total cellular chl a content associated with the reaction center complexes); strain B92 preferentially changed PSU size over the cellular content of reaction complexes. Neither strategy was specifically consistent with the majority of previous studies from other microalgal species. For both strains, cellular light absorption for PSII and PSI was maintained close to unity across the range of growth PFDs since changes of r PSII and r PSI were reciprocated by those of RCIIs and RCIs per cell. Our results demonstrate a significant adaptive flexibility of E. huxleyi to photoacclimate. Finally, we calculated the amount of chl a associated with either photosystem to consider our interpretations of photoacclimation based on conventional determinations of PSU size.

Responses ofEmiliania huxleyi(Prymnesiophyceae) to step changes in photon flux density

“…This research complements earlier studies of the steady-state acclimation of the photosynthetic apparatus of E. huxleyi grown over a range of PFDs (Nielsen, 1997;Harris et al, 2005;Suggett et al 2007) by documenting the rate of response of E. huxleyi to reciprocal shifts between low and high PFD. We provide observations of growth rate, biomass (C, N, protein, RNA), pigment content, light absorption, photosynthetic electron transfer rates based on pulse amplitude modulated fluorescence, rbcL gene expression and ribulose bisphosphate carboxylase-oxygenase (RuBisCO) protein abundance.…”

Light histories influence the impacts of solar ultraviolet radiation on photosynthesis and growth in a marine diatom, Skeletonema costatum

“…6), which play an important role in photoprotection. Adjustment of photosystem (PSI and PSII) size might also have contributed to the acclimation [ [28][29][30]. Accumulation of UV-absorbing compounds, such as mycosporine-like amino acids, is known to be protective against UVR.…”

Photoacclimation, growth and distribution of massive coral species in clear and turbid waters

“…The proportion of absorbed photon energy used to drive photochemistry (photochemical quenching, qP) or converted to heat (non-photochemical quenching, qN or NPQ) can thus be calculated from these RLCs. However, even on a relative basis, RLCs can prove difficult to interpret since, under different environmental conditions or between coral species, the amount of photosynthetically useable radiation (PUR) available to the organism may differ (MacIntyre et al 2002, Suggett et al 2007), but will not be not accounted for by RLCs. In order to account for PUR, absorbance of the coral and Symbiodinium would have to be quantified (Kirk 1994, Enriquez et al 2005, Stambler & Dubinsky 2005.…”

“…Under ambient light, fluorescence indicates the capacity of cells to utilise absorbed excitation energy (Falkowski & Raven 1997, Ralph & Gademann 2005. Both approaches are therefore particularly important depending on the time scale of interest, as the light environment on a reef changes over the long term (wet season: OctoberMarch; dry season: April-September) and the short term (diurnal variation).Conventionally, the photosynthesis light-response can be characterised from changes of O 2 evolution (Falkowski & Raven 1997) or fluorescence (Suggett et al 2003, Suggett et al 2007) following a gradient of increasing actinic light intensity. Photosynthesis is said to have reached steady state once constant with light intensity (over a time scale of minutes) (Suggett et al 2003, Ralph & Gademann 2005.…”