Abstract
Photoacclimation by strains of Haslea “blue” diatom species H. ostrearia and H. silbo sp. nov. ined. was investigated with rapid light curves and induction–recovery curves using fast repetition rate fluorescence. Cultures were grown to exponential phase under 50 µmol m−2 s−1 photosynthetic available radiation (PAR) and then exposed to non-sequential rapid light curves where, once electron transport rate (ETR) had reached saturation, light intensity was decreased and then further increased prior to returning to near growth light intensity. The non-sequential rapid light curve revealed that ETR was not proportional to the instantaneously applied light intensity, due to rapid photoacclimation. Changes in the effective absorption cross sections for open PSII reaction centres (σPSII′) or reaction centre connectivity (ρ) did not account for the observed increases in ETR under extended high light. σPSII′ in fact decreased as a function of a time-dependent induction of regulated excitation dissipation Y(NPQ), once cells were at or above a PAR coinciding with saturation of ETR. Instead, the observed increases in ETR under extended high light were explained by an increase in the rate of PSII reopening, i.e. QA− oxidation. This acceleration of electron transport was strictly light dependent and relaxed within seconds after a return to low light or darkness. The time-dependent nature of ETR upregulation and regulated NPQ induction was verified using induction–recovery curves. Our findings show a time-dependent induction of excitation dissipation, in parallel with very rapid photoacclimation of electron transport, which combine to make ETR independent of short-term changes in PAR. This supports a selective advantage for these diatoms when exposed to fluctuating light in their environment.
Highlights
Variability in the light environment is likely a key parameter dictating diatom species distribution (Lavaud et al 2007), and to our understanding of the ability of diatoms to acclimate to changes in their light environment
We investigated photoacclimation in the form of both non-photochemical downregulation used to dissipate excess excitation and inducible upregulation of electron transport rate (ETR) in two species of Haslea diatoms, H. ostrearia and H. silbo sp. nov. ined
Upregulation of the relative electron transport rate by diatoms has been reported within rapid light curve (RLC) measurements, where an increase in rETR occurred as a result of rapid photoacclimation to the immediate light history applied during the RLC itself (Perkins et al 2006, 2010; Lefebvre et al 2011)
Summary
Variability in the light environment is likely a key parameter dictating diatom species distribution (Lavaud et al 2007), and to our understanding of the ability of diatoms to acclimate to changes in their light environment. We investigated photoacclimation in the form of both non-photochemical downregulation used to dissipate excess excitation and inducible upregulation of electron transport rate (ETR) in two species of Haslea diatoms, H. ostrearia and H. silbo sp. Upregulation of the relative electron transport rate (rETR) by diatoms has been reported within rapid light curve (RLC) measurements, where an increase in rETR occurred as a result of rapid photoacclimation to the immediate light history applied during the RLC itself (Perkins et al 2006, 2010; Lefebvre et al 2011) The mechanisms underlying this rapid photoacclimation are not fully understood (Perkins et al 2010), in part because rETR is based solely on changes in the quantum yield of the pool of PSII, without considering possible changes in the effective absorption cross section for photochemistry, or changes downstream of electron transport
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