1 School of Earth and Ocean and Sciences, Cardiff University, Cardiff, UK.
2 Mount Allison University, 101 Barclay, 63B York St., Sackville, NB, E4L 3G7, Canada.
3 FR CNRS 3473 IUML, Mer-Molécules-Santé (MMS), Université du Maine, Ave O. Messiaen, 72085 Le Mans cedex 9, France.
Abstract
Diatoms are well known for their highly efficient down regulation of photochemistry in the form of non-photochemical quenching (NPQ) utilising the xanthophyll cycle. We investigated this further using fast rapid-rate fluorescence (FRRF) to investigate electron transport rate (ETR) and NPQ, for four strains of the blue diatom Haslea, isolated from samples taken from four widely spaced geographic localities. Cultures were grown at 50 µmol m-2 s-1 PAR until in logarithmic growth phase, at which point modified rapid light curves (RLCs) were applied. These RLCs initially followed a conventional incremental increase in light step intensity, but after reaching 505 µmol m-2 s-1 PAR, the intensity was reduced back to 310 µmol m-2 s-1 PAR, followed by increasing the intensity further through 505, to 610 and then back to the growth irradiance of 50 µmol m-2 s-1 PAR. We investigated the induction of NPQ alongside changes in reaction centre connectivity (ρ) and photosystem II cross section (σPSII). Despite decreasing the light intensity after reaching 505 µmol m-2 s-1 PAR, ETR was observed to increase, failing to saturate at 610 µmol m-2 s-1 PAR. Up to 310 µmol m-2 s-1 PAR, no NPQ was induced, but after this RLC step, there was a steady increase in NPQ that was not reversed by decreasing light intensity, even when reduced to the growth intensity of 50 µmol m-2 s-1 PAR. We present changes in ρ and σPSII that are hypothesised to account for this rapid photoacclimation, and hence the unusual pattern in ETR observed, which was concomitant to the non-light intensity dependent NPQ induction.
