PHOTOSYNTHESIS AND COLD ACCLIMATION: MOLECULAR EVIDENCE FROM A POLAR DIATOM1

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Abstract
The psychrophilic diatomFragilariopsis cylindrus(Grunow) Krieger in Helmcke & Krieger was used to investigate photosynthesis and growth under freezing temperatures. Gene expression during a temperature shift from +5° C to −1.8° C was studied under 3 and 35 μmol photons·m−2·s−1by using a macroarray. These measurements were paralleled by determination of fluorescence induction at PSII and pigment analysis. The shift to −1.8° C at 35 μmol photons·m−2·s−1caused a marginal decrease of photosynthetic quantum yield (Fv/Fm) from 0.61 to 0.52 with fast recovery after 1 day. The ratio of chlcto chlaincreased from 3.1 to 5.5, and the ratio of diatoxanthin to diadinoxanthin increased from 0.7 to 5.0. Genes encoding proteins of PSII (psbA, psbC) and for carbon fixation (rbcL) were down‐regulated, whereas genes encoding chaperons (hsp70) and genes for plastid protein synthesis and turnover (elongation factorEfTs, ribosomal proteinrpS4,ftsHprotease) were up‐regulated. In contrast, cold exposure at 3 μmol photons·m−2·s−1induced a marginal increase in Fv/Fmfrom 0.61 to 0.63 and a strong increase in fucoxanthin concentrations from 0.04 up to 0.12 pg·cell−1. This was paralleled by up‐regulation offcpgenes. The ratio of chlcto chlaalso increased from 3.1 to 4.2, as did the ratio of diatoxanthin to diadinoxanthin from 0.7 to 2.2. Down‐regulation ofpsbA,psbC, andrbcL could also be measured but not up‐regulation ofhsp70,EfTs,rpS4, and theftsHprotease. The latter genes are probably necessary to avoid cold shock photoinhibition only at higher light intensities.