Cultures of the obligate psychrophilic diatom Fragilariopsis cylindrus (Grunow) were grown for 4 months under steady-state conditions at -1°C and +7°C (50 µmol photons m-2 s-1) prior to measurements in order to investigate long-term acclimation of photosynthesis to both temperatures. No differences in maximum intrinsic quantum yield of PS II (F V/F M) and relative electron transport rates could be detected at either temperature after 4 months of acclimation. Measurements of photosynthesis (relative electron transport rates) vs. irradiance (P vs. E curves) revealed similar values for relative light utilization efficiency (a = 0.57 at -1°C, a = 0.60 at +7°C) but higher values for irradiance levels at which photosynthesis saturates (E K) at -1°C and, therefore, higher maximum photosynthesis (P MAX = 54 (relative units) at -1°C, P MAX = 49 at +7°C). Nonphotochemical quenching (NPQ) measurements at 385 µmol photons m-2 s -1 indicated higher (37%) NPQ for diatoms grown at -1°C compared to +7°C, which was possibly related to a 2-fold increase in the concentration of the pigment diatoxanthin and a 9-fold up-regulation of a gene encoding a fucoxanthin chlorophyll a,c-binding protein. Expression of the D1 protein encoding gene psbA was ca. 1.5-fold up-regulated at -1°C, whereas expression levels of other genes from Photosystem II (psbC, psbU, psbO), as well as rbcL, the gene encoding the Rubisco large subunit were similar at both temperatures. However, a 2-fold up-regulation of a plastid glyceraldehyde-P dehydrogenase at -1°C indicated enhanced Calvin cycle activity. This study revealed for the first time that a polar diatom could efficiently acclimate photosynthesis over a wide range of polar temperatures given enough time. Acclimation of photosynthesis at -1°C was probably regulated similarly to high light acclimation.