The photosynthetic rates in two psychrophilic diatoms, Chaetoceros sp. strain K3-10 and Nitzschia sp. K3-3 for cells grown at 0 degrees C were 8 to 10 microliters O(2) evolved per milligram dry weight per hour, and 10-fold higher, about 80 for cells grown at 10 degrees C. The respiration rates followed the same pattern, with a value of around 1 microliter dark uptake per milligram dry weight per hour for both organisms grown at 0 degrees C, and 6 to 10 for cells grown at 10 degrees C. When cells grown at 0 degrees C were immediately shifted to 10 degrees C or cells grown at 10 degrees C were shifted to 0 degrees C, the respiratory rates quickly adapted to values characteristic of cells grown at the shift temperature. On the other hand, the light-saturated rate of O(2) evolution showed much less immediate adaptation, especially on the up shift, 0 degrees to 10 degrees C. The chlorophyll a content of 0 degrees C grown cells was about 0.5% of dry weight, in 10 degrees C grown cells 1.3% (strain K3-10) and 2.2% (strain K3-3). In addition to a diminished chlorophyll a content in 0 degrees C grown cells, there seemed proportionally (by absorbance and calculation) less c to a than in 10 degrees C grown cells. The relative fluorescence excitation spectra of 680-nm emission also showed a lower contribution by both chlorophyll c and fucoxanthin in 0 degrees C grown cells of Chaetoceros sp. strain K3-10 as compared to 10 degrees C grown cells. The data at hand suggest that in psychrophilic diatoms continuously growing at 0 degrees C there may be problems associated with synthesis of an effective accessory pigment system, and as a working hypothesis it is suggested this is related to restriction of synthesis of one or several accessory pigment proteins.