Respiratory losses in the light in a marine diatom: Measurements by short‐term mass spectrometry

Mass spectrometric analysis of gas exchange by the diatom Thalassiosira weisflogii grown under 12 : 12 L/D cycles indicates that mitochondrial respiration is highest in the light, averaging 54 µ mol O 2 (mg Chl a ) −1 h −1 while gross photosynthesis was 162 µ mol O 2 (mg Chl a ) −1 h −1 . In the dark, mitochondrial O 2 consumption slowly decreased to a minimum of 29 µ mol(mg Chl a ) −1 h −1 at the end of scotophase. When cells were poised at the DIC (dissolved inorganic carbon) compensation point in the light (DIC concentration where net photosynthesis equals zero), mitochondrial O 2 consumption was only slightly higher than in the dark. Adding DIC to cells at the compensation point resulted in a rapid increase in both photosynthetic O 2 evolution and mitochondrial O 2 consumption, indicating that the higher mitochondrial respiration rates observed in the light are probably due to an increase in substrate supply from photosynthesis. If estimates of primary production are based on oxygen exchange, this effect is accounted for; however if radiocarbon is used to measure production, especially in short‐term measurements, net production may be significantly overestimated.