The temperature response of photosynthetic capacity was compared to that of 3 carboxylating enzymes in Arctic marine phytoplankton. Only the activity of ribulose-1.5-bisphosphate carboxylase (RuBPC) consistently exhibited an apparent activation energy equivalent to that of photosynthetic capacity. This is the first indication from field samples of phytoplankton that changes in the activity of RuBPC may be closely associated with changes in photosynthetic capacity. The maximum photosynthetic capacity attained by Arctic phytoplankton at a given in situ temperature appeared to increase from about 0.4 mg C mg Chl a-' h-' at -1.5 ' C to an apparently constant value of about 2.0 mg C mg Chl a-' h-' at temperatures equal to and greater than 0.0 "C.
When assayed near respective in situ temperatures, ribulose blsphosphate carboxylase (RuBPC) exhibits a greater response to increasing temperature in arctic than in tropical phytoplankton. This was also true when the Arrhen~us activation energy (E,) was compared for a range of intermediate temperatures. This difference in temperature response and E, for RuBPC is consistent with the temperature response of photosynthetic capacity, but is contrary to the general observation that coldenvironment ectotherms (organisms whose internal temperature conforms to that of the environment) have physiological processes with lower E,'s. Analysis suggests that the genotypic adaptation of qualititative enzyme temperature responses in marine phytoplankton is related to the vertical structure of the environment and to a suite of variables of which temperature is only one. Quantitatively, however, arctic phytoplankton do not appear to adapt to cold by increasing RuBPC levels per unit pigment biomass.
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