The influence of salinity on the kinetics of NH inf4sup+ uptake in Spartina alterniflora.

The effects of short- and long-term exposure to a range in concentration of sea salts on the kinetics of NH inf4sup+ uptake by Spartina alterniflora were examined in a laboratory culture experiment. Long-term exposure to increasing salinity up to 50 g/L resulted in a progressive increase in the apparent Km but did not significantly affect Vmax (mean Vmax=4.23±1.97 μmole·g-1·h-1). The apparent Km increased in a nonlinear fashion from a mean of 2.66±1.10 μmole/L at a salinity of 5 g/L to a mean of 17.56±4.10 μmole/L at a salinity of 50 g/L. These results suggest that the long-term effect of exposure to total salt concentrations within the range 5-50 g/L was a competitive inhibition of NH inf4sup+ uptake in S. alterniflora. No significant NH inf4sup+ uptake was observed in S. alterniflora exposed to 65 g/L sea salts. Short-term exposure to rapid changes in salinity significantly affected both Vmax and Km. Reduction of solution salinity from 35 to 5 g/L did not change Vmax but reduced Km by 71%. However, exposing plants grown at 5 g/L salinity to 35 resulted in an decrease in Vmax of approximately 50%. Exposure of plants grown at 35 g/L to a total sea salt concentration of 50 g/L for 48h completely inhibited uptake of NH inf4sup+ . For both experiments, increasing salinity led to an increase in the apparent Km similar to that found in response to long-term exposure. Our data are consistent with a conceptual model of changes in the productivity of S. alterniflora in the salt marsh as a function of environmental modification of NH inf4sup+ uptake kinetics.