Reinfelder, J. R. Dept of Environmental Sciences,
Morel, F. R. Dept of Geosciences,

The enzyme phosphoenolpyruvate carboxylase (PEPC) catalyzes the first step in the transfer of CO2 from air to the ultimate site of fixation by ribulose-1,5-bisphosphate carboxylase in the bundle sheath cells of terrestrial C4 plants. PEPC activity has been measured in a number of unicellular marine algae, but the enzyme's function, anapleurotic or photosynthetic, has not been definitively established. In the marine diatom Thalassiosira weissflogii, PEPC activity is inversely related to the concentrations of CO2 and Zn in acclimated cultures. PEPC activity is highest in Zn-limited diatoms that have the lowest activities of carbonic anhydrase (CA). In marine diatoms, CA facilitates inorganic carbon transport through the catalysis of HCO3- dehydration. PEPC may therefore support inorganic carbon transport in Zn-limited cells, in addition to other functions. In order to test this hypothesis, Zn-replete diatoms were grown in the presence of the membrane-permeable CA inhibitor ethoxyzolamide (EZ) to assess the specific effects of depressed CA on PEPC activity independent of Zn nutrition which may have many effects on cell physiology. PEPC activities in EZ exposed diatoms increased on a per cell (three-fold) and per unit chlorophyll (seven-fold) basis over a 24 hour period during which time the specific growth rate dropped from 1.4 to 0.8 d-1. The results suggest that increased PEPC activity partially compensates for lowered CA and that PEPC is less efficient than CA in facilitating inorganic carbon transport. The role of PEPC as part of a C4 carbon pump is supported by 14C pulse-chase results showing the transfer of carbon from malate to phosphoglyceric acid and sugars and measurements of phosphoenolpyruvate carboxykinase (a malate decarboxylase) activity in Zn-limited, marine diatoms.
Day: Thursday, Feb. 4
Time: 12:00 - 12:15pm
Location: Sweeney Center
Code: CS62TH1200S