Calcification and measurements of net proton and oxygen flux reveal subcellular domains in Acetabularia acetabulum

Vegetative adults of Acetabularia acetabulum (L.) Silva were studied as a m odel system for subcellular patterning in plants, and a description of seve ral phenotypic and physiological characteristics that reveal patterns of su bcellular differentiation in this unicellular macroalga was undertaken. Ini tially, calcification patterns were studied. Under favorable conditions, th e rhizoid and most of the stalk calcified. Only the apical 10-20% of the st alk and a small region adjacent to the rhizoid remained uncalcified. Calcif ication in algae has been reported to result from a biologically mediated l ocal increase in alkalinity. To test this model extracellular pH and extrac ellular hydrogen ion gradients were examined with ion-selective, self-refer encing, electrodes. In the light, A. acetabulum displayed a general pattern of extracellular alkalinity around the entire alga, although in some indiv iduals the region near the rhizoid and the rhizoid itself displayed extrace llular acidity. Acetabularia acetabulum also displayed net hydrogen ion inf lux at the rhizoid and the apical half of the stalk, variable flux in the l ower part of the stalk, and net hydrogen ion efflux at the base of the stal k next to the rhizoid. The lack of complete correlation between external pH patterns and calcification suggests that other factors contribute to the c ontrol of calcification in this alga. To examine whether net hydrogen ion f lux patterns correlated with photosynthetic or respiration patterns, oxygen flux was measured along the stalk using self-referencing O-2 electrodes. P hotosynthetic oxygen evolution occurred at comparable levels throughout the stalk, with less evolution in the rhizoid. Respiration mainly occurred mar and in the rhizoid, with less O-2 consumption occurring more apically alon g the stalk. Our studies of calcification patterns, net hydrogen ion flux a nd O-2 flux revealed several overlapping patterns of subcellular differenti ation in A. acetabulum.