Free calcium in Micrasterias: local gradients are not detected in growing lobes

Intracellular free Ca2+ ([Ca2+]) has been measured in growing unicells of two species of the green alga, Micrasterias, which have been injected with the indicator dye fura-2-dextran. Ratiometric imaging of Micrasterias denticulata yields levels of 170 to 200 nM [Ca2+] but fails to reveal a significant [Ca2+] gradient associated with the tips of growing lobes, or in any other region of the cell. In Micrasterias muricata slight elevations from a basal value of 350 to 500 nM have been observed, but these might be due to a general inward leakage of Ca2+ at the plasma membrane which is enhanced at the narrow lobes of this cell because of their greater relative surface to volume ratio. Experimental perturbation of the intracellular [Ca2+] with injection of the ion or the addition of the non-fluorescent ionophore, Br-A23187, reveal that [Ca2+] elevations can be generated and indicate that if they naturally occurred, the image system would have detected them. Further evidence that [Ca2+] gradients are lacking derives from studies with BAPTA-type buffers. Injection of 5,5'-dibromo BAPTA and 4,4'-difluoro BAPTA, which in several other systems are the most effective at dissipating intracellular [Ca2+] gradients, have no effect on development of Micrasterias. Taken together, these studies indicate that lobe outgrowth in Micrasterias does not occur in association with marked localized [Ca2+] gradients.