Absence of the Pigments of Photosystem II of Photosynthesis in Heterocysts of a Blue–Green Alga

Abstract

ALL filamentous blue–green algae capable of fixing elementary nitrogen have heterocysts. Stewart et al.¹ have strong evidence that these differentiated cells are the sites of nitrogen fixation. They did not, however, show that photosystem II, responsible for the evolution of molecular oxygen (O₂), is not functional in heterocysts. Because high oxygen tension inhibits nitrogen fixation, heterocysts should not possess the pigments of photosystem II. Fay² produced evidence in vitro suggesting that heterocysts do not contain c-phycocyanin (c-PC), a principal constituent of photosystem II. Stewart et al.¹ found much less c-PC in heterocysts than normal cells and believed that photosystem II could be absent in heterocysts. Others^3,4, however, have demonstrated the evolution of O₂ from blue–green algae in the absence of c-PC. Thus the points to be resolved are whether heterocysts in vivo contain c-PC and the other pigments comprising photosystem II, and whether, in the absence of one or more of these, photosystem II is functional in heterocysts. A comparison of the in vivo pigment composition of normal cells and heterocysts has indicated that heterocysts lack a functional photosystem II.