Photosynthate Limitation of Nitrogen Fixation in the Blue-Green Alga, Anabaena Variabilis

Abstract

Nitrogen fixation in nodulated legumes can be increased by CO2 enrichment of the air surrounding leaves of soybean and peanut plants (Hardy and Havelka, 1974). In the above symbiotic associations organic compounds from CO2 fixation of the plant are transported to the root nodules to supply energy for N2 fixation. In heterocystous blue-green algae under conditions of nitrogen limitation filaments differentiate into heterocysts and vegetative cells (see Wolk, this volume and references therein for discussion of the physiology and biochemistry of heterocysts). The resultant two cell types carry different functions. Heterocysts contain only photosystem I producing ATP by cyclic photophosphorylation and lack photosystem II. The localization of nitrogenase in heterocysts in connection with an efficient O2 uptake system provides protection for nitrogenase against denaturation by oxygen. The absence of oxygen evolution in heterocysts makes them highly dependent on the neighboring vegetative cells that contain both photosystems and can fix CO2. Thus vegetative cells are equipped to deliver reductant supplied as an organic compound to the hetero-cyst (see Wolk, 1968). This raises the possibility that the generation of reductant by photosynthesis and CO2 fixation as well as the transport of reductant (photosynthate) may limit nitrogen fixation and nitrogenase-mediated H2 evolution in heterocystous blue-green algae. The experiments discussed in this communication show that nitrogenase activity and growth of Anabaena variabilis is limited by photosynthate availability.