Azolla-Anabaena Relationship VII . DISTRIBUTION OF AMMONIA-ASSIMILATING ENZYMES , PROTEIN , AND CHLOROPHYLL BETWEEN HOST AND SYMBIONT 1

The N2-fixing Azolla-Anabaena symbiotic association is characterized in regard to individual host and symbiont contributions to its total chlorophyll, protein, and levels of ammonia-assimilating enzymes. The phycocyanin content of the association and the isolated blue-green algal symbiont was used as a standard for this characterization. Phycocyanin was measured by absorption and fluorescence emission spectroscopy. The phycocyanin content and total phycobilin complement of the symbiotic algae were distinct from those of Anabaena cylindrica and a free-living isolate of the Azoila endophyte. The algal symbiont accounted for less than 20% of the association's chlorophyll and protein. Acetylene reduction rates in the association (based solely on the amount of algal chlorophyll) were 30 to 50% higher than those attained when the symbiont was isolated directly from the fern. More than 75% of the association's glutamate dehydrogenase and glutamine synthetase activities are contributed by the host plant. The specific activity of glutamate dehydrogenase is greater than that of glutamine synthetase in the association and individual partners. Both the host and symbiont have glutamate synthase activity. The net distribution of these enzymes is discussed in regard to the probable roles of the host and symbiont in the assimilation of ammonia resulting from N2 fixation by the symbiont. The Azolla-Anabaena azollae symbiosis is an N2-fixing association between a eukaryotic fern and a prokaryotic alga, both of which exhibit a higher plant type of photosynthesis (18, 19). As an approach toward the characterization of the role of the host and symbiont4 in the association, it was considered important to determine the contribution of the partners to the association's Chl, protein, and ammonia-assimilating enzymes. While the alga could be isolated free of the fern, its removal from the host was never complete. Thus it was necessary to find an independent quantitative estimate for the amount of alga in the association. While nitrogenase is unique to the alga (21) its lability makes it unsuitable for quantitation. Chl a/b ratios provided another estimate, but this was based on the absence of Chl b from the alga (20) and was therefore not as sensitive. A third method, the quantitation of phycobilins which are unique to the alga in the association, was employed. A procedure involving fluorescence emission spectroscopy at 77 K permitted quantitative measurement of phycobilins in the association, where the fern's Chl b ' Supported in part by NSF National Science Foundation Grant DEB 74-11679 AOl. 2Contnbution No. 620 from the Charles F. Kettering Research Laboratory. 3To whom reprint requests should be sent. 4"Symbiont" is used throughout this paper in reference to the algal partner. would have precluded the use of absorption spectroscopy (1, 13). The phycocyanin content of the isolated symbiont and that of the association were used to determine the contributions of the individual partners to the association's protein and Chl. It was then possible to estimate the distribution of principal ammoniaassimilating enzymes between the host and symbiont. This allowed an initial assessment of the probable roles of the individual partners in the assimilation of ammonia resulting from nitrogen fixation. For comparative purposes data are also presented on the nonsymbiotic blue-green alga Anabaena cylindrica, the free-living endophyte from Azolla caroliniana (16), and algal-free Azolla plants. MATERIALS AND METHODS