Mie Mie Aung, Ulrich Eberhardt, Paul L. G. Vlek:
Studies on the Photosynthetic Carbon Acquisition of Azolla-Anabaena Symbiosis

MIE MIE AUNG¹, ULRICH EBERHARDT², PAUL L. G. VLEK^3
1Georg-August University Göttingen, Institute of Agronomy and Animal Production in the Tropics and Subtropics, Germany
²Georg-August University Göttingen, Former Central Isotope Laboratory, Germany
³University of Bonn, Center for Development Research, Germany

Azolla spp. is small ferns floating on water surfaces. They contain cyanobacterial microsymbionts (Anabaena azollae) in leaf cavity are able to fix elemental nitrogen. The combination of photosynthetic acquisition of carbon and energy and biological nitrogen fixation provides a basis for potentially high productivity of the symbiotic system. Consequently, Azolla have been used for centuries to fertilize rice paddies without need for supplementary manure. When combined with urea as a fertilizer, Azolla layers reduce the volatilization of ammonia originating from urea hydrolysis by decreasing the pH of floodwater. This effect is accomplished by reduction of algal photosynthetic activity during the day. More insight into the physiology of Azolla system and into the interactions between Azolla and Anabaena may increase the benefit from using Azolla in agriculture and in this way contribute to a sustainable land use. Photosynthesis is the ultimate source of ATP and reductant required for nitrogen fixation and there is a close relationship between both processes. Thus carbon fluxes into and out of Azolla and between the symbiotic partners are of interest. Studying Azolla carolinia and A. pinnata var. imbricata involving the measurement of ¹⁴C incorporated from ¹⁴CO₂ into symbiotic Anabaena filaments and Azolla tissues, we found that less than [15]% of the fixed carbon in the system was located in the Anabaena. KAPLAN and PETERS (1988), comparing net photosynthesis rates of symbiotic association and Anabaena-free Azolla plants, suggested that Anabaena contributes little to carbon fixation. Sucrose is assumed to be the photosynthetic product transferred from Azolla to Anabaena (KAPLAN & PETERS, l.c.). Some appraoches to verify the existence of carbon metabolism in Anabaena symbionts are presented.

Keywords: Azolla, carbon acquisition

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Contact Address: Mie Mie Aung, Georg-August University Göttingen, Institute of Agronomy and Animal Production in the Tropics and Subtropics, Grisebachstraße 6, 37077 Göttingen, Germany, e-mail: maunggwdg.de