ACCLIMATION OF CYANOBACTERIAL COMMUNITIES IN RICE FIELDS AND RESPONSEOF NITROGENASE ACTIVITY TO LIGHT REGIME

The short-term and long-term effects of light regime on nitrogenase ac tivity (NA) and cyanobacterial communities in rice fields (Valencia, S pain) were examined. Daily variation in nitrogen fixation was measured during three periods of the crop cycle: tillering (formation of secon dary stems in the rice plants), heading (formation of reproductive str uctures), and maturity. Two locations were examined over two consecuti ve years (1994 and 1995). Despite differences in the crop-cycle period s, location, and year, a consistent pattern of nitrogen fixation was o bserved, with a main activity peak in the morning and another in the l ate evening. Short-term experiments, performed on two cyanobacterial b looms (Nostoc sp. and Anabaena sp.) exposed to natural light under pla nt canopy (7% incidence irradiance), and to different light intensitie s under neutral density screens without plant cover (full sunlight, 43 %, 26%, and 13% of incident irradiance), indicated that nitrogenase ac tivity (NA) was dependent on both light intensity and quality. In long -term experiments, where natural communities of cyanobacteria were exp osed to one month of different light intensities, changes in the speci es composition of the three main genera of heterocystous cyanobacteria (Nostoc, Anabaena, and Calothrix) were observed. The light intensity at which communities were exposed for one month became the optimum irr adiance for NA for each cyanobacterial community. Assays performed at higher or lower irradiances showed lower NA. Nitrogen fixation followe d a pattern of seasonal variation along the crop cycle. Values were lo w at the beginning of the crop (May), reached a maximum value at the e nd of the tillering stage (June), and declined thereafter until the en d of the cultivation cycle (September).