The light response of nitrogen fixation in Lake Valencia, Venezuela [Anabaena spp., Cylindrospermopsis, photosynthesis]

Water samples from six depths in the upper water column of Lake Valencia, Venezuela, were incubated in situ with acetylene at several dephts corresponding to different light exposures. On five dates there was sufficient nitrogen fixation to define the light-response curve for samples taken at 2 and 100 cm. The light response of nitrogen fixation was modeled successfully with an equation previously developed for the light response of photosynthesis. All parameter values of the response curves for samples originating at a given depth were relatively stable across dates, but differed between depths. Rates of increase of N fixation with irradiance in the subsaturation range $(\alpha)$ were much lower than is typical of photosynthesis. While the general shape of nitrogen fixation curves was similar to that of photosynthesis curves, critical parameters were more sensitive to the light history (depth of origin) of cells than would be expected for photosynthesis.