Nitrogen and carbon fixation byAnabaena sp. isolated from a rice paddy and grown under P and light limitations

Anabaena sp., isolated from a rice paddy, was investigated for its nitrogen fixation as measured by acetylene reduction activity (ARA) in P-limited continuous and light-limited semi-continuous cultures. Growth rate (μ) under P limitation was a function of cell P content (q _p). Both the photosynthetic capacity (P_max) and photosynthetic efficiency (α) increased with μ when expressed per cell, but not per unit chla. The ARA of steady-state cells under P limitation increased with μ and was linearly related to C-fixation rate. This was apparently a consequence of the control of C-fixation by P limitation. In light-limited cells, steady state ARA, both at the culture light intensity and in the dark, increased asymptotically with μ, but the activity in the dark was only about 51% of that in the light. When the light level of steady-state cells grown at a high in intensity was switched to a low level, ARA decreased exponentially with time. Dark ARA activity also showed a similar decline, but at much lower levels. Thus, ARA depended not only on light history, but also immediate photosynthesis. Steady-state ARA at the ambient intensity or in the dark showed a strong correlation with¹⁴C-fixation rate. ARA of light-limited cells showed the same light-saturation characteristics as their¹⁴C-fixation, with the same initial saturation intensity,I _k. The ratios of P_max to the maximum ARA (ARA_max), and α to the slope of ARA (α_ara) were identical. A comparison of gross to net photosynthesis and N₂ fixation suggested that there was little leakage or excretion of fixed C or N.