Effects of O2 on N2 fixation in heterocystous cyanobacteria from the Baltic Sea

Abstract

The effect of O 2 on nitrogenase activity in natural samples of heterocystous cyanobacteria from the Baltic Sea was studied using on-line laser photo-acoustic trace-gas detection. This technique records nitrogenase activity in near real-time and allows measurements in continuously changing O 2 concentrations. Our results showed that under non-steady state conditions the optimum concentration of O 2 for N 2 fixation differed from that at steady-state O 2 levels. The optimum O 2 concentration depended upon whether the O 2 concentration was increasing or decreasing, with decreasing concentrations yielding higher O 2 optima for dark nitrogenase activity than increasing O 2 concentrations. The cyanobacteria rapidly adapt to changes in O 2 , and therefore measurements also reflect the history of O 2 concentrations to which organisms have been exposed. Steady-state and nonsteady-state O 2 concentrations both decreased their optimum O 2 concentration for nitrogenase activity rates with increasing irradiance. However, the optimum O 2 concentration was always higher than zero, even at saturating irradiances. Hence, it appears that low levels of O 2 are an obligatory requirement for maximum nitrogenase activity in the field. Low levels of respiration served as a source of additional energy, suggesting that even at light saturation, photosynthetic energy generation in the heterocyst can not saturate the demand of nitrogenase for ATP. The large changes in nitrogenase activity due to the combined effect of variations in O 2 concentration and light emphasize the necessity of including these effects in models that calculate the daily integral of N 2 fixation.