AME 33:261-270 (2003)  -  doi:10.3354/ame033261

ABSTRACT: The effect of O₂ 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 O₂ concentrations. Our results showed that under non-steady state conditions the optimum concentration of O₂ for N₂ fixation differed from that at steady-state O₂ levels. The optimum O₂ concentration depended upon whether the O₂ concentration was increasing or decreasing, with decreasing concentrations yielding higher O₂ optima for dark nitrogenase activity than increasing O₂ concentrations. The cyanobacteria rapidly adapt to changes in O₂, and therefore measurements also reflect the history of O₂ concentrations to which organisms have been exposed. Steady-state and non- steady-state O₂ concentrations both decreased their optimum O₂ concentration for nitrogenase activity rates with increasing irradiance. However, the optimum O₂ concentration was always higher than zero, even at saturating irradiances. Hence, it appears that low levels of O₂ 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 O₂ concentration and light emphasize the necessity of including these effects in models that calculate the daily integral of N₂ fixation.

KEY WORDS: Heterocystous cyanobacteria · Nitrogen fixation · Oxygen · Light-response · Laser photo acoustics · Baltic Sea · Cyanobacteria blooms