Inter-Research > AME > v33 > n3 > p261-270  
Aquatic Microbial Ecology

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

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

Marc Staal1,*, Sacco te Lintel Hekkert2, Frans J. M. Harren2, Lucas J. Stal1

1Department of Marine Microbiology, Netherlands Institute of Ecology-KNAW, PO Box 140, 4400 AC Yerseke, The Netherlands
2Life Science Trace Gas Exchange Facility, Department of Molecular and Laser Physics, University of Nijmegen, Toernooiveld, 6525 ED Nijmegen, The Netherlands

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

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