The marine cyanobacterium Trichodesmium uses an unidentified mechanism to protect its nitrogenase enzyme from deactivation by oxygen. Several cellular oxygen consuming reactions may minimize the in vivo oxygen concentration during periods of photosynthesis. Analysis of the mechanisms proposed to date indicates that there remains to be discovered a significant oxygen consuming process in the Trichodesmium colonies. The hypothesis being tested is that the photoreduction of oxygen (Mehler reaction) removes a significant fraction of photosynthetically evolved oxygen. Tests of this hypothesis and measurement of other oxygen consuming reactions require stable isotope techniques. Two related O-18 techniques are capable of determining photosynthetic oxygen evolution and oxygen consumption rates in the light: membrane inlet mass spectrometry and O-18 isotope dilution techniques. The methods to be used are either new or have been confined to laboratories before. With modern, reliable and compact mass spectrometers it is feasible to conduct stable isotope techniques at sea. %%% The nitrogen-fixing cyanobacterium Trichodesmium must possess some mechanism whereby its nitrogenase enzyme is not deactivated by oxygen produced during photosynthesis. The photoreduction of oxygen (Mehler reaction) is a possible pathway which will be investigated in thei project. A newly developed mass spectrometer and oxygen 18 isotopes will be used at sea in the Caribbean to test the hypothesis that the Mehler reaction of general importance in Trichodesmium.
