As it is difficult to reconcile concomitant oxygenic photosynthesis and oxygen labile nitrogen fixation cyanobacteria have evolved different adaptation strategies to overcome this anomaly. A confinement of nitrogenase into a special micro-aerobic cell is seen for heterocystous species. Cyanobacteria lacking this cell type confide in other behavioral strategies. For these, the most common approach is to separate nitrogen fixation and photosynthesis in time. As photosynthesis by necessity must occur during the light phase, nitrogen fixation accordingly takes place during the dark phase.
The only exception to the above strategies is Trichodesmium. This marine cyanobacterium fixes nitrogen aerobically only during the light phase. Nitrogenase is confined to a specific cell type, diazocytes, arranged consecutively and constituting 7-20% of all cells.
In the present study a method was developed which enabled detection of these nitrogenase-containing diazocytes using an epi-fluorescence microscope. This enabled screening studies of their distribution in a large number of trichomes and within colonies of natural and cultured Trichodesmium. Groups of diazocytes were found randomly spread over colonies, and the trichomes could contain more than one group of these zones. The unique nitrogen fixation behavior of Trichodesmium was also examined in relation to the photochemical quantum yield [variable fluorescence/maximal fluorescence (Fv/Fm)], oxygen production, and C uptake. This unveiled that Trichodesmium not only practices a spatial separation of the two incompatible processes, but also a temporal separation between N2 fixation and photosynthesis during the photo phase. Nitrogen fixation peaked around midday, and varied inversely with oxygen evolution, 14C-uptake and Fv/Fm. When examining Fv/Fm two- dimensionally, regions with lower yields were seen along trichomes, and a larger proportion of the trichomes displayed lower Fv/Fm at the peak of nitrogen fixation. Prior to this study, this nitrogen fixation strategy was exclusively found in Trichodesmium. However, another marine cyanobacterium, Katagnymene, was here discovered to share the same localization of nitrogenase into diazocytes and daytime nitrogen fixation. Though, phylogenetic analyses performed revealed that the two species of Katagnymene were in fact one, and had such a high sequence similarity to Trichodesmium that they were re-affiliated to this genus. Also, a novel species of Trichodesmium was found, assigned T. aureum sp. nov. The nitrogen fixation behavior and phylogeny of yet another common marine cyanobacterium, Lyngbya majuscula was examined. Like Trichodesmium, L. majuscula has previously been reported to fix nitrogen during the photo phase. Our data show that under natural conditions nitrogenase activity is only observed during the dark phase and that is the case for the nitrogenase enzyme as well, and nitrogenase was detected in all cells. This thesis provides novel insights into the nitrogen fixing behavior of Trichodesmium, and unveils novel members of the genus, including those previously characterized as the genus Katagnymene. It also reveals that Lyngbya majuscula does not share this same nitrogen fixing behavior, but fixes nitrogen at night as many other non-heterocystous cyanobacteria. Hence, Trichodesmium is still the only non-heterocystous genus capable of aerobic daytime nitrogen fixation.
Publicly defended January 25th 10:00 in the lecture hall at the Department of Botany, Stockholm University
Faculty opponent: Professor John R. Gallon, School of Biological Sciences, University of Wales Swansea, Singleton Park, Swansea, SA2 8PP, U.K.
ISBN 91-7265-398-1 pp 1-43