Nitrogen fixers

A relatively uncomplicated case is the use of nitrogen fixers as food items. There is abundant archaeological evidence for the importance of pulses in the Mediterranean in Neolithic and later periods. Because pulses have 5 N values of around 0%o, the use of pulses as food stuffs should be detectable. Recently, we have measured human 5 N values in some Greek sites that indicate that a few individuals in the cemetery were almost completely dependent on pulse protein (Van Klinken and Triantaphyllou, in prep.). 

In this sen.se, in natural environments, the plant needs can be fulfilled by less strict, but more homogeneously extended plant-microbe interactions, such as those that cereals undertake with free nitrogen fixers (e.g., azospirilli or cyanobacteria), and that often involve not only nitrogen exchange but also hormones and other signals. 

The evolutionary history of symbiotic nitrogen fixers is therefore a tale of coevolution, which occurred in the shadow of their hosts, chasing their growing roots, and striving for adaptation. It is an example of how bacterial genetics has managed to keep pace with the creative power of eukaryotic sexual recombination. Mobile replicons, insertion elements, and symbiotic islands prone to move have helped rhizobia to succeed in their pursuit. The race, naturally, is not over and, looking at it from a distance, what we have. seen, compared to what we have yet to see, is probably just a cloud of dust. 

Oxygenic photosynthetic bacteria include the nitrogen fixer Trtchodesmtum and the cyanobacteria Synechococcus and Prochlorococcus. The latter represent a major fraction of the microbial biomass in the ocean and are probably the most abimdant primary producers in the ocean. They were not discovered until the late 1980s, probably because of their small size as they are members of the picoplankton. These cyanobacteria are photosynthetic. They use bacteriochlorophyll a and plastocyanin, a blue Cu-based pigment, for light harvesting. 

Diazotrophs Nitrogen-fixing bacteria. In the marine environment, some nitrogen fixers live as symbionts in phytoplankton, particularly diatoms. 

Little is known about the potential to exploit mutualism in agricultural systems. Almost all the work to date has concentrated on symbiotic nitrogen-fixers and mycorrhizal associations. Availability of soil nitrogen and phosphorus is a severe problem in many areas of the world. Allelochemicals may impact the availability of these nutrients through effects on the symbiotic microbes. 

We should also learn which organisms and allelochemicals adversely affect the microbial symbionts. Some have already been shown to suppress growth of bacterial nitrogen fixers and nitrifiers as well as mycorrhizal fungi. Compounds that inhibit nitrification could prove to be important agriculturally. 

Azad, H.R., Davis, J.R., Schnathorst, W.C., Kado, C.I. Influence of Verticillium wilt resistant and susceptible potato genotypes on populations of antagonistic rhizosphere and rhizoplane bacteria and free nitrogen fixers. Appl Microbiol Biotechnol 1987 26 99-104. 

In aquatic habitats, cyanobacteria are the principal nitrogen fixers. Rates of nitrogen fixation by cyanobacteria are generally one or two orders of magnitude higher than those by free-living, nonphotosynthetic soil bacteria. Under favorable conditions, cyanobacteria may fix up to 1600 kg of N/ha/year in a rice paddy. 

Barcelos e Ramos, J., Biswas, H., Schrdz, K. G., LaRoche, J., and RiebeseU, U. (2007). Effect of rising atmospheric carbon dioxide on the marine nitrogen fixer Trichodesmium. Glob. Biogeochem. Cycles... 

Moisander, P. H., Piehler, M. F., andPaerl, H. W. (2005). Diversity and activity of epiphytic nitrogen-fixers on standing dead stems of the salt marsh grass Spartina alternijlora. Aquat. Microb. Ecol. 39, 271— 279. 

Scranton, M. I. (1984). Hydrogen cycling in the waters near Bermuda The role of the nitrogen fixer, Oscillatoria thiebautii. Deep Sea Res. 31, 133—143. 

---