Biological Nitrogen Fixation for Food and Fiber Production

Binding characteristics of n(2)-fixing bacteria to cereal roots

The attachment of Rhizobium japonicum 61A89 and Rhizobium spp. 32H1 to the roots of wheat and rice seedlings is analyzed in terms of an equilibrium model. A Langmuir adsorption isotherm describes the binding. Strain 61A89 binds to a greater extent than does strain 32H1, and the equilibrium constants for each strain binding to wheat are strongly temperature dependent. Both time-dependent dissociation and association, predicted by an equilibrium model, have been found. The dissociation rate constant for 32H1 is approximately twice that of 61A89, and each is weakly temperature dependent. The rate equation for the binding of exponentially growing 61A89 to wheat roots has been solved as a function of time. Theory and experiment both indicate that the binding at very short times is much less than the equilibrium values. The binding of Azotobacter vinelandii 12837 to wheat roots has also been measured. Root-associated Azotobacter fixes nitrogen, whereas under aerobic growth conditions, root-associated 61A89 and 32H1 do not. The effect of metabolic inhibitors and antibiotics on the binding of Rhizobia and Azotobacter was examined.

Atmospheric nitrogen deposition from a remote source enriches macroalgae in coral reef ecosystems near Green Turtle Cay, Abacos, Bahamas

Over the past several decades, the fixation of "new" nitrogen to the biosphere has doubled. For the early 21st century, the most significant rate increases in atmospheric nitrogen deposition are predicted for developing nations. Wet nitrogen deposition was assessed on the remote island of Green Turtle Cay, Bahamas in a dry and wet season from January to July 2000. Episodic deposition of nitrate (approximately 1-137 microM) and ammonia (approximately 2-122 microM) represented a mean deposition rate of approximately 0.2 mgDIN m(-2) yr(-1). Wet deposition of nitrogen to the climatologically-linked east coast of Florida is approximately 4 times greater than the estimated annual wet nitrogen deposition value at Green Turtle Cay, suggesting the continental US as a principal airshed for this loading source. Short-term bioassays of macroalgal productivity with a 5% rainfall solution caused depressed net productivity and increased dark respiration, well known "transient metabolic" responses by nutrient-limited tropical macroalgae. Wet deposition of inorganic nitrogen from episodic rainfall events may provide up to 20% of the "new" nitrogen necessary to meet growth demands of macroalgae on coral reefs near Green Turtle Cay.

Synthesis of nitrogenase and heterocysts by Anabaena sp. CA in the presence of high levels of ammonia

Anabaena sp. CA fails to synthesize heterocysts and nitrogenase when grown with KNO3 as the nitrogen source. By contrast, both heterocysts and proheterocysts are synthesized in NH4Cl-containing media to a level nearly commensurate with cells grown in the absence of combined nitrogen. The growth rate of the organism in NH4Cl-containing media was similar to that obtained with KNO3 as the nitrogen source and was independent of the presence of N2 in the atmosphere. Thus, our results indicate that the organism assimilated nitrate and ammonium nitrogen equally well to meet the nitrogen requirements for growth. Moreover, in contrast to previous studies with other cyanobacteria, the repressor singal for heterocyst differentiation in Anabaena sp. CA is not derived from the metabolism of ammonia but appears to be involved with nitrate metabolism. Nitrogenase activity was partially expressed in NH4Cl-grown cultures. Increasing the level of nitrogenase activity to a value representative of a N2-grown culture required both the inhibition of ammonia assimilation and de novo protein synthesis. An increase in the number of mature heterocysts was not required. The fact that high levels of exogenous ammonia only partially repress the synthesis of proteins required for the maximum expression of nitrogenase activity in Anabaena sp. CA has important implications.

Expression of Rhizobial Nitrogenase: Influence of Plant Cell-Conditioned Medium

Conditioned medium was obtained from suspension cultures of soybean ( Glycine max L. Merrit) cells after incubating them for 4 to 8 days with rhizobia which were separated from the soybean cells by two dialysis bags, one within another. This conditioned medium from the plant cell side (PCM) of the two membranes was used to elicit and influence nitrogenase activity (acetylene reduction) in rhizobia. When conditions for obtaining PCM from the soybean cell suspension cultures were varied, it could be shown that freshly grown rhizobia were able to induce active compounds in the PCM. These compounds caused acetylene reduction activity in test rhizobia under conditions where control rhizobia, containing various substrates, showed little or no acetylene reduction activity. Rhizobia that were already capable of acetylene reduction could not induce such compounds in the PCM when this was included with test rhizobia. The PCM from soybean cultures was also found to aid the expression of nitrogenase activity in suspension cultures of rhizobia normally associated with either peas, lupins, broad beans, or clovers. This is the first communication indicating nitrogenase activity in freeliving cultures for various species of rhizobia.