Effect of oxygen on formation and structure of Azotobacter vinelandii alginate and its role in protecting nitrogenase

The activity of nitrogenase in the nitrogen-fixing bacterium Azotobacter vi nelandii grown diazotrophically under aerobic conditions is generally consi dered to be protected against O-2 by a high respiration rate. In this work, we have shown that a high rate of respiration is not the prevailing mechan ism for nitrogenase protection in.-l. vinelandii grown in phosphate-limited nitrogen-free chemostat culture. Instead, the formation of alginate appear ed to play a decisive role in protecting the nitrogenase that is required f or cell growth in this culture. Depending on the O-2 tension and cell growt h rate, the formation rate and composition of alginate released into the cu lture broth varied significantly. Furthermore, transmission electron micros copic analysis of cell morphology and the cell surface revealed the existen ce of an alginate capsule on the surface of A. vinelandii. The composition, thickness, and compactness of this alginate capsule also varied significan tly. In general, increasing O-2 tension led to the formation of alginate wi th a higher molecular weight and a greater L-guluronic acid content. The al ginate capsule was accordingly thicker and more compact. In addition, the f ormation of the alginate capsule was found to be strongly affected by the s hear rate in a bioreactor, Based on these experimental results, it is sugge sted that the production of alginate, especially the formation of an algina te capsule on the cell surface, forms an effective barrier for O-2 transfer into the cell. It is obviously the quality, not the quantity, of alginate that is decisive for the protection of nitrogenase.