Microaerophilic production of alginate by Azotobacter vinelandii

In this study, growth characteristics of the nitrogen fixing bacterium Azotobacter vinelandii as well as some factors controlling the alginate production by this strain were quantitatively investigated. From a quantitative examination of growth parameters, respiration and morphological changes, relationships between the different essential processes, i.e. nitrogen fixation, respiration intensity, PHB biosynthesis and alginate formation, were established. The first part of this study was focused on the production of alginate under controlled-microaerophilic conditions to examine the suitable oxygen concentration profile in respect to quantitative alginate formation for this strain. Testing a pO2 range of 0-10 air saturation, showed that intermediate pO2 values (2.5-5) were optimal for alginate and biomass production. Above and below these values, the bacterium either wasted the carbon and energy source in respiration (CO2) or in PHB biosynthesis (44-58 of the biomass). Thus, efficient conversion of carbon source to alginate is achieved only if the DOT is accurately controlled at an intermediate value. To lower the extreme O2 sensitivity, different nitrogenous rich compounds were added to the mineral medium. Surprisingly, the cells exhibited a pronounced oxygen sensitivity even in the presence of complex nitrogenous materials. Furthermore, higher alginate yield (Yalg/X) was obtained in nitrogen free medium, and for the purpose of medium simplicity the addition of nitrogenous compounds was not considered. To gain more insights toward a better understanding of the inhibitory effect of oxygen towards nitrogenase and its relation to alginate biosynthesis, respiratory protection - one of the two mechanisms for nitrogenase protection - was impaired through phosphate limitation. This was performed in a pO2 controlled bioreactor (2.5-3) as well as in shaked flasks.