(627b) Algal Culture with a Nitrogen Fixing Bacterium for Sustainable Biomass Production

Authors: 
Barney, B. M., University of Minnesota

Macronutrients such as nitrogen and phosphorus are directly tied to biomass yields. There is currently a significant and growing interest in next generation biofuel targets such as algae, which promise higher biomass yields in combination with increased biofuel production. Yet, these increased biomass yields for algae require an increase demand for certain nutrients. In conventional crops, yield increases over the past few decades are highly correlated to increased application of nitrogen fertilizers derived from the Haber-Bosch industrial process. This energy-intense nitrogen-fixing process requires substantial inputs of pressure and heat, often tied to natural gas consumption. Thus, the realization of improved yields of algal biomass is likely to be accompanied by an increased demand for Haber-Bosch derived nitrogen. As a result, a portion of the energy obtained thought the production of algal biofuels will need to be diverted back to Haber-Bosch processes in order to sustain the accompanying biomass.

Though the increased demand for industrially produced nitrogen would significantly impact the overall economics of algal biofuel production, nature provides us with examples from current agricultural practices where symbiotic relationships can eliminate the need for externally provided nitrogen. Model relationships between plants such as legumes and the nitrogen-fixing bacteria that are associated with them can provide a blueprint for methods of developing similar relationships with algae. However, while legumes have evolved these relationships in a manner that supports the symbiosis, this is accomplished through compartmentalization, where nitrogen fixation is generally done in a low oxygen environment of the roots, while oxygen-producing photosynthesis is isolated to pigmented tissues. A similar system may be difficult to recreate in large-scale cultures with microalgae.

Studies in our laboratory focus on the aerobic nitrogen-fixing bacterium Azotobacter vinelandii, which is a unique and model organism for nitrogen fixation. We have recently made progress toward the development of alternative routes to increase nitrogen production by this bacterium through the application of broad screens, developing terminal nitrogen products and deregulating nitrogen fixation. We have further demonstrated the successful culture of this bacterium with various algae strains by providing a supplemental source of sugar to support the nitrogen fixation in A. vinelandii. Many algae are known to produce extracellular sugars and polysaccharides, and our current efforts aim to increase carbohydrate production by the algae during photoautotrophic growth, to establish the mutually beneficial relationship that will support both species.