Haloalkaline Bioconversions for Methane Production from Microalgae Grown on Sunlight

Microalgal biomass can be converted to biofuels to replace nonsustainable fossil fuels, but the widespread use of microalgal biofuels remains hampered by the high energetic and monetary costs related to carbon dioxide supply and downstream processing. Growing microalgae in mixed culture biofilms reduces energy demands for mixing, maintaining axenic conditions, and biomass concentration. Furthermore, maintaining a high pH improves carbon dioxide absorption rates and inorganic carbon solubility, thus overcoming the carbon limitation and increasing the volumetric productivity of the microalgal biomass. Digesting the microalgal biomass anaerobically at high pH results in biogas that is enriched in methane, while the dissolved carbon dioxide is recycled to the phototrophic reactor. All of the required haloalkaline conversions are known in nature.

TrendsExtreme habitats such as soda lakes host organisms that cycle nitrogen, sulfur, and carbon while maintaining their integrity in a hostile environment. These organisms have great potential for biotechnological applications where extreme conditions such as high pH and salinity have an engineering advantage.When microorganisms are subjected to cyclical regimens in which the supply of carbon and the supply of nutrients are uncoupled, only organisms that are able to accumulate storage lipids or sugars are able to survive. This selective pressure has been successfully applied to mixed-culture bioreactors to enrich organisms that accumulate desired compounds.The biomass productivity of phototrophic microalgae in attached-growth systems (biofilms) is similar to that of suspended-growth systems but biofilms have the added advantage that the biomass is already more concentrated.