Scaling up bioethanol production from the farmed brown macroalga Macrocystis pyrifera in Chile

Abstract

Interest in third-generation biomass such as macroalgae has increased due to their high biomass yield, absence of lignin in their tissues, lower competition for land and fresh water, no fertilization requirements, and efficient CO2 capture in coastal ecosystems. However, several challenges still exist in the development of cost-effective technologies for processing large amounts of macroalgae. Recently, genetically modified micro-organisms able to convert brown macroalgae carbohydrates into bioethanol were developed, but still no attempt to scale up production has been proposed. Based on a giant kelp (Macrocystis pyrifera) farming and bioethanol production program carried out in Chile, we were able to test and adapt this technology as a first attempt to scale up this process using a 75 L fermentation of genetically modified Escherichia coli. Laboratory fermentation tests results showed that although biomass growth and yield are not greatly affected by the alginate:mannitol ratio, ethanol yield showed a clear maximum around a 5:8 alginate:mannitol ratio. In M. pyrifera, a much greater proportion of alginate and lower mannitol abundance is found. In order to make the most of the carbohydrates available for fermentation, we developed a four-stage process model for scaling up, including acid leaching, depolymerization, saccharification, and fermentation steps. Using this process, we obtained 0.213 Kg ethanol Kg(-1) dry macroalgae, equivalent to 9.6 m(3) of ethanol hectare(-1) year(-1), reaching 64% of the maximum theoretical ethanol yield. We propose strategies to increase this yield, including synthetic biology pathway engineering approaches and process optimization targets.