Microbial Utilization of Hydrothermally Treated Algae Biomass Aqueous Co-Product

Hydrothermal liquefaction is a promising technology for producing viable bio-fuels and research is underway around the world looking into its potential use on several feedstocks, including algae.  However, the majority of recent studies focus primarily on the characteristics and potential uses of the organic “bio-crude oil” product of hydrothermal treatment, while the aqueous phase of the product remains largely uninvestigated.  While not directly upgradable to usable fuel, this aqueous co-product contains a significant fraction of the initial carbon of the biomass feedstock and the majority of other components such as nitrogen and phosphorous.  Disposal of this material as a waste stream would be an energetic and economic burden on a large-scale process, both from a nutrient loss and waste water processing standpoint.   Efficient utilization of this co-product is a key factor in the overall sustainability of the process. This work investigates the composition of hydrothermally treated aqueous algae product (AqAl) and its utility as a media component for bacterial cell culture production.

There are several assumed uses for AqAl, such as animal feed and fertilizer supplements, that have been mentioned in literature, but few have actually been investigated in detail. Recycling AqAl as a media component for algae feedstock growth is one potential method for utilizing this product and has been investigated recently.  However, algae growth inhibition was observed at dilute levels of AqAl concentration.  An alternate AqAl utilization method is to use it as a carbon and nutrient source for model heterotrophic organisms such as Escherichia coli and Pseudomonas putida.  While genetic manipulation of algae is in its infancy, there is a wealth of genetic engineering technology established for these model organisms.  These microbes could potentially be designed to provide an assortment of functions for an algae-based fuel process, such as bio-flocculant or additional biomass production. However, before any beneficial functionality can be obtained from these organisms, they must first be engineered to efficiently metabolize the components of AqAl.  This task is investigated here.    

Two model organisms, Escherichia coli and Pseudomonas putida, were utilized in this study.  Their growth characteristics were observed on various formulations of minimal media made with AqAl.  The AqAl was derived from the hydrothermal treatment of a Nannochloropsis algae strain.  Chemical analysis and high-performance liquid chromatography (HPLC) were used to quantify the components of the AqAl.  Carbon sources (particularly organic acids), ammonia, total organic carbon and total nitrogen in the media were measured before and after cell cultivation to determine uptake selectivity and efficiency.  It was shown that these organisms can grow using AqAl as the only carbon and nitrogen source, and optimal growth conditions were found.  These results will serve as a basis for the genetic manipulation of these microbes for the benefit of an algae-based bio-fuel process.