Microbial Utilization of Aqueous Co-Products from Hydrothermal Liquefaction of Microalgae Nannochloropsis oculata.

Abstract

Algae biomass is a promising source of liquid biofuels. Hydrothermal liquefaction can convert wet algae biomass into a “biocrude” oil with similar properties to crude petroleum, but it also produces an aqueous co-product (AqAl) that contains most of the nitrogen and phosphorus from the initial biomass and up to 40% of the carbon. Efficient recycle of these components within the system is crucial, yet direct feeding of AqAl back to algae ponds has proven problematic. In this work, we investigate the use of an intermediate bacterial culture to initially utilize this challenging product. In the first part, we investigate the properties of products formed from Nannochloropsis oculata algae biomass processed under a variety of hydrothermal reaction conditions. We then use AqAl as the sole C/N/P source for bacterial culturability studies on bacteria Escherichia coli and Pseudomonas putida. We determine that bacterial culture is feasible and the bacterial strains studied can utilize up to 43% of the organic carbon present. However, there is an inhibitory effect imposed by the AqAl. In the second part, we improve the bacterial strains’ growth phenotypes through adaptive evolution in media of increasing AqAl concentration. Evolved E. coli and P. putida strains can grow at rates 104% and 260% faster and reach cell concentrations 24% and 61% higher, respectively, in AqAl media. The full genomes of improved strains are determined and mutations conferring benefits are identified. Some of these mutations are individually introduced to E. coli to investigate their specific effects. Also, gene expression levels between a top-performing evolved strain of E. coli and its parent strain are compared. Finally, we investigate the utility of a microbial processing step on an integrated algae biofuel process in two ways. First, we investigate the possibility of using microbial biomass as a supplemental feedstock for hydrothermal reactions, observing oil yields upwards of 75% of that of algae. Second, we investigate the effects bacterial pre-culture has on AqAl pertaining to its suitability as an algae growth medium component. We conclude that an intermediate bacterial culture could contribute to the overall efficiency and sustainability of an algae biofuel process.