Comparing in planta accumulation with microbial routes to set targets for a cost-competitive bioeconomy

Abstract

Plants and microbes share common metabolic pathways for producing a range of bioproducts that are potentially foundational to the future bioeconomy. However, in planta accumulation and microbial production of bioproducts have never been systematically compared on an economic basis to identify optimal routes of production. A detailed technoeconomic analysis of four exemplar compounds (4-hydroxybenzoic acid [4-HBA], catechol, muconic acid, and 2-pyrone-4,6-dicarboxylic acid [PDC]) is conducted with the highest reported yields and accumulation rates to identify economically advantaged platforms and breakeven targets for plants and microbes. The results indicate that in planta mass accumulation ranging from 0.1 to 0.3 dry weight % (dwt%) can achieve costs comparable to microbial routes operating at 40 to 55% of maximum theoretical yields. These yields and accumulation rates are sufficient to be cost competitive if the products are sold at market prices consistent with specialty chemicals ($20 to $50/kg). Prices consistent with commodity chemicals will require an order-of-magnitude-greater accumulation rate for plants and/or yields nearing theoretical maxima for microbial production platforms. This comparative analysis revealed that the demonstrated accumulation rates of 4-HBA (3.2 dwt%) and PDC (3.0 dwt%) in engineered plants vastly outperform microbial routes, even if microbial platforms were to reach theoretical maximum yields. Their recovery and sale as part of a lignocellulosic biorefinery could enable biofuel prices to be competitive with petroleum. Muconic acid and catechol, in contrast, are currently more attractive when produced microbially using a sugar feedstock. Ultimately, both platforms can play an important role in replacing fossil-derived products.

Keywords: bioeconomy; in planta accumulation; microbial production; technoeconomic analysis (TEA); value-added bioproduct.

Fig. 1.

MSPs obtained from plant systems (highest demonstrated accumulation) and microbial routes (demonstrated and maximum theoretical yield scenarios). In the plant systems, three ethanol selling prices are assumed: 1) ethanol produced in the integrated biorefinery is sold at $1.44/LGE, as quantified in the base case model; 2) ethanol is sold at the target fuel selling price of $0.66/LGE ($2.50/gal gasoline equivalent), as set by the U.S. DOE (24); and 3) ethanol selling price is equivalent to the 1940 to 2020 historical average U.S. gasoline rack sales price ($0.40/LGE or $1.53/gal of gasoline) (25). In the microbial system, the demonstrated case is developed based on the state-of-the-art reported yield for these bioproducts, and the theoretical case is built assuming the maximum theoretical yield. Detailed input parameters used for technoeconomic modeling are documented in SI Appendix, Table S1. Numerical results are listed in SI Appendix, Table S2.

Fig. 2.

Comparison of MSPs ($/kg) between in planta accumulation and microbial routes to bioproducts, using the base case ethanol selling price ($1.44/LGE) for lignocellulosic biorefineries that convert residual biomass after bioproduct extraction from plants. The solid circles represent the highest reported mass accumulation rates in plants, and the empty circles represent the corresponding yields required to reach cost parity using microbial routes for the same products. The solid triangles indicate the highest reported yields as a fraction of maximum theoretical yields in microbial systems. For catechol, 4-HBA, and muconic acid, the theoretical maximum yield is determined by flux analysis; no theoretical maximum yield has been reported for PDC using flux analysis, so we use reaction stoichiometry to determine its theoretical maximum. The empty triangles represent in planta mass accumulation rates needed to reach cost parity with the best-reported microbial yields for each bioproduct. Breakeven microbial production yields for 4-HBA and PDC exceed the theoretical maximum and, therefore, are not shown. (Inset) The in planta accumulation rates for muconic acid and catechol, which are less than 0.08 dwt%.

Fig. 3.

Bioproduct selling price ($/kg) versus mass accumulation rate (dwt%) in engineered biomass sorghum utilized as feedstock in lignocellulosic biorefineries. The bar on the right side of each subplot shows the MESP ($/LGE) obtained in lignocellulosic biorefineries. The dot symbol in each subplot is the bioproduct selling price at each mass accumulation rate for reaching cost parity with the base case biorefineries developed in this study ($1.44/LGE, as calculated in our base case model). The “x” in each subplot refers to the product selling price and mass accumulation rate required to reach a target ethanol selling price ($0.66/LGE [24]). The plus symbol in each plot corresponds to a product selling price and mass accumulation rate needed for ethanol to reach parity with the U.S. long-term historical gasoline rack price from 1940 to 2020 ($0.40/LGE [25]).

Fig. 4.

Simplified process diagrams for bioproduct production in plant systems and microbial systems. In plant systems, the bioproduct is accumulated in the engineered biomass sorghum and then extracted onsite before the remaining biomass sorghum is used to produce ethanol.