Lignified Biomass Utilization: the Lignin Challenge

The major potential source of renewable energy/biofuels is that from plant biomass, for example through fermentation of polymeric carbohydrates to provide bioethanol. In this context, bioethanol production levels in the United States have steadily grown over the last decade, from approximately 1.4 to 4.26 billion gallons between 1995 and 2005 (Henniges and Zeddies, 2006), with this predominantly being obtained from the partial fermentation of corn. Yet this represents only approximately 4% of the current U.S. annual gasoline consumption (~100 billion gallons) and 7% of that needed (60 billion gallons) by 2030. There are two major scientific hurdles, however, that have not been technically overcome for the facile utilization of this and other plant renewable resources, both of which involve the polymeric lignins.

The first results from their intractable nature, since lignin removal has long been a limitation in the processing of wood both for pulp/paper manufacture and for forage digestibility by ruminants. This is largely due to the lack of isolated enzymes and/or proteins that can efficiently degrade lignin macromolecules, in contrast to reports in the 1980s that indicated that this problem had been solved (Glenn et al., 1983; Kirk et al., 1986; Tien and Kirk, 1983; Tien and Tu, 1987). That is, nearly 20 years ago, it was reported that several productive routes for lignin removal from wood had been both discovered and attained via utilization of lignin-degrading enzymes in fungi/bacteria, and where three candidates ultimately emerged (lignin peroxidase, manganese peroxidase, and laccase). However, this ‘‘lignin peroxidase’’ or ‘‘ligninase’’ (Tien and Tu, 1987) was assayed initially only with an aqueous acetone extract of spruce wood (Tien and Kirk, 1983), which does not actually extract the lignins from wood. Twenty years later, none of these enzymes is (routinely) utilized in biotechnological applications for lignin removal/separation, and their roles in enzymatic lignin biodegradation are still in question, as we had noted earlier (Sarkanen et al., 1991). Today, more than 50 million tons of lignin-derived substances are generated annually as byproducts of pulp/paper manufacture within the United States alone (Committee on Biobased Industrial Products; Board on Biology; Commission on Life Sciences; National Research Council, 2000). Interestingly, other possibilities now perhaps considered as being more likely to be useful are putative true lignin depolymerases targeting specific interunit linkages in lignin macromolecules (Chen et al., 2001).

From a structural perspective, the lignins, nature’s second most abundant organic substances after cellulose, are amorphous cell wall polymers that make up approximately 20–30% of all plant stem biomass (Lewis and Yamamoto, 1990; Lewis et al., 1999). More specifically, vascular plant species have different lignin contents, with values ranging from approximately 30% in conifers (softwoods) to lower amounts (~20–25%) in hardwoods (such as poplar) and herbaceous species, to even smaller levels in various ‘‘primitive’’ plant species. The physiological roles of lignins are to engender structural support to the vascular apparatus, thereby enabling such organisms to stand upright, as well as providing conduits for water and nutrient transport, and to provide physical barriers against opportunistic pathogens. It is currently not known, however, what actual (i.e., minimal) lignin contents and/or compositions are needed for a particular plant to avoid any deleterious effects for growth/development/stem structural integrity, etc.

The second technological hurdle is that lignins cannot readily be converted into either ethanol and/or other liquid/gaseous fuels using currently available fermentation processes. Indeed, the polymeric lignins themselves are a formidable physical barrier to an efficient fermentation of carbohydrate biomass for ethanol generation, and thus their presence represents a critical problem in making these technologies more economical. Therefore, an approach whereby the carbon allocated toward lignification is redirected, resulting in inherently useful and/or more easily tractable materials, could potentially facilitate the generation of biofuels from the remaining plant biomass. One such strategy would be the generation in planta of allyl/propenyl phenols, such as eugenol and chavicol. In addition, these liquid/combustible phenolic products could themselves be potentially utilized for (nonethanol) biofuel/bioenergy purposes.