Neutron Technologies for Bioenergy Research

Production of deuterated biomass by cultivation of Lemna minor (duckweed) in D2O

Common duckweed Lemna minor was cultivated in 50% D₂O to produce biomass with 50-60% deuterium incorporation containing cellulose with degree of polymerization close (85%) to that of H₂O-grown controls. The small aquatic plant duckweed, particularly the genus Lemna, widely used for toxicity testing, has been proposed as a potential source of biomass for conversion into biofuels as well as a platform for production of pharmaceuticals and specialty chemicals. Ability to produce deuterium-substituted duckweed can potentially extend the range of useful products as well as assist process improvement. Cultivation of these plants under deuterating conditions was previously been reported to require addition of kinetin to induce growth and was hampered by anomalies in cellular morphology and protein metabolism. Here, we report the production of biomass with 50-60% deuterium incorporation by long-term photoheterotrophic growth of common duckweed Lemna minor in 50% D₂O with 0.5% glucose. L. minor grown in 50% D₂O without addition of kinetin exhibited a lag phase twice that of H₂O-grown controls, before start of log phase growth at 40% of control rates. Compared to continuous white fluorescent light, growth rates increased fivefold for H₂O and twofold for 50% D₂O when plants were illuminated at higher intensity with a metal halide lamp and a diurnal cycle of 12-h light/12-h dark. Deuterium incorporation was determined by a combination of ¹H and ²H nuclear magnetic resonance (NMR) to be 40-60%. The cellulose from the deuterated plants had an average-number degree of polymerization (DP_n) and polydispersity index (PDI) close to that of H₂O-grown controls, while Klason lignin content was reduced. The only major gross morphological change noted was root inhibition.

Ultrastructure and Enzymatic Hydrolysis of Deuterated Switchgrass

Neutron scattering of deuterated plants can provide fundamental insight into the structure of lignocellulosics in plant cell walls and its deconstruction by pretreatment and enzymes. Such plants need to be characterized for any alterations to lignocellulosic structure caused by growth in deuterated media. Here we show that glucose yields from enzymatic hydrolysis at lower enzyme loading were 35% and 30% for untreated deuterated and protiated switchgrass, respectively. Lignin content was 4% higher in deuterated switchgrass but there were no significant lignin structural differences. Transmission electron microscopy showed differences in lignin distribution and packing of fibers in the cell walls that apparently increased surface area of cellulose in deuterated switchgrass, increasing cellulose accessibility and lowering its recalcitrance. These differences in lignification were likely caused by abiotic stress due to growth in deuterated media.

Development of approaches for deuterium incorporation in plants

Soon after the discovery of deuterium, efforts to utilize this stable isotope of hydrogen for labeling of plants began and have proven successful for natural abundance to 20% enrichment. However, isotopic labeling with deuterium ((2)H) in higher plants at the level of 40% and higher is complicated by both physiological responses, particularly water exchange through transpiration, and inhibitory effects of D2O on germination, rooting, and growth. The highest incorporation of 40-50% had been reported for photoheterotrophic cultivation of the duckweed Lemna. Higher substitution is desirable for certain applications using neutron scattering and nuclear magnetic resonance (NMR) techniques. (1)H(2)H NMR and mass spectroscopy are standard methods frequently used for determination of location and amount of deuterium substitution. The changes in infrared (IR) absorption observed for H to D substitution in hydroxyl and alkyl groups provide rapid initial evaluation of incorporation. Short-term experiments with cold-tolerant annual grasses can be carried out in enclosed growth containers to evaluate incorporation. Growth in individual chambers under continuous air perfusion with dried sterile-filtered air enables long-term cultivation of multiple plants at different D2O concentrations. Vegetative propagation from cuttings extends capabilities to species with low germination rates. Cultivation in 50% D2O of annual ryegrass and switchgrass following establishment of roots by growth in H2O produces samples with normal morphology and 30-40% deuterium incorporation in the biomass. Winter grain rye (Secale cereale) was found to efficiently incorporate deuterium by photosynthetic fixation from 50% D2O but did not incorporate deuterated phenylalanine-d8 from the growth medium.

Production of deuterated switchgrass by hydroponic cultivation

The bioenergy crop switchgrass was grown hydroponically from tiller cuttings in 50 % D 2 O to obtain biomass with 34 % deuterium substitution and physicochemical properties similar to those of H 2 O-grown switchgrass controls. Deuterium enrichment of biological materials can potentially enable expanded experimental use of small angle neutron scattering (SANS) to investigate molecular structural transitions of complex systems such as plant cell walls. Two key advances have been made that facilitate cultivation of switchgrass, an important forage and biofuel crop, for controlled isotopic enrichment: (1) perfusion system with individual chambers and (2) hydroponic growth from tiller cuttings. Plants were grown and maintained for several months with periodic harvest. Photosynthetic activity was monitored by measurement of CO2 in outflow from the growth chambers. Plant morphology and composition appeared normal compared to matched controls grown with H2O. Using this improved method, gram quantities of switchgrass leaves and stems were produced by continuous hydroponic cultivation using growth medium consisting of basal mineral salts in 50 % D2O. Deuterium incorporation was confirmed by detection of the O-D and C-D stretching peaks with FTIR and quantified by (1)H- and (2)H-NMR. This capability to produce deuterated lignocellulosic biomass under controlled conditions will enhance investigation of cell wall structure and its deconstruction by neutron scattering and NMR techniques.

Why genetic modification of lignin leads to low-recalcitrance biomass

Molecular dynamics simulations show genetically modified lignin to associate less with hemicellulose than does wild type.

Advances in solid-state NMR of cellulose

Nuclear magnetic resonance (NMR) spectroscopy is a well-established analytical and enabling technology in biofuel research. Over the past few decades, lignocellulosic biomass and its conversion to supplement or displace non-renewable feedstocks has attracted increasing interest. The application of solid-state NMR spectroscopy has long been seen as an important tool in the study of cellulose and lignocellulose structure, biosynthesis, and deconstruction, especially considering the limited number of effective solvent systems and the significance of plant cell wall three-dimensional microstructure and component interaction to conversion yield and rate profiles. This article reviews common and recent applications of solid-state NMR spectroscopy methods that provide insight into the structural and dynamic processes of cellulose that control bulk properties and biofuel conversion.