Role of Microorganisms in Lignocellulosic Biodegradation

Introduction

Petroleum-based refineries have been the way to produce fuels and chemicals for decades due to the low cost of fossil fuel. However, depletion of fossil fuel reserves, increase in hazardous waste generation, and global warming have led to extensive research into the production of alternative liquid transportation fuels from renewable resources. The US Department of Energy has set a target for a 20 % reduction in the 2007 gasoline use predominantly through the use of biofuels (US DOE Biomass Multi-Year Program, 2008) and 30 % of 2006 crude oil demand by 2030 (Foust et al. 2007) while the EU has mandated that biofuel will account for 10 % of transportation fuel requirement by 2020 (Trostle 2008) and 25 % by 2030 (Himmel et al. 2007). Lignocellulosic biomass has been predicted to replace the use of corn and sugarcane for biofuel production as these food crops has conflicting uses. Lignocellulosic biomass is an excellent source of biofuel production as it is very rich source of fermentable sugars (75 %) (Bayer et al. 2007) and further is present in large abundance. The most promising lignocellulosic feedstocks for biofuel production in the United States, South America, Europe, and Asia are corn stover, sugarcane bagasse, wheat straw, and rice straw, respectively (Kadam and McMillan 2003; Kim and Dale 2004; Knauf and Moniruzzaman 2004; Cheng et al. 2008).

Bioconversion of lignocellulosic biomass is done by pretreatment, enzymatic hydrolysis followed by fermentation. Lignocellulosic biomass is composed of mainly cellulose, hemicellulose, and lignin in an intricate structure which is recalcitrant to enzymatic degradation. Table 2.1 includes some examples of lignocellulosic biomass. The best way to break the rigid structure of lignin and make the polysaccharides amendable to enzymes is through pretreatment of biomass which is conducted at elevated temperature, pressure, with or without chemicals.

Biocatalysis is the biochemical platform for bioconversion of lignocellulosic biomass to biofuel. However, high cost of enzymes is a major bottleneck in production of biofuels at industrial scale. Therefore, sustainable production of low-cost © The Author(s) 2017

V. Rana, D. Rana, Renewable Biofuels, SpringerBriefs in Applied Sciences and Technology, DOI 10.1007/978-3-319-47379-6_2

Table 2.1 Approximate composition (% dry wt) of various lignocellulosic feedstocks

Composition (% dry wt)

Lignocellulosic residue

Cellulose

Hemicellulose

Lignin

References

Agricultural residues

Rice straw

29.2-39

15-25.9

10-19

Brylev et al. (2001). Merino and Cherry (2007). and Prasad et al. (2007)

Wheat straw

33-39

22-30

12-25.5

Merino and Cherry (2007). Prasad et al. (2007). and Kootstra et al. (2009)

Sugarcane bagasse

25^15

25-32

12.2-25

Dijkerman et al. (1997). Singh et al. (2009). and Alves et al. (2010)

Com stover

35.1-39.5

19.1-24.6

11-19.1

Lee (1997) and Mosieret al. (2005)

Rapeseed stover

27.6

20.2

18.3

Li et al. (2009)

Oilseed rape

27.3

20.5

14.2

Peterssonet al. (2007)

Oat straw

27-35

20-37

10-19

Rowell et al. (1992) and Rowell Roger (1992)

Com cob

32.3^45.6

35-39.8

6.7-15

McKendry (2002). Howard et al. (2003). and Prasad et al. (2007)

Rice husk

24-36.1

12-29.3

11-20

Okeke and Obi (1994) and Abbas and Ansumali (2010)

Wheat bran

10.5-14.8

35.5-39.2

8.3-12.5

Miron et al. (2001)

Sorghum straw

32-35

24-27

15-21

Herrera et al. (2003) and Vazquez et al. (2007)

Cotton stalk

31

11

30

Rubio et al. (1998)

Cotton seed hairs

80-95

5-20

0

Howard et al. (2003)

Nut shells

25-30

22-30

30^10

Sinner et al. (1979) and Howard et al. (2003)

Bamboo

49-50

18-20

23

Alves et al. (2010)

Rye straw

33-35

27-30

16-19

Rowell Roger (1992) and Stewart et al. (1997)

Jute fibers

45-53

18-21

21-26

Mosihuzzaman et al. (1989)

Barley straw

31^13

24-33

6.3-15

Rowell Roger (1992) and Garcfa-Aparicio et al. (2006)

Switchgrass

25^15

22-31.4

12-20

Howard et al. (2003) and Merino and Cherry (2007)

Alfalfa

21.8

12.4

9.7

Dijkerman et al. (1997)

Woody biomass

Loblolly pine

35

16.8

29

Rana et al. (2012)

Lodgepole pine

44.9

22.6

26.8

Pan et al. (2008)

Monterey pine

41.7

20.5

25.9

Merino and Cherry (2007)

Douglas fir

44.6

19.4

31.5

Pan et al. (2008)

Hybrid poplar

40

22

24

Merino and Cherry (2007)

Willow

49.3

14.1

20

Bridgeman et al. (2008)

Hardwood stems

40-55

24^10

18-25

Malherbe and Cloete (2002) and Howard et al. (2003)

Softwood stems

45-50

25-35

25-35

Malherbe and Cloete (2002) and Howard et al. (2003)

Eucalyptus

45-51

11-18

29

Morais and Pereira (2012)

Municipal solid waste

Swine waste

6

28

NA

Howard et al. (2003)

Solid cattle manure

1.6—4.7

1.4-3.3

2.1-5.1

Howard et al. (2003)

Waste paper from chemical pulp

60-70

10-20

5-10

Howard et al. (2003)

Primary waste water solids

8-15

NA

24-29

Howard et al. (2003)

Banana waste

13.2

14.8

14

Monsalve et al. (2006)

and highly efficient enzyme complex is important as a research field. Current biorefineries are dependent on commercially produced cellulase for converting lignocellulosic biomass to fermentable sugars, but their high cost is often prohibitive for making a commercial biofuel production.

There are three categories of lignocellulose-degrading enzymes: cellulase, hemi- cellulase, and ligninase. The synergistic action of these enzymes plays a pivotal role in the hydrolysis of lignocellulose. The desired characteristics of enzymes for bioconversion of lignocellulosic biomass are high specific activity on biomass; high yield with complex substrate such as biomass; high thermostability; resistance to pH and shear tolerant; decreased susceptibility to enzyme inhibition by end products such as glucose, cellobiose, and xylooligomers; selective adsorption on cellulose; and synergy between different enzymes (Knauf and Moniruzzaman 2004; Maki et al. 2009; Viikari et al. 2012). These attributes can be achieved by genetic manipulation, protein engineering, and selection of cellulase-producing robust microbial strains. Other strategies to improve the yield and rate of enzymatic hydrolysis are optimization of enzyme cocktail, ratio of individual enzymes in enzyme mixture, substrate concentration, and reuse of enzymes by recycling or recovery. Another research focus has been on finding extremophilic microorganisms producing enzymes that can tolerate acid, alkali, and high heat (Miller and Blum 2010; de Carvalho 2011). Despite extensive research in the past decades, the enzyme hydrolysis step still remains a major techno-economic bottleneck in lignocellulose conversion to ethanol.

 
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