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Biodegradation of lignocellulose in soil: basic understanding of degradation mechanism.
Abstract:
"In all things of nature there is something of the marvelous.”—Aristotle.
Soil is a natural reservoir for all kinds of living being which controls the biogeochemical cycles
through the regenerative and degradative process. Microorganisms living in the soil environment are
responsible for moderating the microenvironment by their enzymatic activity. The enzymes released
by the microorganism not only affect the biomass directly but also indirectly act as inhibitors and
activators for other microorganisms. These groups of organisms have the ability to modify the
microenvironment as the enzymes and other byproducts’ released due to the metabolic activity effects
the temperature, pH and other elements which act as key enhancers for degradative reactions.
Basically, lignocellulosic materials are the plant cell wall composites which is composed of 40-55% of
cellulose, 24-40 % of hemicelluloses and lignin of about 18-25 % in hardwood (Howard et.al
,2003).In wheat straw the amount of lignin is about 18-20%(Tuomela et al ,1999)and about 25-35% in
soft wood(Nausbaumer et al ,1996).Understanding the lignocellulosic breakdown in the soil system
could be a valuable source which can be of great benefit for the utilization of lignocellulosic materials
to extract the valuable chemicals for the welfare of human beings. This paper discusses about the
current knowledge on the soil biodegradation system particularly in the lignin degradation. It can be
important for improvement of the biological pretreatment of the lignocellulosic biomass for making
Biofuel and chemicals.
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1. Introduction:
As the cellulose is protected by the lignin and hemicelluloses matrix the disruption of these
would release the C-6 sugars which can be used for the production of Biofuels.Pretreatment is
the process of releasing the cellulosic sugars by using various technologies such as Physical,
physico-chemical, chemical, and biological processes have been used for pretreatment of lignocellulosic (paper 2
)materials Acid treatment, Ammonia fiber/freeze explosion treatment, Liquid hot water treatment,
Lime pretreatment etc. The Acid, Lime and AFEX pretreatment technologies remove lignin
significantly, decrystallizing the cellulose which can be utilized for the Biethaol production.
(N.Mosier et al, 2005).During the acid treatment dilute sulfuric acid is mostly used (Grohmann
et al, 1985).Alkali pretreatment where is lime is mostly used for wheat straw pretreatment
(Chang et al,) For the AFEX pretreatment, it is suitable for only hardwood rather than soft wood
(McMillan,1994) (N.Mosier et al, 2005). All the present technologies which are being used are
mostly thermo chemical technologies; though the pretreatment is efficient enough to release the
cellulose sugars significantly the removed lignin and hemicelluose are not utilized as beneficial
by products thus the technology is not being cost effective. Biochemical research has also
focused on the use of enzymes (cellulases) produced by bacteria or fungi (e.g., Trichoderma
reesei) to hydrolyze cellulose (Hamelinck et al., 2005). The present scope lies mainly in
pretreatment technology which forms the key to cost-effective bioethanol production in the long
term. Lignin is a structure polymer of the vascular plants which helps in protecting the plant cell
wall .Degradation of this lignin compound is a major task because covalent lignin carbohydrate
linkage prevents the enzymatic degradation of lignocellulose and this is due to the fact that lignin
can depolymerize to oxidative units or co –polymerize and form complex aromatic structure.
During the process of natural degradation all the complex materials in nature degrade into the
elemental forms through the process of composting. The net desired compost is a result of micro
environmental factors such as variations in temperature, PH, pressure, and also due to the
microbial interactions which have a direct or indirect effect on the enzyme production system of
the microorganism. Diversity in soil exceeds beyond that of eukaryotic organisms, they play a
very important role during nutrient cycle apart from their role in formation of soil aggregates,
and they form the major contributors’ in the complex interlinked food webs. These have the
potential to degrade any complex organic substance into simpler and more natural elemental
forms in nature. There potential for degradation can be observed in the cases where soil
microcosm has shown to degrade soil applied pesticides. The microbe could degrade the
chemical before the chemical showed its effect- enhanced degradation.
In the current context, bioethanol production by using the cellulosic (source of C-6 sugars) and
hemicellosic (source for C-5 sugars) components, useful organic components from the lignin
such as for making bioplastics, dispersants in cement industry, additives in agricultural
chemicals, textile dyes, carbon fibers (Howard R.L, et al, 2003).However, breaking the chemical
barrier of the lignocellulosic material is a great challenge. At the same time, lignocellulosic
materials are the cheapest materials which can be converted in to the useful sugar components.
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Therefore, the study in the lignocellulosic biodegradation system is important. According to the
latest survey made by Thomson Reuters (2009), the U.S. ethanol consumption is forecast to
increase from 5.6 billion gallons last year to 13.5 billion gallons in 2012, far more than the 7.5
billion gallons in 2012 originally estimated. The carbon dioxide emissions estimated increase
from 5,890 million tons in 2006 to 7,373 million tons in 2030. When there is such rapid increase
in the demand for Biofuel, the need for a breakthrough is necessary to meet the demands and
reduce the green house gas emissions (These are the gases responsible for the global warming).
I. Soil as a potential source for a novel pretreatment technology.
Soil consists of different kinds of elements which support all kinds of life forms. For plant growth, it has
14 elements which are considered to be essential because they are absorbed by plants in relatively large
amounts. The organic component of the soil is mainly distinguished into undecomposed, decomposed and
decomposing organic matter, where in the newly formed organic matter is the end product. The dead
organic matter in the soil system is subjected to different kinds of thermo chemical and pressure induced
mechanisms where in microbial activity also takes place. In microenvironment of the soil, temperature is
dependent not only on the top layer of the soil, but mainly on the gaseous exchange in the soil which
takes place with the help of water in the soil system. Apart from different kinds of elements, various
organic compounds present in soil undergo the degradation reaction in the soil though they are not soluble
in water. This is possible due to the reason that the H+ concentration in the soil helps in solubility of
different kinds of elements and compounds present in soil. Therefore pH plays an important role in soil
reactions. The main contributors of H+ ions in soil are the compounds which have the hydrogen on
reaction with water release hydrogen into the soil resulting in an acidic environment. Organic acids, such
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as humic acid and fulvinc acid which form an intermediary compound of organic matter decomposition
also react in the similar way.
The decomposition of cellulose in soil occurs at pH 6.8 to 7.5, therefore the formation of spring turf in
acidic soils takes place (Soil and its fertility –H.Teusher & R.Adler.).The properties of organic soil
components manly depend on pH, as cations such as H+,Ca++,Mg ++,K+,Na+ are attached to colloids which
changes the charge of that colloids and thus form aggregates.
Colloid humus compounds consist of humic acids and water insoluble salts which have very high affinity
towards calcium and magnesium humate which act as polyelectrolyte which makes the compound attach
to more cations .During the process of organic decomposition cellulose produce polyurinoides which are
mucilaginous substances, aid in humus congregation. Lignin is a major part of humus composition which
is required for the production of humic acid.
The process of decomposition of organic matter, in turn forming new compounds is mainly due to four
reactions which occur in soil they are Oxidation, Reduction, hydrolysis and carbonation. Microorganisms
play an important role during this process as in their absence accumulation of organic matter would take
place till the total nitrogen, potassium and phosphorous, sulfur, and carbon would be locked up
unavailable in the form of rock or gas. Due to the presence of microbes the elements from the organic
matte are released, which adds them back into the circulations that they can be used again by the plant and
animal life. The activity of the soil microbes is limited to availability of the energy, environmental
conditions, and formation of certain detrimental substances which would create a resistance for their
growth. They are mainly dependent on the supply pH oxygen, amount of organic matter present and the
amount of inorganic compounds present with respect to the pH of the soil ( The soil and its fertility
,Tescher &Adler) Cellulose is the most abundant biological polymer where about every year
approximately 28 Billion tons of cellulose are formed as a result of photosynthesis where in it forms
about 6% of atmospheric carbon dioxide fixed by land and sea plants.(Smith ,JE .Biotechnology (1981)
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In a mixed microbial community there are potential environmental factors which can induce or repress the
enzyme activity ,The extracellular endocellulase of agaricus bisporus is induced by high concentrations of
cellobiose where as the high glucose concentration in the environment strongly inhibit the enzyme
activity. Whereas the low glucose concentration, the enzyme is weakly induced. (Soil and plants vol 1Dilip K.arora, bhart rai, k.g.mukherji, Guy r.knudse)
Different kinds of microorganisms which can degrade lignocellulosic biomass in soil
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Soil biology and the biological micro-environment
Organic matter, microscopic and macroscopic organisms (e.g., fungal hyphae and invertebrates),
detritus from fungi and animals, and bacteria, and biological exudates, all assist in stabilizing soil
structure. The role of each part of the biomass differs according to its size Broadly, large
aggregates greater than 250  m diameter (macro-aggregates), are stabilized by their inherent
physical structure (Chapter 2), wetting and drying cycles, and organic matter. Micro-aggregates
(< 250  m) are stabilized by live or dead roots, fungi, invertebrates and micro-organisms
The populations of soil organisms of all sizes are linked functionally through their roles in the
degradation of various forms of organic material. The latter includes live and dead plant material
and other live or dead organisms. This shows that animals such as nematodes and some fungi
feed directly on live plants while other fungi and bacteria feed predominantly on litter.
Earthworms and other large invertebrates create, and inhabit, burrows and pores, and are very
mobile. The most notable of these are termites, which are divided into three groups according to
the structure of their nests: those that build mounds (a) above ground, (b) on the soil surface, and
(c) below ground. Small arthropods, microfauna and fungi live mostly in larger voids and in
association with roots. Foster (1988) reviewed the location of the various types of soil-dwelling
organisms and found that fungi, which constitute about 80% of the biomass in many soils, tend
to be restricted to the rhizosphere of roots, to larger pores between aggregates and to the surface
of aggregates. Bacteria, by contrast, are found on roots in the rhizosphere, in small colonies in
the larger micropores, within aggregates and on and within cell debris. For more information on
location refer to Foster (1988). Smiles (1988) describes the physics of the micro-environment of
small soil organisms.
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Tables of microorganisms as separate sets of bacteria,fungi
Lignin mechanism -soil as its potential source.
Soil consists of different kinds of materials which undergo degradation continuously.
Lignocellulosic materials form one of the major component of the soil degradation system as it
forms the main component for the cell wall composition of most of the plant cells. During the
biodegradation process, among the lignocellulosic biomass which mainly consist of cellulose,
hemicelluloses, pectin, and lignin the cellulose and hemicelluloses are comparatively easily
degraded into monomer sugar units. Cellulose during the process of degradation forms
polyurinoid components which act as mucilaginous units for humus formation.
Lignin is a complex aromatic polymer which is composed of hydroxyl, methoxy and carboxyl
groups. The random synthesis of lignin formation makes it more strenuous task for its
degradation as the bond formed between the two lignin monomers is not similar to others.
During the biodegradation process of lignin, the lignin structure is modified with the help of
enzymes such as laccase, peroxidases and esterases which are released by lignin degrading fungi
initially as the breakage of bonds is not a feasible. Apart from the lignin molecule there are many
kinds of other complex structures present in soil .The part of the soil components which are
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chemically modified along with modified or partially degraded lignin undergo a dehydrative
condensation to form humus aggregate. These aggregates are colloids which are composed of
mainly polyuriods, partially degraded lignin, and other complex aromatic structures. After the
humus formation, it undergoes dehydration and demethylation reactions which lead to decrease
hydrogen to carbon ration .During the dehydration and demethylation process, the humic acids
and the protein get separated. The complex of humus aggregate consists of humic acids, fulvic
acids and humin. The lignin compound mostly is a part of humic acids. During the process of
dehydrative condensation of the humus formation the chemically modified lignin further
undergoes a chemical modification which results in a compound made of the other aromatic
structures .These basically has the capability to provide the necessary factors for further lignin
degradation process .
The further understanding of different compounds which are involved in the humic acids ,their
activity towards the further degradation of lignin ,whether it is forming a more complex or
simpler structure would give us a chance of understanding the chemical structure, its nature of
reactivity with other aromatic compounds and in turn its degradation .
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5. Conclusion: to be edited
Bioethanol production comparatively decrease the green house gas emissions because the
amount of carbon released during the fuel combustion is proportional to the amount of carbon
dioxide the plant consumes during the process of pHotosynthesis. Brazilian Bioethanol which is
produced from bagasse shows a reduction of 90% for green house gas emissions. In Europe
Bioethanol production is from different source like sugar beet, wheat comparatively has lower
GHG emissions when compared to gasoline. The most promising area for interest for the
production of bioethanol is lignocellulosic biomass in terms of GHS, availability of raw material
(Jeremy Woods, http://www.best-europe.org/Pages/ContentPage.aspx?id=482_)????
6. References:
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