Biomedical Engineering Reference
In-Depth Information
In the carbon cycle of Earth's ecosystems, microorganisms play an important
role. Carbon elements stored in organic matter are mineralized and released by
microorganisms as soon as possible, making the biosphere an environment with
a good balance of carbon. It is estimated that 90 % of carbon dioxide on Earth
is formed by microbial decomposition. Most of the carbon dioxide is fixed by
photosynthesis and accumulated in woody and herbaceous bodies in the form of
glycans, especially on land; about 60 % of fixed carbon dioxide constitutes wood.
In the wood, 75 % of the carbon is made up of polysaccharides (i.e., cellulose,
hemicellulose, starch, pectin, and arabinose). More than an additional 20 % of
carbon is made up of lignin and xylan; the protein content is only about 15 %.
The polysaccharide contents in herbaceous plants and shrubs are even more than in
timber. Such complex organic matter that has a huge yield is difficult to be broken
down, and it is decomposed by some special microorganisms in the soil.
The decomposition of natural lignocellulosic feedstock produced by green plants
through photosynthesis constitutes an extremely important component of matter and
energy transformation in the natural ecological system. It is fulfilled by microbial
degradation. Study of how the microorganisms in nature coordinate to complete the
degradation of natural lignocellulosic feedstock lays the foundation for obtaining
a high yield of bacteria and useful fermentable products. Although the microbial
conversion function is an important part of the carbon cycle in the ecosystem,
it is a huge waste in terms of the use of natural resources by human beings.
Microorganisms play an important role in the natural carbon cycle. They participate
not only in CO
2
fixation and photosynthesis but also in the decomposition of
regenerated CO
2
.
1. Photosynthesis: Photosynthetic microorganisms mainly include algae, cyanobac-
teria, and photosynthetic bacteria. They transform the CO
2
in the atmosphere and
water bodies to organic carbide through photosynthesis. Especially in most of
the aquatic environment, the main photosynthetic organisms are microorganisms.
In the aerobic zone, cyanobacteria and algae dominate; in the anaerobic region,
photosynthetic bacteria dominate.
2. Decomposition: The organic compounds in nature are mainly decomposed by
microorganisms in the land and water. In aerobic conditions, they are completely
oxidized to CO
2
by aerobic microbes; in anaerobic conditions, they are incom-
pletely oxidized into organic acids, methane, hydrogen, and carbon dioxide by
anaerobic fermentation.
Many microorganisms can decompose the organic compounds, such as fungi,
actinomycetes, bacteria, and protozoa. Among them, the decomposition ability of
fungus is especially strong, including some ascomycetes; imperfecti fungi and
basidiomycetes, such as
Polyporus
spp.,
Agaricales
spp.,
Trichoderma
spp., and
Myrothecium
spp.; bacterial species of
Sporocytophaha myxococcoides
; actino-
mycetes species of
Streptomyces antibioticus
; and so on. Fungi are more active
in the early stages of decomposition of hemicellulose. Then, later in decompo-
sition they mainly rely on the role of actinomycetes. Fungi that can decompose
