CHAPTER 2 • Marine Organic Geochemistry: A General Overview
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Amino acids undergo several microbiologically-mediated reactions (Meister 1965),
some of which are extracellular (see Chapter 6). While some reactions result in the
depletion of amino acids (e.g. fermentation, deamination, respiration), others lead to
the formation of new amino acids (Lee and Cronin 1982). For example, decarboxylation of aspartic acid and glutamic acid produces f3 -alanine and y-aminobutyric acid,
respectively. The relative abundances of f3 -alanine and y-aminobutyric acid have been
used to assess the extent of degradation of natural organic mixtures, since they are
consistently enriched in diagenetic sequences (Cowie and Hedges 1994).
2.2.3
Carbohydrates
Carbohydrates are organic compounds that contain only carbon, hydrogen, and oxygen, in which hydrogen and oxygen occur in the same proportion as they do in water.
They are polyhydroxy aldehydes and ketones whose general formula is Cn(H20)w They
exist as monosaccharides, disaccharides, trisaccharides, etc. A polysaccharide contains
many monosaccharide molecules. Monosaccharides can be classified as aldoses or
ketoses, depending on the occurrence of an aldehyde or a keto group. According to
the number of carbon, they are trioses, tetroses, pentoses, hexoses, etc. Thus, a C 6 monosaccharide can be an aldohexose or a ketohexose. Naturally occurring monosaccharides are usually of D-configuration at C-5. The most common are hexoses, pentoses
and deoxy sugars (Fig. 2.14). Modified polysaccharides, liked chitin, are also common.
In chitin, the hydroxyl group in C2 of the monosaccharide (amino sugar) has been
replaced by an amino group.
Carbohydrates are important components of marine organisms. For example, they
usually comprise 20 to 40% of dry biomass of phytoplankton (Parsons et al. 1961).
Carbohydrates are contained in cell walls of plants, bacteria, and fungi, providing structural support. Chitin is the marine equivalent of cellulose. Chitin (a polymer of Nacetyl-D-glucosamine) is present as a structural component in arthropods, mollusks,
most fungi and some algae. In bacteria, peptidoglycans in cell walls can account for
up to 75% of dry biomass. Peptidoglycans are polysaccharide chains of N-acetyl-Dglucosamine and N-acetylmuramic acid cross-linked by peptides.
Carbohydrates are also involved in energy reserve. D-glucose is stored in the form
of polysaccharides: starch in plants and glycogen in animals.
A very high range in reactivity occurs in carbohydrates. Glucose, for instance, is
readily metabolized by most organisms, but cellulose (a polymer of glucose) is not.
Moreover, starch, a cellular storage material, is water soluble and degradable, whereas
the structural component cellulose is insoluble and very resistant to degradation. They
are both made of many units of D-glucose and the only difference is the position of
the glycoside bond among them. In cellulose, D-glucose units are linked via an f3 -glycoside linkage whereas in starch all glycoside linkages are a (axial).
Bacteria must initially hydrolyze polysaccharides outside the cell in order to provide substrates small enough to fit bacterial porins and thus enter the cell where glycolysis and respiration occur. Hydrolysis is carried out by bacterial cell surface and
extracellular enzymes. This step, previous to complete mineralization of carbohydrates,
appears to be dependent on chemical structure of substrates. For instance, the polysaccharide pullulan (a(I,6-linked maltotriose[a(I,4) linkages] units) is preferentially
61
Amino acids undergo several microbiologically-mediated reactions (Meister 1965),
some of which are extracellular (see Chapter 6). While some reactions result in the
depletion of amino acids (e.g. fermentation, deamination, respiration), others lead to
the formation of new amino acids (Lee and Cronin 1982). For example, decarboxylation of aspartic acid and glutamic acid produces f3 -alanine and y-aminobutyric acid,
respectively. The relative abundances of f3 -alanine and y-aminobutyric acid have been
used to assess the extent of degradation of natural organic mixtures, since they are
consistently enriched in diagenetic sequences (Cowie and Hedges 1994).
2.2.3
Carbohydrates
Carbohydrates are organic compounds that contain only carbon, hydrogen, and oxygen, in which hydrogen and oxygen occur in the same proportion as they do in water.
They are polyhydroxy aldehydes and ketones whose general formula is Cn(H20)w They
exist as monosaccharides, disaccharides, trisaccharides, etc. A polysaccharide contains
many monosaccharide molecules. Monosaccharides can be classified as aldoses or
ketoses, depending on the occurrence of an aldehyde or a keto group. According to
the number of carbon, they are trioses, tetroses, pentoses, hexoses, etc. Thus, a C 6 monosaccharide can be an aldohexose or a ketohexose. Naturally occurring monosaccharides are usually of D-configuration at C-5. The most common are hexoses, pentoses
and deoxy sugars (Fig. 2.14). Modified polysaccharides, liked chitin, are also common.
In chitin, the hydroxyl group in C2 of the monosaccharide (amino sugar) has been
replaced by an amino group.
Carbohydrates are important components of marine organisms. For example, they
usually comprise 20 to 40% of dry biomass of phytoplankton (Parsons et al. 1961).
Carbohydrates are contained in cell walls of plants, bacteria, and fungi, providing structural support. Chitin is the marine equivalent of cellulose. Chitin (a polymer of Nacetyl-D-glucosamine) is present as a structural component in arthropods, mollusks,
most fungi and some algae. In bacteria, peptidoglycans in cell walls can account for
up to 75% of dry biomass. Peptidoglycans are polysaccharide chains of N-acetyl-Dglucosamine and N-acetylmuramic acid cross-linked by peptides.
Carbohydrates are also involved in energy reserve. D-glucose is stored in the form
of polysaccharides: starch in plants and glycogen in animals.
A very high range in reactivity occurs in carbohydrates. Glucose, for instance, is
readily metabolized by most organisms, but cellulose (a polymer of glucose) is not.
Moreover, starch, a cellular storage material, is water soluble and degradable, whereas
the structural component cellulose is insoluble and very resistant to degradation. They
are both made of many units of D-glucose and the only difference is the position of
the glycoside bond among them. In cellulose, D-glucose units are linked via an f3 -glycoside linkage whereas in starch all glycoside linkages are a (axial).
Bacteria must initially hydrolyze polysaccharides outside the cell in order to provide substrates small enough to fit bacterial porins and thus enter the cell where glycolysis and respiration occur. Hydrolysis is carried out by bacterial cell surface and
extracellular enzymes. This step, previous to complete mineralization of carbohydrates,
appears to be dependent on chemical structure of substrates. For instance, the polysaccharide pullulan (a(I,6-linked maltotriose[a(I,4) linkages] units) is preferentially