THE GEOCHEMISTRY OF NATURAL WATERS

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THE GEOCHEMISTRY OF NATURAL WATERS

• BIOTRANSFORMATIONS OF ORGANIC COMPOUNDS - I
• CHAPTER 7 - Kehew (2001)

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LEARNING OBJECTIVES

• Classify microorganisms according to their
  characteristics and metabolic processes.
• Understand the implications of various metabolic
  processes for natural and accelerated
  bioremediation of contaminated aquifers.

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CLASSES OF MICROORGANISMS - I

• Procaryotes - Include bacteria and cyanobacteria
  (blue-green algae). Have a simple cellular
  structure that lacks a nucleus. Widely
  distributed in surface and subsurface
  environments.
• Eucaryotes - Have a more complex structure
  including a nucleus. Includes algae, fungi and
  protozoa. Less abundant in subsurface than
  procaryotes.

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CLASSES OF MICROORGANISMS - II

• Archaebacteria - Inhabit inhospitable
  environments that may have been present in the
  early history of the earth. Include methanogens
  (metabolize H2 and CO2 to form CH4). The
  methanogens are limited to anaerobic
  environments. Other archaebacteria live in
  brines, hydrothermal solutions and acid
  solutions.
• Viruses - Although containing DNA, they are
  parasites and cannot reproduce by themselves.
  They must inject DNA into other microorganisms.
  Because of their small size, they may be mobile
  in aquifers.

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E. coli Salmonella
Streptococcus Staphylococcus
Vibrio cholerae
Common shapes of bacterial cells (from Chappell,
1993).
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METABOLIC PROCESSES

• Two basic types of microorganisms in the
  subsurface.
• Heterotrophs - Obtain both carbon and energy from
  existing organic carbon.
• Chemolithotrophs - Obtain energy by oxidizing
  inorganic compounds in the environment. To obtain
  carbon, they must reduce CO2 to synthesize
  organic compounds.
• Autotrophs - Include the chemolithotrophs and
  photosynthetic organisms.

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OXIDATION OF ORGANIC MATTER BY HETEROTROPHS

• Fermentation - An anaerobic process in which the
  substrate (food) is partially oxidized to other
  organic compounds (including carboxylic acids and
  alcohols). Fermentation of milk yields lactic
  acid. Fermentation of sugars in fruits and grains
  yields ethanol.
• Respiration - When electron acceptors are
  available, organic substrates are oxidized all
  the way to CO2

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ENZYMES - I

• Oxidation of organic matter provides energy for
  living organisms because such reactions are
  thermodynamically favored
• 1/4CH2O 1/2O2 ? 1/4CO2 1/4H2O
• ?G? -119.98 kJ/electron equivalent
• However, the rates of such reactions are
  generally very slow at low temperatures.
• Microorganisms employ catalyzing enzymes to
  surmount kinetic barriers.
• Enzymes function by forming a complex with the
  reactants, bringing them in close contact.

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ENZYMES - II

• Oxygenase - An enzyme that catalyzes oxidation
  reactions.
• Reductase - An enzyme that catalyzes reduction
  reactions.
• Enzymes are proteins, but they may require
  non-protein compounds called cofactors.
• Cofactors include metal ions and organic
  compounds called coenzymes (e.g., NAD -
  nicotinamide adenine dinucleotide).

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ENZYMES - III

• Enzyme induction - The ability of microorganisms
  to synthesize new enzymes to make use of
  substrates not previously available.
• Xenobiotic chemicals - Chemicals not naturally
  found in the environment. Another word for
  contaminants.
• Enzyme induction is important in the
  biodegradation of xenobiotics.
• Acclimation period - Time between initial
  encounter of microorganisms with xenobiotics and
  the development of their ability to use the
  xenobiotics as a substrate.

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MICROBIAL HABITATS

• Other than a growth substrate, water is the most
  basic requirement for microorganisms.
• Halophiles - Bacteria adapted to high-salinity
  brines in sedimentary basins.
• Osmotic pressure - If the salinity inside and
  outside the cell are greatly different, a water
  pressure may build up that stresses the organism.
• Microbes are known that can survive temperatures
  from 0C to at least 100C (thermophilic
  bacteria).

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Relationship between temperature and growth rate
for microorganisms. Growth rate increases with
temperature to some optimum value, and decreases
thereafter (from Chappelle, 1993).
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MICROORGANISMS AND pH

• Microorganisms in general have adapted to life
  at pH ranging from lt2 (acid mine waters) to gt10
  (alkaline lakes).
• Individual strains of bacteria tolerate
  relatively narrow ranges in pH.
• Enzyme activity is what is most affected by pH.

Effect of pH on the activity of four enzymes
(from Sawyer et al., 1994).
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MICROBES AND OXYGEN

• Obligate aerobes - Microorganisms for which the
  presence of oxygen is essential. Oxygen is the
  only electron acceptor that these species can
  employ.
• Facultative anaerobes - Can use oxygen if it is
  available but are able to switch to alternate
  electron acceptors when oxygen is depleted.
• Obligate anaerobes - Use alternate electron
  acceptors exclusively. Oxygen is toxic.
• Sometimes aerobes and anaerobes can be found in
  the same aquifer (micro-environments).

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HETEROTROPHIC METABOLISM AND GROUND WATER