10 Exercise Solutions

Solution Exercise 1

Prokaryotic microorganisms lack membranous nuclei, whereas eukaryotes have membrane-bound nuclei.

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Solution Exercise 2

The cell walls of prokaryotes staining Gram-positive consist of peptidoglycan, which is a meshwork of mucopolysaccharides cross-linked in three dimensions by peptide bridges, and a variety of secondary polymers (teichoic or teichuronic acids and proteins). On the other hand, the cell walls of prokaryotes that stain Gram-negative contain lipopolysaccharides, phospholipids, and proteins arranged in a membrane bilayer (the outer membrane). Sandwiched between the outer membrane and cytoplasmic membrane is a thin layer of peptidoglycan.

Bonus – Some bacteria with cell walls are neither Gram-positive nor Gram-negative according to the Gram-stain. The cell walls of Archaea lack the kind of peptidoglycan that is found in prokaryotes and instead contain either pseudopeptidoglycan, glycoproteins, or proteins alone.

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Solution Exercise 3

Extracellular polymeric substances (EPS).

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Solution Exercise 4

Doubling time for exponential microbial growth is given by the equation T = ln(2)/μ = 0.693/μ in which the frequency of cell division is specified by the exponential growth rate constant μ. This gives a doubling time of 138.6 s, which is equivalent to 2.3 min.

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Solution Exercise 5

In groundwater systems, length scales extend over large distances of 103 m (km) all the way down to 10-6 m (μm) levels, depending on how habitat size is defined. Length scale is critical because it defines other important physical properties for habitability such as surface areas and relative volumes of solids, water, air, and other fluids. These factors not only determine where it is possible for microbial life to take refuge but also influence water movement, groundwater chemistry, and reactive mass transport processes.

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Solution Exercise 6

Being below the surface, groundwater is insulated from the daily cycles of solar irradiance and seasonal climate changes above ground. The insulating properties of subsurface material and high specific heat capacity of water result in shallow groundwater retaining a fairly constant temperature, roughly equal to the mean annual air temperature of the location. Deeper groundwater is warmed by the geothermal gradient of the region.

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Solution Exercise 7

The expected temperature in a deep groundwater system at 2000 m depth would be around 50 to 60°C higher than the temperature at the surface which averages about 15°C, which corresponds to the temperature range of thermophilic microorganisms.

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Solution Exercise 8

~35 million cells. In near-surface groundwater systems, prokaryotic microbial cell numbers average 107 cells/g. Therefore, a 3.5 g sample would contain 107 cells/g × 3.5 g = 3.5 × 107 (35 million) cells.

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Solution Exercise 9

  1. Oxygen – water
  2. Nitrate – nitrogen
  3. Fe(III)/Mn(IV) – Fe(II)/Mn(II)
  4. Sulfate – sulfide
  5. Carbon dioxide – methane

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Solution Exercise 10

Autotrophs rely on chemical compounds of inorganic (i.e., lithic) substances as their source of energy for reduction of carbon dioxide, so they are classed as Chemolithoautotrophs.

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Solution Exercise 11

Heterotrophs (often synonymous with organotrophs) rely on organic molecules that already exist in the system to promote their growth, so they are classed as Chemoorganoheterotrophs.

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Solution Exercise 12

The pore size diameters in shales and clays (typically < 107 m) are too small to provide enough room to accommodate prokaryotic microorganisms with cell diameters greater than 10-7 m.

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Solution Exercise 13

Only one (prokaryotes). Life in the subsurface is limited by pore diameter. The smaller size of prokaryotes allows them to live within the pore diameter of carbonate rocks.

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Solution Exercise 14

Prokaryotic microorganisms contribute to chemical reactions in groundwater systems in two ways. First, metabolic enzyme activity can speed up (catalyze) slow reactions and force corresponding reaction quotients to shift rapidly towards or away from equilibrium. This affects many aspects of groundwater chemistry including pH, redox conditions, mineral dissolution and precipitation processes, and the chemical speciation of solutes. Second, prokaryotic cells behave as microscopic reactive solids owing to the chemical reactivity of functional groups, such as carboxyl or phosphoryl substituents, in the macromolecular components of cell walls, external sheaths, and EPS. As reactive solids, bacteria not only contribute to the sorption of dissolved ions but also serve as heterogeneous nucleation templates for mineral precipitation.

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Solution Exercise 15

As pH increases, sorbent solids tend to develop a more negative surface charge that favors increased (positively charged) cation sorption and decreased (negatively charged) anion sorption.

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Solution Exercise 16

Microbes are used to clean up groundwater pollutants in processes known as ‘bioremediation’. Bioremediation uses microorganisms to reduce pollution through the biological degradation of pollutants (e.g., petroleum hydrocarbons and chlorinated compounds) into non-toxic substances. The pollutants can act as either electron donors (e.g., BTEX) or electron acceptors (e.g., TCE) in microbial metabolism.

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Solution Exercise 17

Rationale: Microorganisms can use BTEX compounds as electron donors in their metabolism. As long as there are available nutrients and electron acceptors in the groundwater, microorganisms can metabolize and therefore degrade target contaminants.

Three possible strategies include:

  1. natural attenuation – natural microbial community is left to eliminate the target contaminant without human intervention;
  2. biostimulation – essential nutrients are added to stimulate the natural microbial community to eliminate the target contaminant; and
  3. bioaugmentation – nutrients and select strains of bacteria are injected into the subsurface to promote the elimination of the target contaminant.

Among commonly available electron acceptors, oxygen yields more energy than any other oxidant in aerobic respiration. To enhance microbial degradation of BTEX compounds, you could increase the availability of oxygen in groundwater. Oxygen can be added to groundwater either directly to the subsurface by air sparging or through injection of a chemical oxidant (e.g., hydrogen peroxide) that decomposes to release oxygen.

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Solution Exercise 18

Mineral dissolution processes tend to consume protons as reactants, which forces a release of sorbed cations into solution to conserve electroneutrality. The most common source of protons in mineral dissolution reactions is carbonic acid, which is generated from the degradation of organic matter by heterotrophic microbial activity. Other inorganic and organic acids are produced by microorganisms as well. These include sulfuric acid from the oxidation of sulfide minerals, as well as a wide variety of carboxylic acids such as acetic acid and oxalic acid.

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Solution Exercise 19

Hydrous iron and manganese oxides are utilized by dissimilatory Fe(III)- and Mn(IV)-reducing bacteria as solid-phase electron acceptors for anaerobic respiration. These oxide minerals typically occur as thin coatings on other mineral grains, as well as particulate organic materials. Dissolution of these coatings by microbial reduction under low oxygen conditions frequently results in gleyic (gray-blue-green) color characteristics, evident in hand samples of borehole cuttings and cores from Mn(IV)- and Fe(III)-reduction zones in groundwater systems.

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Solution Exercise 20

Microbial carbonate mineral precipitation can be used in cementing unconsolidated sediments and infilling of empty pore spaces to improve shear strength and stiffness of loose deposits and/or reduce permeability to control groundwater flow.

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Groundwater Microbiology Copyright © 2021 by F. Grant Ferris, Natalie Szponar, and Brock A. Edwards. All Rights Reserved.