Table of Contents:
Types of Bacteria in Groundwater
Population Distribution of Bacteria Versus Soil Depth
Reduction/Oxidation of Iron and Manganese
The source of bacteria in groundwater is unknown (Bower, 1978). Bacteria may be indiginious or be spread by contaminated surface water (i.e. sewage effluent, leachate from garbage dumps, and other polluted waters) to groundwater. No matter what the source of bacteria; bacteria and their biological processes affect the quality of our groundwater. The purpose of this web page is to introduce the topic bacteria and its associated roles in groundwater.
Aerotolerant - an anerobe which will not be inhibited by oxygen and can still grow at sub-optimal rates.
Anaerobes - Bacteria that can grow only in the absence of O2.
Autotroph - In reference to carbon source - an organism which uses carbon dioxide (CO2) as the sole carbon source.
Barotolerant - An organism able to tolerate high hydrostatic pressure, although growing better at normal pressures.
Chemolithotroph - An organism which obtains its energy from the oxidation of inorganic compounds.
Chemoorganotroph - An organism which obtains its energy from the oxidation of organic compounds.
Facultative aerobe - An organism which is normally anaerobic but can also grow in the presence or oxygen (O2).
Facultative anaerobes - Bacteria that can grow in the presence or absence of O2.
Halotolerant - An organism capable of growing in the presence of NaCl but not requiring it.
Heterotroph - In reference to carbon source - an organism that uses reduced, preformed organic molecules as its principal carbon source.
Lithotroph - In reference to energy source (electron donor) - An organism which uses an inorganic substrate (such as ammonia, hydrogen) as an electron donor in energy metabolism.
Mixotroph - An organism able to assimilate organic compounds as carbon sources while using inorganic compounds as electron donors.
Obligate aerobes - Bacteria that can not grow in the absence of O2.
Organotroph - In reference to energy source (electron donor) - An organism which obtains energy by the metabolism of organic substrates (as electron donors).
Psychrophile - An organism able to grow at low temperatures and showing a growth temperature optimum of < 15 °C.
Psychrotolerant - An organism able to grow at low temperature but having a growth temperature optimum of > 15 °C.
(Umbreit, 1991)
Orginal by San Mach
The composition/bacterial content are influenced by changes in the environment, such as percolation of nutrient-rich surface waters, or by pollution of groundwater. The depth to which bacterial activity is possible is determined by the nutrient supply, and in addition to pH, Eh, salt content, groundwater temperature, and the permeability of the aquifer. Microbiological activity can and will exist in many subsurface regions: Carbon sources necessary for microbial life are carbonates and other inorganic carbon that is present in most underground materials; temperatures favorable for microbiological activity can and will exist in many subsurface regions (temperatures favorable for microbial life extend to a depth about 2000 m); water pressures to this depth are not high enough to deter microbial activity, and many bacteria can live under the high osmotic pressures of saline water; and oxidation-reduction levels often are within the tolerance of bacteria. Subsurface microbe colonization is usually greatest in the (nutrient-rich) humic upper soil layers and decreases generally with falling nutrient supply at greater depths. Since molecular oxygen is usually absent in deeper regions, anaerobic bacteria will prevail. These bacteria use sulfate, carbon dioxide, nitrate, and simple organic compounds rather than oxygen as electron acceptors in their metabolism. Such bacteria are sulfate reducers, denitrifiers, methane formers, sulfur oxidizers, and hydrocarbon utilizers. In the presence of other energy sources, particularly organic substances, bacterial metabolism can also take place below the humus layer. Whereas aerobic bacteria are tied to free oxygen availability for their distribution, which generally decreases downward, anaerobes and facultative anaerobes are found at various depths. (Matthess, 1982)
The population density depends on the amount of organic debris available and on the suitability of environmental factors, and indirectly on the effect of incoming dissolved organic substances that are nutrients of the established bacteria colonies. For the same solution content, fast-flowing water supplies more nutrients and removes more poisonous excreta and therefore permits a greater density of colonization than slow-flowing or stagnant water. The composition of the species and their number of individuals are constantly adjusting to the arrival of organic matter (Matthess, 1982). Population density is an important consideration because the amount of bacterial activity is directly related to the density of the bacteria population.
Denitirication is an anaerobic bacterial process in which nitrate is reduced to free nitrogen gas and nitrogen oxides. The reduction of nitrate (denitification) begins in the presence of organic substances having oxygen contents of approximately 0.5 mg/L, through action of nitrate-reducing and denitrifying microorganisms. Nitrite is formed as the first product of reduction, through both physiological groups, and the denitrifying bacteria utilize No3- as the electron acceptor and reduce it to NO2-, N2O, or N2. The elemental nitrogen escapes into the ground air. Other types of bacteria form ammonia from the nitrite. Desulfovibrio desulfuricans can reduce nitrate directly to ammonia. Ammonia can also be formed by microbial breakdown of organic compounds containing nitrogen. Under aerobic conditions ammonia is oxidized to nitrite and nitrate by nitrogen bacteria, such as Nitrosomonas, Nitrococcus, and Nitrobacter.
NH3 + 2O2 -> HNO3 + H2O
4NH3 + 3O2 -> 2N2 + 6H2O
Because denitrifying bacteria require organic carbon as an energy source, dentirification occurs primarily in soils where organic matter is readily available. Once nitrate reaches the groundwater it tends to move with the groundwater without further attenuation other than dispersion. (Matthess, 1982)
The nonbiological reduction of sulfate with the organic substance presupposes temperatures of 700-1000 Celsius. Bacteria make sulfate reduction possible at normal temperatures that would otherwise require much higher temperatures. The Desulfovirio group, which are chiefly represented by D. desulfuricans, are adapted to the specific salt content and temperature of the aquifer.
Subsurface hydrogen sulfide, sulfur, sulfides, and other sulfur compounds such as thiosulfate, terrathionate, and sulfites can be oxidized by colorless bacteria of the genera Thiobacillus, Beggiatoa, Crenothrix, Thiotrix, and Thoploca, which as autotropic organisms obtain their energy for carbon assimilation by chemosynthesis. H2S is first converted to free sulfur, which can be temporarily stored inside or outside the cell before it oxidizes to sulfuric acid. Many types oxidize only the hydrogen sulfide, to elemental sulfur, which is then oxidized by other species (Matthess, 1982).
2H2S + O2 -> 2H2O + 2S
2S + 3O2 + 2H2O ->2H2SO4
2FeS2 + 2H2O + 3O2 ->2FeSO4 + 2H2S
Na2SO3 + H2O + 2O2 ->Na2SO4 + H2SO4
Reduction/Oxidation of Iron and Manganese:
A variety of microorganisms can reduce ferric (Fe3+) iron to the ferrous (Fe2+) state, which can be a major form of anaerobic respiration. Both chemoorganotrophic and chemolithotrophic bacteria can use ferric iron as an electron acceptor for energy metabolism. Fe3+ reduction can be coupled to the oxidation of a wide variety of both organic and inorganic electron donors because the reduction potential of the Fe3+/Fe2+ couple is very electropositive.
Only a small amount of energy is available from the oxidation of iron from the ferrous (Fe2+) to the ferric (Fe3+) state, so iron bacteria must oxidize large amounts of iron in order to grow. Most iron-oxidizing bacteria are obligate acidophiles because at neutral pH ferrous iron rapidly oxidizes to the ferric state and is only stable under anoxic conditions. Thiobacillus ferrooxidans is the best-known iron-oxidizing bacterium and can grow autotrophically using either ferrous iron of reduced sulfur compounds as electron donors. The overall reaction of ferrous iron oxidation is:
Fe2+ + 1/4 O2 + H+ -> Fe3+ + 1/2 H2O Fe3+ + 3 H2O -> Fe(OH)2 + 3H+
Other iron bacteria that live at near-neutral pH are commonly found where ferrous iron is moving from anoxic to oxic conditions. These bacteria include Gallionella feruginea, Sphaerotilus natans, and Leptothrix ochraceaChemoorganotrophs can carry out anoxic reduction of Mn4+ to Mn2+. Shewanella putrefaciens and some other bacteria show anoxic growth on acetate and several other nonfermentable carbon sources occurs with Mn4+ as an electron acceptor. The reduction potential of the Mn4+/Mn2+ couple is very high and several compounds should be able to donate electrons to Mn4+ reduction.
Leptothrix and a few other bacteria can oxidize Mn2+. Biological oxidation of Mn2+ is possible at pH values below 8 because Mn2+ does not oxidize spontaneously. There is no evidence that Leptothrix obtains energy from the oxidation process and it is not known what benefits the organism derives.
Hydrocarbon-oxidizing bacteria are responsible for the decomposition of petroleum and petroleum products. If the oil is carried into anoxic regions it will decompose very slowly and may stay there for many years. Significant aliphatic hydrocarbon oxidation only occurs in the presence of oxygen and thus in only the vadose zone of an aquifer. Hydrocarbon-oxidizing bacteria can attach to oil droplets and eventually decompose the oil to CO2. Many microorganisms can degrade petroleum, including pseudomonads, corynebacteria and mycobacteria. Under ideal conditions up to 80% of the nonvolatile components are oxidized by bacteria within 6 months to a year of the oil spill, but most oil that travels into an aquifer can remain there much longer.
Some pesticides can be used as carbon sources and electron donors for certain soil microorganisms. Decomposition of contaminants can be effected by a variety of environmental factors, such as pH, aeration, temperature, and organic matter content of the soil. In anoxic environments biodegradation is linked to reductive dechlorination of the molecule, the dechlorinated derivative being much less toxic than the original chlorinated molecule. A sulfate-reducing bacterium, Desulfomonile, reduces 3-chlorobenzoate to benzoate and Cl-:
C7H4O2Cl- + 2 H -> C7H5O2- + HCl
The reductive dechlorination of 3-chlorobenzoate is a type of anaerobic repiration. A variety of bacteria can reductively dechlorinate dichloroethylene, trichloroethylene, tetrachloroethylene, chloroform, dichloromethane, and certain brominated and fluorinated compounds. Many of these compounds are detected as groundwater contaminants because of their use as degreasing agents and industrial solvents.
Bioventing technology is based on the principle that soil contains vast microbial population of bacteria, fungi and other microorganisms. Bacteria can adapt to and metabolize a wide range of petroleum hydrocarbons when an underground spill occurs. Using oxygen as a preferred electron acceptor, bacteria consume the fuel as a food source and convert it to energy and material for cell growth. Most contaminated sites become anaerobic because the oxygen utilization rate is higher than the rate of diffusion of oxygen into the soil. When the supply of oxygen is exhausted, the bacteria degrade the petroleum hydrocarbons at a much slower rate. To speed the biodegadation up, air is pumped into the subsurface and bacteria populations switch back to aerobic biodegradation. Emissions monitoring is required at sites contaminated with volatile petroleum products such as gasoline or aviation gasoline, since air injection bioventing may transport petroleum hydrocarbon vapors into breathing zones or confined spaces. Based on soil sampling results from over 100 sites, BTEX levels were reduced by 97% and TPH levels were reduced by 24%. (Ratz, 1997)
Adapted from Civil Engineering (67, 9) 1997 by Matt Kaiser
Bacteria that live on or in a second organism and that can cause harm to the host are known as pathogens. Clostridium tetani, which causes tetanus, usually inhabits soil and lives off dead matter. Infection of humans and animals is accidental and infection is not necessary for C. tetani to live. Clostridium botulinum, which causes botulism, also inhabits soil and does not need to infect to survive. Infection into humans occurs by eating soil-contaminanted food that has been improperly prepared. Vibrio cholerae, Shigella dysenteriae, Escherichia coli, Campylobacter spp., Salmonella paratyphi, and Samonella typhi are examples of bacteria that can occur in groundwater if it is contaminated by wastwater. Groundwater usually has a small number of these types of bacteria but infection can still occur if the right conditions exist. If water is properly filtrated and disinfected most water born diseases do not occur.
Courtesy of Robert Lampe, (Virginia Tech Biology Department) used with permission
Bouwer, Herman. Groundwater Hydrology, McGraw-Hill Book Company. 1978.
Downey, D, E. Marchand, and J. Ratz. Civil Engineering, (67, 9), 1997.
Madigan, M., J. Martinko., and J. Parker. Biology of Microorganisms. Prentice Hall. Upper Saddle River, NJ., 1997.
Matthess, Georg. The Properties of Groundwater, John Wiley & Sons. New York, New York, 1982.
Neidhardt, F. C., J. L. Ingraham, and M. Schaechter. Physiology of the Bacterial Cell. Sinauer Associates, Inc., 1990.
Umbreit, W.W. Essentials of Bacterial Physiology, Dowden, Hutchinson & Ross, Inc. Stordsburg, Pennsylvania, 1991.
Student authors: Bill Dzeda, Matt Kaiser, and San Mach
Faculty Advisor: Daniel Gallagher, dang@vt.edu
Copyright © 1998 Daniel Gallagher
Last Modified: June 7, 1998
