BIOCHEMISTRY IN PERSPECTIVE

SUMMARY: Extremophilic organisms utilize an array of oxidation-reduction reactions to generate the energy required to sustain lifein hostile environments.

The Extremophiles: OrganismsThat Make a Living in HostileEnvironmentsHow can living organisms sustain life in veryhostile environments? The extremophiles, the uniqueorganisms that live in harsh or extreme environments, areclassified according to the specific conditions to which theyare adapted. Examples include thermophiles and hyper-thermophiles (high temperature), acidophiles (low pH),piezophiles (high pressure), and halophiles (high salt con-centration). Such organisms, many of which are archaeans,often thrive in habitats with several extreme conditions. Forexample, the hot water near hydrothermal vents is undervery high pressure. As with all living organisms, theextremophiles require energy. Energy generation involvesoxidation-reduction (redox) reactions (described in Chapter8) in which free energy is released as electrons are trans-ferred from an electron donor to an electron acceptor.

Briefly, there are three energy-generating mechanisms:photosynthesis, chemoorganotrophy, and chemolithotrophy.Photosynthesis (Chapter 13) is a process that converts lightenergy into chemical energy (ATP). Most photosyntheticorganisms produce O2 as a by-product. In certain microor-ganisms O2 evolution does not occur. Chemoorganotrophsand chemolithotrophs generate ATP by oxidizing organicand inorganic compounds, respectively.

Chemotrophs have two possible mechanisms of syn-thesizingATP: fermentation and respiration. Fermentationis a biochemical process in which energy capture occursvia the oxidation of organic molecules. ATP is synthesizedfrom ADP by substrate-level phosphorylation, the directtransfer of a phosphate group from a phosphorylatedorganic molecule. Fermentation is inefficient because theproduct molecules released as waste are only partially oxi-dized. Respiration is a more sophisticated process inwhich a series of electron carrier molecules reversiblyaccept and donate electrons. The energy captured is usedto create an electrochemical gradient that drives ATPsynthesis. In aerobic respiration, the low-energy electrons

that result from this processare donated to O2. In anaerobic respiration, terminalelectron acceptors other than O2 are used. Examples of thelatter include ferric iron (Fe3�), nitrate (NO3

�), or certainorganic molecules. The following examples of energy-generating mechanisms used by selected extremophilesprovide a hint of the diversity among these remarkableorganisms.

Methanococcus janaschii is a hyperthermophile. It livesnear hydrothermal vents at temperatures of 80�C or greater.It is also a piezophile. As its name suggests, this archaeanis a methane (CH4) producer. Under strictly anaerobicconditions, M. jannaschii obtains energy by convertingCO2 to CH4 in a series of complex electron transfer reac-tions. The electron donor is hydrogen gas (H2), a productof geochemical processes. The net reaction is

CO2 � 4H2 → CH4 � 2H2O

Bacillus infernus (called the “bacillus from hell”) is athermophile that grows at 60�C. This halotolerant (0.6 MNa�) bacterium was discovered 2700 m below the earth’ssurface. B. infernus, isolated in nutrient-deficient environ-ments, has a very slow rate of reproduction. It derivesenergy from the fermentation of the sugar glucose or theanaerobic respiration of electron donors such as formate(HCOO�) and lactate (CH3CHOHCOO�). Electron accep-tors include manganese dioxide (MnO2), ferric iron (Fe3�),and nitrate (NO3

2�).Thiobacillus ferroxidans is an aerobic, acidophilic (pH

2–4) chemolithotroph that is commonly found in sulfate-containing acid drainage from coal mines. It derives energyfrom ferrous iron (Fe2�) and reduced forms of sulfur suchas H2S and iron sulfide (FeS). The products of this processare Fe3� and sulfuric acid.Acid drainage pollutes lakes andrivers and kills aquatic life.