B. NUTRITIONAL DIVERSITY (continued)
3. Chemoautotrophs: selected examples
Chemoautotrophs derive energy from chemical reactions (redox reactions) and fix CO2 as their
carbon source. Because there is little energy to be gained by most of these redox reactions, many
chemoautotrophs:
(a) are aerobes, taking advantage of O2 as their Terminal Electron Acceptor (TEA) so as to derive the
maximum energy from each reaction. (A notable exception are the methanogens, which are strictly
anaerobic and grow extremely slowly. See Methanogens outline.)
(b) use a lot of reduced energy source (electron source) and produce a lot of oxidized waste product
(e.g. S2- used and SO42- excreted; Fe2+ used and Fe3+ excreted)
(c) inhabit unusual environments where more energy-effective chemoheterotrophs cannot live.
Two aerobic examples are given below: S-oxidizers and Iron-oxidizers.
a) Sulfur-oxidizing Bacteria
10E: pp. 360-363, 568-571; 9E: pp. 595-598, 670-675
Different forms of reduced sulfur can serve as electron (energy) source, ultimately producing sulfate:
H2S Æ S° Æ S2O32-Æ SO3-Æ SO42Sometimes S° (elemental Sulfur) accumulates as intracellular granules; sometimes complete oxidation to
sulfate occurs, depending on the organism and on its environment.
Complete oxidation of H2S releases a total of 8e-, which are typically passed to O2, the TEA. These
electrons are used to generate energy (chemotrophy), and to fix CO2 to organic C (autotrophy)
There are two main groups of sulfur-oxidizers:
(i) those living at neutral pH that accumulate S° in the cytoplasm as granules, such as the free-living
and symbiotic sulfur-oxidizers at hydrothermal vents
(ii) those living at acid pH such as Thiobacillus spp. that oxidize sulfide completely to H2SO4 and can
live at pH<2. Some of these are also Fe2+ oxidizers (see section on Iron-reducers, below).
Examples of each type are described below.
(i) Hydrothermal vents:
10E: pp. 647-651; 9E: pp. 670-675
• At areas of ocean floor where spreading of the crust releases hot upwellings of minerals (including
H2S), temperatures can reach >350°C, with the surrounding seawater being around 2°C. Because they
are so deep in the ocean, the vents have no photosynthetic carbon and life at these vents relies on
microbial chemoautotrophs as the base of the food chain, primarily those oxidizing H2S. Free-living
species such as Thiobacillus hydrothermalis and others make the vent waters turbid with biomass and
serve as a food source for higher organisms such as clams, shrimp and worms.
• Tube worms (e.g. Riftia pachyptila) have endosymbiotic sulfur-oxidizers, housed in a specialized
tissue called the trophosome. The tube worm provides H2S, O2 and CO2 (all dissolved in the vent
waters) to the bacteria, which oxidize the sulfide and produce biomass and organic wastes for the
worm’s C source. S° granules tend to accumulate in the bacteria/trophosome as storage material.
• Many higher life forms around the vent have epibionts (bacterial symbionts on their external surfaces)
that are primarily sulfide-oxidizers; they appear to be a food source for the higher organisms.
• Other environments exist that are totally dependent on chemoautotrophic bacteria: e.g. isolated caves,
deep subterranean environments, probably Lake Vostok beneath the Antarctic ice sheet, and possibly
Mars and Europa (??).
(ii) sulfur-oxidizers growing at acidic pH: e.g. Thiobacillus thiooxidans
• Produce H2SO4 (sulfuric acid) as their waste product from sulfide oxidation, acidifying their
environment as they grow.
A practical example is corrosion of concrete, where biologically produced H2SO4 reacts with the cement
in concrete (primarily calcium oxides and hydroxides) to produce gypsum (CaSO4), causing it to
deteriorate.
b) Iron (Fe2+) oxidizing Bacteria
e.g. Thiobacillus ferrooxidans
10E: pp. 571-572, 666-669; 9E: pp. 462-464, 598-601
oxidizes Fe2+ (ferrous iron) Æ Fe3+ (ferric iron)
These organisms tend to live in acidic environments so that:
(a) the Fe2+ (electron/energy source) remains soluble and available to them
(b) they can use the “free” Proton Motive Force (PMF) provided by the acidic environment, as long as
they can keep their cytoplasm near neutrality (~ pH 6) to maintain pH (∆pH) and charge (∆Ψ) gradients
They have a very short electron transport chain that is insufficient to generate ATP, but is used to
(a) reduce O2 to H2O (thus removing protons from the cytoplasm and maintaining neutral pH for ∆pH)
(b) help maintain PMF by placing more electrons in the cytoplasm (the ∆Ψ


F
(c) reduce (fix) CO2 to organic carbon by running part of the electron transport chain “backwards”, or
“uphill” by allowing protons to enter the cytoplasm along their concentration gradient. NADP+ can then
be reduced to be used in Calvin cycle reactions.
These processes use lots of electron source (Fe2+) and generate lots of oxidized product (Fe3+) . This is a
problem where iron ores are exposed to air, soil and water, creating an acidic runoff called acid mine
drainage. This occurs by a combination of spontaneous chemical reactions and biological reactions:
1) upon first exposureof FeS ores (pyrites) to air, a slow chemical rxn occurs:
FeS2 + 3.5 O2 + H2O Æ Fe2+ + 2 SO42- + 2H+
2) this provides acidic conditions and Fe2+ ions for the biological reaction (iron oxidation)
Fe2+ + 1/2 O2 + 2H+ Æ Fe3+ + H2O
3) and the Fe3+ stimulates further chemical oxidation:
FeS2 + 14 Fe3+ + 8H2O Æ 15 Fe2+ + 2SO42- + 16H+
(i.e. H2SO4)
Progressive, rapid acidification of runoff waters can occur, with high concentrations of dissolved
minerals (e.g. Fe2+, Al3+) leading to toxic conditions downstream of ore beds and tailings piles.
However, this process can also be exploited through the commercial application of:
Bioleaching (biological oxidation of metal ores) 10E: pp. 669-672; 9E: pp. 689-694
e.g. Thiobacillus ferrooxidans (which is also a sulfur-oxidizer) is used to solubilize copper and gold
from waste ore in commercial processes used worldwide. A combination of biological and
electrochemical reactions are required, including those involved in acid mine drainage (above) and
chemical reactions with copper sulfides.