Environmental Engineering Reference
In-Depth Information
earth's interior, which cools and forms reduced sulfur compounds [
2
]. The sulfate
in seawater then reacts to form hydrogen sulfide as well as sulfur-containing
minerals such as pyrite (FeS
2
), chalcopyrite (CuFeS
2
), and pyrrhotite (Fe
1
x
S)
[
2
], which form the surface chimney structure that is characteristic of hydrothermal
vents. The hydrogen sulfide provides the energy, rather than light, for the
chemoautrophic microorganisms that form the base of the hydrothermal vent food
web [
33
]. Some species of microorganisms can operate in aerobic conditions, using
oxygen as the electron acceptor, while others have the ability to carry out this
reaction in anaerobic conditions, using nitrate, sulfate, or sulfur as the electron
acceptor [
2
]. These organisms survive in symbiotic relationships with other
organisms living near the hydrothermal vents. The microorganisms, which are
endemic to hydrothermal vent environments, allow unique communities to develop
and help maintain the oceanic sulfur cycle by transforming hydrogen sulfide
released by the hydrothermal vents into sulfate [
33
].
The Biogeochemical Cycle of Oxygen
Oxygen is involved in all nutrient cycles, and its presence or absence dictates the
reactions that will occur in a specific marine environment. Oxygen gas can be
introduced into marine environments across the air-sea interface (e.g., by diffu-
sion). However, oxygen concentration is controlled by the biological processes of
photosynthesis and respiration, and by physical processes such as mixing within the
water column. In the euphotic zone, phytoplankton photosynthesis produces
oxygen, which is then used as the electron acceptor to conduct aerobic respiration.
This process is carried out by both autotrophic and heterotrophic organisms
throughout the water column.
Oxygen concentration generally decreases with depth in the ocean. Photosynthesis
can only be carried out in the lighted parts of the water column, but respiration
continues throughout the water column. As the organic matter from the surface layers
sinks, it is taken up by organisms and used to conduct respiration, depleting oxygen
levels. Some marine environments, particularly in marine sediments, are suboxic, with
oxygen concentrations less than 0.2 ppm (but still detectable), or anoxic, with oxygen
concentrations below detectable levels [
2
]. Organisms survive in these environments
by using anaerobic respiration, in which compounds such a nitrate, sulfate, iron, or
even organic matter are used as alternative electron acceptors to oxygen [
2
].
Anoxic zones are not limited to marine sediments, with increasing attention
being paid to decreasing oxygen concentrations in previously oxygen-rich areas of
the ocean. Hypoxic zones, marine environments with oxygen concentrations below
2mgL
1
, typically form when primary productivity is high, leading to increased
organic matter in the system and increased respiration, and when mixing throughout
the water column is low, preventing the oxygen in the upper water column from
reaching lower layers [
34
-
36
]. Hypoxic zones have been increasing in frequency,
including the Gulf of Mexico and Chesapeake Bay [
34
,
35
]. Factors such as
