obligate aerobe Any organism that must utilize
atmospheric oxygen in its metabolic pathways and for
cellular respiration, and cannot survive without it. The
adjective obligaterefers to an environmental factor.
See alsoAEROBE.
obligate anaerobe Any organism where atmospheric
oxygen is toxic to its growth; grows only in an anaero-
bic environment. The adjective obligate refers to an
environmental factor.
See alsoANAEROBE.
oceanic zone The deep water of the oceans beyond
the shelf break. The ocean is divided into zones. The
whole mass of water is called the pelagic. This is
divided into major subzones, including the neritic,
which covers all water to a depth of 600 feet, and the
oceanic zone, which covers all water below 600 feet.
The oceanic zone is further divided into subzones. The
mesopelagic, semidark waters, covers the depths from
650 feet to 3,200 feet, which is the middle layer
between the upper (sunlit 650 feet) epipelagic and the
lower cold and dark bathypelagic.
See alsoAPHOTIC ZONE; LITTORAL ZONE.
ocean thermal vents A hydrothermal vent is an area
where a major fissure occurs between plates making up
the Earth’s crust. As the plates gradually separate,
underlying volcanic activity reaches the surface, and
the molten volcanic rock meets ultracold seawater,
causing physical and chemical reactions. These vents
are the habitat of deep-sea animals formerly unknown
to science.
See alsoAPHOTIC ZONE.octahedral Having symmetry of a regular octahedron.
Molecules and polyatomic ions having one atom in the
center and six atoms at the corners of an octahedron.octahedral hole A cavity or space between six atoms
or ions in a crystal in which these atoms represent the
corners of an octahedron.octahedron SeeCOORDINATION.octane number(octane rating) A number indicating
the relative antiknock value of a gasoline. Octane num-
bers are based on a 100 scale on a comparison of refer-
ence fuels. Isooctane is 100 (minimal knock) and
n-heptane is 0 (bad knock).octet rule A principle that bonded atoms have eight
outer electrons (including those shared with other
atoms), although there are exceptions.199