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; growth can occur only in
an anaerobic environment. The adjective obligaterefers
to an environmental factor.
See alsoANAEROBE.
oceanic zone The ocean is divided into zones. The
whole mass of water is called the pelagic. This is divid-
ed into two major subzones: the neritic zone, 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 alsoPELAGIC ZONE.
octahedron SeeCOORDINATION.
ODMR See OPTICALLY DETECTED MAGNETIC
RESONANCE.
OEC SeeOXYGEN-EVOLVING COMPLEX.olfaction The process of smell. In humans, chemore-
ceptors are located in a patch of tissue about the size of
a postage stamp, called the olfactory epithelium, that is
located high in the nasal cavity.oligochaete Worms of the phylum Annelida, class
Oligochaetae. Chiefly terrestrial and freshwater worms
with distinct body segmentation and no apparent head.
The earthworm is a familiar example.oligogyny Multiple egg-laying queens that each have
their own territory within a colony.oligonucleotide Macromolecules composed of short
SEQUENCEs of NUCLEOTIDESthat are usually syntheti-
cally prepared and used, for example, in site-directed
MUTAGENESIS.oligotrophic lake Acondition of a lake that has
low concentrations of nutrients and algae resulting in
clear blue conditions. Contrast with mesotrophic
lakes, which have a moderate nutrient condition,
and eutrophic lakes, which have excessive levels of
nutrients.253