| OBJECTIVE
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A magnifying lens of a microscope. Most microscopes carry a selection of objectives. The objective is located near the object being viewed. [Link to this definition]
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| OCULAR
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The same as eyepiece, refers to the lens of a microscope into which one looks when trying to view an image of an object. [Link to this definition]
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| ODONTOSTOME
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A type of polyhymenophoran (spirotrich) ciliate distinguished by flattened bodies, few somatic cilia and the sculpting of the body (e.g. Epalxella, Fig. **). [Link to this definition]
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| OFF-GREEN
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A term used to describe the colour of some chloroplasts which lack chlorophyll B, but usually have an olive or browny hue. [Link to this definition]
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| OLIGOHYMENOPHORA
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A type of ciliate with buccal ciliature comprised of an undulating membrane and three membranelles; e.g. peritrichs, Colpidium, etc. [Link to this definition]
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| OLIGOTRICH
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A kind of polyhymenophoran ciliate (STEP **) in which the adoral zone of membranelles is used not only in the acquisition of food but also serves to propel the cell (e.g. Halteria, tintinnids, Figs **, **). [Link to this definition]
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| OOMYCETES
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A type of fungus, the mature organism is in the form of walled mycelia but produces flagellated stages in the life cycle, related to the chrysomonads. [Link to this definition]
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| Operculum
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Literally, lid or bowl-shaped flap. Term used for diverse protist structures including (1) the “plug” of an emergence pore of a cyst or spore, (2) the whole or partial covering of certain peritrich loricae , (3) the so-called “epistomial disc” (an anterior part of the body of certain other peritrich ciliates), or (4) the circumscissile bowl-shaped flap dissolved from a fungal sporangial wall allowing spore discharge. In fungi, “operculum” is sometimes synonymized with “exooperculum”. Operculate (= exooperculate) discharge occurs on sporangia releasing zoospores among some chytrids. This term is also used among fungi describing asci-releasing ascospores. [Link to this definition]
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| ORGANELLE
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A discrete structure found within eukaryotic cells. [Link to this definition]
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| ORGANIC
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Chemical compounds which have been produced by organisms, said mostly of sugars and polysaccharides, proteins etc., essentially the hydrocarbons. Does not include inert chemical compounds, such as silica, produced by some organisms. [Link to this definition]
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| OSMOTROPHIC
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A form of nutrition in which soluble compounds are taken up by the organism, either by pinocytosis or by mechanisms capable of transporting one or a few molecules at a time (membrane pumps). [Link to this definition]
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| Outer rodlet layer
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Distinct zones of rods scattered in spore walls of ascospores and conidia; best visualized with freeze-etch techniques. In hydrophobic conidia the structure initiating interface with host insect cuticle of invertebrate fungal pathogens. [Link to this definition]
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| OVERLOADED
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Used in reference to sewage works which have more incoming sewage than they are designed for. Overloaded systems are equivalent to polluted natural environments, and usually have a poor quality effluent. [Link to this definition]
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| OXIDISE
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To add oxygen. [Link to this definition]
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