Etymology
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algae (n.)
(plural), 1794, from alga (singular), 1550s, from Latin alga "seaweed," which is of uncertain origin. Perhaps connected to Latin ulva "grass-like or rush-like aquatic plant," or perhaps from a PIE root meaning "to putrefy, rot," but de Vaan considers this unlikely and suggests it might be a foreign loan-word.
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alga (n.)
1550s, singular of algae (q.v.).
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algal (adj.)

"of or pertaining to the algae," 1846, from Latin alga (see algae) + -al (1).

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algologist (n.)
"student of seaweed," 1830, from Latin alga (see algae) + -logist (see -logue). Related: Algology.
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phytoplankton (n.)

"microscopic marine algae, the plant parts of the plankton community," 1897, from phyto- + plankton.

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phycology (n.)

"study of seaweeds, the department of botany that treats of algae or seaweed," 1847, from phyco- + -logy. Related: Phycological; phycologist.

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volvox (n.)
genus of fresh-water algae, 1798, from Latin volvere "to roll," from PIE root *wel- (3) "to turn, revolve." So called from their motion.
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prokaryotic (adj.)

"having no nuclear membrane in its cell" (as bacteria and blue-green algae), 1957, from prokaryote + -ic. Related: Prokaryon.

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Red Sea (n.)
the Greek thalassa erythra; the reason for the name is unknown; speculation has traced it to: 1. algae in coastal waters; 2. sandstone rock formations on the shores; 3. a tribal name; 4. ancient association of "red" with "south" (as "black" with "north").
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thallus (n.)
1829, Latin, from Greek thallos "green shoot, young branch, twig," related to thalia "abundance," thalos "scion, child," ultimately from PIE root *dhal- "to bloom" (source also of Armenian dalar "green, fresh," Albanian dal' "I sprout," Old Irish duilesc, a type of algae).
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