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I was once walking round one our local lakes with my family and there was a sign that read:

Blue-green algae is present in the lake [emphasis added]

I maintained that it should be are, however we have never been sure and I have seen it referenced as both on the web and in biological journals.

Which one is correct?

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I found wiki.answers.com/Q/What_is_the_plural_form_of_algae that a single organism is ALGA and therefore is "are", still uncertain about algae. –  Alex 'Munky' Fell Dec 12 '12 at 17:49
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Algae is a Latin plural which is not perceived as a plural by most people, so it's often employed as a singular or mass noun. Alga/algae is the same problem as datum/data, but not so heatedly argued - perhaps because algae plays a much smaller role in our everyday discourse than data. –  StoneyB Dec 12 '12 at 18:06
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@StoneyB: Speak for yourself! –  Cerberus Dec 12 '12 at 18:19
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@Cerberus Naturally, when I speak of "most people" I mean "mostpeople" who (as e.e.cummings teaches us) are not "you and me". –  StoneyB Dec 12 '12 at 18:26
    
@StoneyB: Yay! All is well, then. –  Cerberus Dec 12 '12 at 19:26

3 Answers 3

up vote 8 down vote accepted

The National Geographic Style Manual at http://stylemanual.ngs.org/home/A/alga-algae has (selected extracts):

Alga is the singular noun and requires a singular verb

Algae is the plural form and requires a plural verb: When the algae die, they fall to the bottom.

Occasionally algae may be considered a collective [mass] noun and treated as singular: A map shows how algae periodically blooms in the bay. Look! There it has drifted under the bridges, and out into the ocean, where it hovers over the reef.

However, there is an inconsistency in their approach; the example they give for the use of the singular noun is:

A colony of Licmophora fans itself across another alga, Bonnemaisonia. This should surely be another mass-noun usage, but they're using the singular, alga. Their example should read: A colony of Licmophora fans itself across a colony of another alga, Bonnemaisonia.

The trouble here is that alga is rarely considered as a single particle - more often as a single strain. One virion or one bacterium are reasonably commonly considered.

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I think that just about sums it up. The fact of the matter is alga is much less common than algae. The NG writer is being somewhat pedantic, to say the least, and it's no surprise he can't even apply his own rule consistently. I think it would be more accurate to say "algae" is often/usually used as a "mass noun", rather than occasionally. –  FumbleFingers Dec 12 '12 at 18:08
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Be aware that algae is will horribly shock the sensibilities of pedants (like me). If you want to keep everybody happy, say algae are. Nobody will object to that. –  Cerberus Dec 12 '12 at 18:22
    
Do you also insist on confetti are, or is your pedantry selective? –  Edwin Ashworth Dec 13 '12 at 8:28
    
@Cerberus: Spaghetti are on the plate? Besides, IMHO, algae are present means that a sample has found at least two organisms, whereas algae is present means you can see a mass of the stuff. –  TimLymington Dec 13 '12 at 11:20
    
@TimLymington: your usages keep me happier. –  Edwin Ashworth Dec 13 '12 at 17:48

Algae are is correct.

Per Dorland's Illustrated Medical Dictionary, 31st Edition (bold mine):

alga — any individual organism of the algae

algae — a large group included in the kingdom Protista, now classifed into several differnt phyla. They are cryptogamous plantlike organisms in which the body is unicellular or consists of a thallus. Algae include the seaweeds and many unicellular fresh-water plants.

I would say in your two links, the more scholarly sites and publications use plural verbs with algae. The public, when asking questions about things like blue-green algae in a backyard pool, seem more likely to use a singular verb.

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The dictionary doesn't seem to include the sense 'two or more individual organisms' when defining algae. Or the 'virtually uncountable tide' (I'd like to use algal mass, as in kelp forest, but mass also has a partly conflicting grammatical sense here, of course) polyseme. –  Edwin Ashworth Dec 13 '12 at 0:54

While algae is plural in form, in English, it is usually not a count noun: for example, we can say one cow or two cows. We don't usually say one alga or two algae: unless you're talking about different types of algae. So, if you're saying something like "the algae covers everything", the singular would be OK. But, if you're saying, "algae are divided into five species...." then it should be plural.

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In your last example, the subject of the sentence would be species (plural), not algae. Algae would be the object of the prepositional phrase. –  JLG Dec 12 '12 at 19:08
    
point taken. I've edited appropriately –  A.Ellett Dec 12 '12 at 19:12
    
Why the minus points? –  A.Ellett Dec 12 '12 at 19:53

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