The English language has a diversity of plural forms. Some of the irregular ones occur quite often in the marine sciences. For many of these, the plural is formed as in Latin. Even if your writing is a perfect example of clarity, and you avoid all unnecessary jargon, you may find that you need to use the word algae or data once in a while.
While neither algae nor data ends with an ‘s’, these words are both plural. Thus, you should not write “my data is confusing”; you should write “my data are confusing.” Don’t say, “I saw a green algae in the intertidal”; say “I saw a green alga in the intertidal.” Writing “data is” is an easy mistake to make, and something to watch for when you are editing your work.
This is what The Elements of Style has to say about data: “Like strata, phenomena, and media, data is plural and is best used with a plural verb. The word, however, is slowly gaining acceptance as a singular” (p. 44). So, while you may see data used from time to time as a singular, for formal writing it is best to treat it as plural. (The Elements of Style seems to be silent on the question of algae.)
Strunk, William Jr., 2000. The Elements of Style, with Revisions, an Introduction, and a Chapter on Writing by E.B. White, 4th Edition. Allyn and Bacon, a Pearson Education Company: Needham Heights, Massachusetts.