Do you use an or a before eutrophic?

Well, you can use either word before I, depending on the sentence. For example: George and I went to the prom. An I is necessary in the word taint. You might be asking whether it is "a" or "an" ......

Use I only when it is the subject of a verb. At all other times, use me.

Telephones that were connected by wire. (Land lines)

There is no word or phrase in English that requires a comma. Commas indicate a pause in speech, or the introduction of a clause, or some other separation of ideas necessary to prevent confusion.

Yes, however it's usually grammatically unnecessary. Consider: "You don't need me, and I don't need you." The comma makes it look/sound better, but it's not really needed.