Answer:
No, the sentence is not a run-on sentence. A run-on sentence is a sentence that contains two or more independent clauses that are not joined by a conjunction or a punctuation mark. An independent clause is a group of words that contains a subject and a verb and can stand alone as a complete sentence.
In the sentence "The honey mushroom, a fungus spreading over more than two thousand acres across eastern Oregon's Malheur National Forest, is thought to be the largest living organism on Earth," there are two independent clauses: "The honey mushroom, a fungus spreading over more than two thousand acres across eastern Oregon's Malheur National Forest" and "is thought to be the largest living organism on Earth." These two independent clauses are separated by a comma, which acts as a conjunction and connects the two clauses into a single sentence. Therefore, this sentence is not a run-on sentence.
Step-by-step explanation: