Biggest Organism: Big Honkin' Mushroom v. Great Barrier Reef?

There's a Big Honkin' Mushroom in Oregon that covers 2,200 acres and is claimed to be the “largest living organism.” My question is whether a mushroom is a single organism or if it's communal and therefore in the same class as the Great Barrier Reef (which extends for more than 2,500 kilometers and has single bommies that cover 250,000 acres!)

(Googling for “mushroom communal organism” is terrifying proof that hippies know HTML.)