http://www.bbc.com/earth/story/20141114-the-biggest-organism-in-the-world
Is it really so far fetched that a larger scale version of this may have evolved with subspace/interdimensional properties in the futuristic Trek universe? We've already been shown that in Trek there are beings that exist in different dimensions out of phase or live in subspace itself. And there are giant Space Amoeba...I mean, how is this so hard to fathom?
The writers deliberately used the word mycelial because of the root structures and how they could pass information and chemicals quickly. They took a real world phenomenon and then expanded on it and transformed it in a way which was pretty inventive. And, in my opinion, much more well thought out than say a roaming Space Amoeba, or giant cat chasing our heroes through a mansion, or crew members devolving (or evolving in the case of Threshold) into giant spiders or salamanders.