[toomanybeersies]

Well you could become a fungitarian I guess? You'd really struggle to get enough calories purely from fungus though, I guess you could get a decent amount from algae if you build some nice large algae reactors to feed you.

[AnimalMuppet]

I've thought about plankton. You ought to be able to make a pretty nourishing stew out of (the right varieties of) plankton.

Note well: I have no idea how restrictive a subset "the right varieties" is, nor how hard it is to obtain plankton that are only from that subset.