IANAL, but I think you can trademark letters, and that similarly to colors, given the letter is known to correspond to your business in a particular market segment, and with the caveat that the trademark would only apply in that market segment.

Theoretically a space/" " is just another character, so maybe. But practically speaking, it would probably be legally filed as "space", and the company's trademark would be on either them naming their company "space", or on them stylizing it as a " ". That's my best guess, at least.