What If We Encounter Intelligent Autotrophs?

There's a common scenario in science fiction where the human race encounter intelligent herbivores who are horrified at eating meat. What if we encounter intelligent autotrophs who are horrified at eating anything?

Photoautotrophs are unlikely to evolve intelligence. They have little need for brains and little surplus energy to power brains. On the other hand, chemautotrophs might be more likely to develop intelligence. I once calculated that a pure carbon monoxide atmosphere would provide close to the amount of energy that would result from using the same volume of air to oxidize ordinary food. (It might also take a clever and mobile ET to locate and move to transient CO sources.) So if you ever wondered if New York or Los Angeles had been taken over by beings from another planet…

This scenario reminds me of the following quote from Joiwind in A Voyage to Arcturus:

Neither will we eat plants and animals, which are our fellow creatures. So nothing is left to us but water, and as one can really live on anything, water does very well.

as well as The Two Thousand Year Old Man:

Nothing but fresh, clean spring water … and a stuffed cabbage.