[moby_click]

As their name implies, carbohydrates consist of carbon and water (hydrogen and oxygen), hydrated carbon. Amino acids need at least nitrogen in addition, which in our digestive tract is probably already most abundantly found in amino acids. The microbiome does consume carbohydrates though, mostly those that are inaccessible to our digestive system.

I'm not an expert on this, but my guess is that by mass their most important products for us are short chain fatty acids. They might assist in breaking down some proteins that we lack enzymes for or modifying amino acids, but the absorption of amino acids happens mostly in earlier sections of our intestines, whereas the microbial biomass and activity increases towards the later stages of the gut.