Alot of bacteria are good.. Primarily aerobic ones.. Soil is packed with them.. You can do symbiotic hydro grows, but it can get rough if you aren't on the ball and/or don't have sufficient O2, but its generally unadvised.. I did a couple goes with goldfish and supplimentary bacteria that convert waste to uptakable forms.. Worked well, but was kind of a pain and killed alot of fish!
Hydro carb suppliments contain the enzymes these bacteria etc produce to do that without the bacteria, and I don't know what to think of that because alot of these things only use carbs to assist in the production of non carb compounds..
I switched to soil though, and like that waay more..
Anyways, bacteria etc can thrive in hydro systems with or without carb addition (the plants do dump sugars from their roots remember).. To reiterate though, carb loading is about helping bacteria etc thrive..