Buttermilk contains lactic acid bacteria, which are necessary for the production of sourdough.
During the production of natural sourdough, the so-called lactic acid fermentation takes place in the sourdough (souring phase). During this process, sugar is fermented by lactic acid bacteria into lactic acid. If you would like to add an alternative lactic acid starter, you can also try natural yoghurt.
Parallel to lactic acid fermentation, alcoholic fermentation takes place via yeasts. Yeast fungi are present almost everywhere, e.g. also in the rye meal itself, so that yeast does not have to be added separately. However, it can be useful to add a small amount of dry or compressed yeast to accelerate the formation of the sourdough.