Would that be from water, soil, plants, funguses,...?
I asked the question in Quora but that, at the time of posting in this forum, haven't been very helpful. I also have searched for the information, but I haven't been able to find it. In Vegan Health it says that
"The vitamin B12 component in B12 supplements and fortified foods is made by bacteria and sourced from bacteria cultures; it is not taken from animal products"
But I wonder where did those cultures come from. One day someone had to collect the bacteria from somewhere in order to start the cultures.