Proto-cooperation
It is an association of mutual benefit between two populations,
but not obligatory and only complementary. Both population are
capable of surviving in their natural environment on their own, although
when offered, the association offers some mutual advantages.
For an example, the mixed culture of Proteus vulgaris (produce
biotin/requiring nicotinic acid) and Bacillus polymyxa (produce
nicotinic acid/ requiring biotin). Both grow as partner bacterium synthesizes
the missing vitamin.