https://www.nytimes.com/2019/12/07/us/dna-bone-marrow-transp...
Maybe not quite a 'long-term' memory in the context you're after, but I wonder how this would compare to something like instinct and how we would tell the difference?
Somewhat related to this is Microsoft Research's project to use DNA for data storage: https://www.microsoft.com/en-us/research/project/dna-storage...