According to a May, 2010 article by Karen Schwarzberg and Mike Gurney in the Microbe Blog Small Things Considered,
I don't have any particular problems digesting nori or the various other seaweeds I ingest on an almost daily basis.the Japanese can digest their nori (Porphyra) courtesy of the particular strains of B. plebeius that they carry, strains that are not found in North Americans. Furthermore, genomic evidence suggests that the initial acquisition of the β-porphyranase genes by B. plebeius was by horizontal transfer from a marine Bacteroidetes. The likely site for the transfer might well have been within the human GI tract since such Bacteroidetes are present on the uncooked seaweeds that have been avidly eaten by the Japanese since at least the eighth century. The transfer would have provided the host with a selective advantage: the ability to metabolize and derive energy from a frequently-eaten carbohydrate source that others in the population could not digest. Interestingly, the paper (Hehemann, Nature) reports an incident of transfer from mother to infant, implying that we can pass those modified bacteria to close family members and perhaps others.