Many Japanese people consume a diet rich in seaweed, including the edible red alga Porphyra, which is used for preparing sushi. Although humans cannot digest the seaweed polysaccharides (porphyran and agarose), certain marine Bacteroidetes do possess the necessary CAZymes. Curiously, Japanese individuals frequently harbor seaweed-digesting Bacteroides plebeius in their gut microbiomes, while individuals from North America do not. Bacteroides plebeius is not a marine bacterium, and its close relatives cannot digest seaweed. What is the mostly likely explanation for how B. plebeius acquired functional porphyranase and agarase genes?