Marine algae contain sulphated polysaccharides that are absent in terrestrial plants. These unique polymers are used as a carbon source by marine heterotrophic bacteria that produce specific CAZymes. In comparison to the accumulating knowledge on the degradation of plant polysaccharides little is known about the enzymes acting on polysaccharides from marine edible algae such as Porphyra (nori), Ulva (sea lettuce) or Undaria (wakame).

Gut microbes supply the human body with energy from dietary polysaccharides through carbohydrate active enzymes, or CAZymes, which are absent in the human genome. These enzymes target polysaccharides from plants that dominated diet throughout human evolution
The array of CAZymes in gut microbes is highly diverse, exemplified by the human gut symbiont Bacteroides thetaiotaomicron, which contains 261 glycoside hydrolases and polysaccharide lyases, as well as 208 homologues of susC and susD-genes coding for two outer membrane proteins involved in starch utilization.
The question of how this diversity evolved by acquiring new genes from microbes living outside the gut. In this paper by Hehemann et al in Nature April 2010 the authors characterize the first porphyranases from a member of the marine Bacteroidetes, Zobellia galactanivorans, active on the sulphated polysaccharide porphyran from marine red algae of the genus Porphyra. They show that genes coding for these porphyranases, agarases and associated proteins have been trans¬ferred to the gut bacterium Bacteroides plebeius isolated from Japanese individuals
Their comparative gut metagenome analyses show that porphyranases and agarases are frequent in the Japanese population and that they are absent in metagenome data from North American individuals.
Seaweeds make an important contri¬bution to the daily diet in Japan (14.2 g per person per day), and Porphyra spp. (nori) is the most important nutritional seaweed, traditionally used to prepare sushi
This indicates that seaweeds with associated marine bacteria may have been the route by which these novel CAZymes were acquired in human gut bacteria, and that contact with non-sterile food may be a general factor in CAZyme diversity in human gut microbes.

Hehemann et al 2010 Transfer of carbohydrate-active enzymes from marine bacteria to Japanese gut microbiota Nature vol 464 pp 908-912

Sonneburg Genetic pot luck 2010 Nature vol 464 837-8

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