The marine red seaweed Pyropia yezoensis is grown on a large scale in Japan using mariculture for production of nori sheets. Here, we optimized the isolation of agar from P. yezoensis and evaluated its yield, physical properties, and product applications. An alkali pre-treatment of seaweeds before agar extraction increased the weight-average molecular weight of the agar and promoted the conversion of L-galactose sulfate to 3,6-anhydrogalactose, which in turn reduced the sulfate contents, thus altering the gel strength of the agar. This ability to adjust agar quality by altering the alkali concentration during material pre-treatment allowed production of agar with properties similar to those of agar from the red seaweed Gelidium sp., which is widely used for industrial agar applications. We demonstrate the suitability of P. yezoensis agar in production of solid plates for bacterial growth. In addition, the P. yezoensis agar was particularly useful as a gel material, with the capacity for excellent size separation of DNA by electrophoresis even without agarose purification, because of quite low contents of sulfate. These findings support the applicability and usefulness of P. yezoensis for agar production. The established large-scale cultivation methods for P. yezoensis can generate sufficient biomass to produce agar to support medical and biological studies.

The red seaweed Pyropia yezoensis has been demonstrated to be a novel resource for the production of high-quality agar. P. yezoensis is grown for the food industry in large-scale Japanese mariculture operations. However, discolored P. yezoensis is mostly discarded as an industrial waste, although it has some kind of utility values. Here, we evaluated the utility of discolored P. yezoensis as a resource for agar production. The quality of agar from the discolored seaweed was comparable to that from normal seaweed. In addition, as a distinguishing characteristic, agar yield was higher from discolored seaweeds than from normal types. Moreover, we successfully used agar from discolored P. yezoensis for bacterial plate media and DNA electrophoresis gels without agarose purification. Thus, our results demonstrate that discolored P. yezoensis is suitable for agar production and use in life science research. Diverting discolored P. yezoensis from disposal to agar production provides a solution to the current industrial waste problem in mariculture, as well as a secure source of agar for research purposes.