Porphyra has an annual value of more than US $1.3 billion and is considered the most valuable maricultured seaweed in the world. According to the Food and Agriculture Organization (FAO), nearly 1 397 660 metric tons (mt) (wet weight) of Porphyra were produced through mariculture. Porphyra has nearly 133 species distributed all over the world, including 28 species from Japan, 30 from the North Atlantic coasts of Europe and America, and 27 species from the Pacific coast of Canada and the United States. Six species of Porphyra (namely P. yezoensis Ueda, P. tenera Kjellman, P. haitanensis Chang et Zhen Baofu, P. pseudolinearis Ueda, P. dentata Kjellman, and P. angusta Okamura et Ueda) are usually cultivated in Asia with the first three being the most commonly cultivated.
Porphyra grows from 5 to 35 cm in length. The thalli are either one or two cells thick, and each cell has one or two stellate chloroplasts with a pyrenoid, depending on the species. Porphyra has a biphasic heteromorphic life cycle with an alternation between a macroscopic foliose thallus (the gametophytic or haploid phase), and a microscopic filamentous phase called the conchocelis (the sporophytic or diploid phase). Porphyra reproduces by both sexual and asexual modes of reproduction. In sexual reproduction, certain mature vegetative cells of the thallus differentiate into carpogonia, and others on the same or different thalli differentiate into spermatangia. After fertilization by ame boid sperm, the resulting fertilized carpogonia divide to form packets of spores called zygotospores (=carpospores).
After release, the zygotospores germinate, usually on a calcareous substrate, and develop into the filamentous 'con chocelis' phase. The conchocelis phase can survive in adverse environmental conditions but eventually gives rise to 'fat filaments', then conchosporangia, and finally conchospores under the appropriate environmental condi tions. The conchospores germinate in a bipolar manner to give rise to a chimeric thalli, thus completing the life cycle. Asexual reproduction happens only in some species (e.g., P. yezoensis), through formation of blade archeospores (previously referred to as monospores). The conchocelis serves as a perennating stage in nature. The conchocelis can also be maintained in laboratory cultures for long periods through vegetative propagation.
Cultivation of Porphyra began in Japan, Korea, and China during the seventeenth century or possibly earlier depending on folk lore from each of those coun tries. Modern techniques for Porphyra cultivation were introduced to these countries in the 1960s. The culture methods of Porphyra in all countries are basically very similar, with minor modifications, such as adaptations to local growing areas and traditional practices of local farm ers. The cultivation technique involves four major steps (Figure 1 ): (1) culture of conchocelis, fat filaments, formation of the conchosporangia and production and release of conchospores; (2) seeding of culture nets with conchospores; (3) nursery rearing of sporelings; and (4) harvesting.
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