Agronomy and Utilization of Sea Lettuce (Ulva)

Green distromatic seaweeds of the genus Ulva (Sea Lettuce) are commercially cultivated in East Asia for the food produce Aonori. Seaweed having resemblance to lettuce are in fact U.Fasciata, U.Lactuca etc.- that are distromatic blades, while commercially cultivated species include U.Prolifera (=Enteromorpha prolifea) and U.intestinalis that are tubular with the walls of tube one cell thick.

Habitat is upper eulitorral zones of subtropics, main causative agent of green tides, including the one that happened along the shores of Quigdao in 2008 during Bejing Olympics. This is a dioecious species having diplomatic lifecycle with isomorphic gametophytic and sporophytic stages.

Thalli are coarsely filamentous that can be branched or unbrached and have dis-like holdfasts developed from basal cells that attach firmly on suitable substratum like pebbles or rocks. Upon maturity, apical region of the haploid gametophytes changes color to orange-yellow (males) or yellow-green (females) and release biflagellate gametes. Fertilization can be isogamic or anisogamic. Zygotes attaches to suitable substrata where it germinates and develop into diploid sporophyte generation. Apical parts of the sporophytes mature in a similar fashion to that of gametophytes and produces quadriflagellate zoospores by meiosis. Released zoospores settle on substratum and germinate to respective gametophytes, completing the life cycle.

Cultivation method

In natural seeding method, rope nets are submerged in places where this alga grows naturally. Once the germination is detected, the nets are taken and brought to the farm. In artificial seeding method, matured thalli are collected and gamete release is induced with a combination of brief drying, followed by floating in seawater with intense illumination. Fragmenting the thalli is found to be an effective way to induce gametogenesis and is used for its artificial seeding. Released gametes are allowed to get settled on culture nets by providing dark conditions. Once germinated, these nets are taken to the farm. Nets with germinated algae are cultivated on poles fixed in shallow, calm areas of ocean or in estuaries, such that the nets periodically get exposed to air with each low-tide.

Tank cultivation using deep seawater, the so called “germling cluster method” have also been developed, although not widely implemented for the commercial cultivation. Information on its phylogenetics and phylogeography are known making it one of the well-studied green algal lineages. While reproduction by fragmentation is not known, settled vegetative fragments of this seaweed was found to be a source for the succession of green tides.

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Monostroma: the Jeweled Seaweed for Future- Felix Bast (Page 47-48)