Monostroma nitidum (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

1 & 2, habit of gametophytes; 3, cross-section of a gametophytic thallus; 4, detail of cells of a gametophytic thallus, surface view; 5, fertile cells with gametes in a gametophytic thallus; 6, gamete with two identical flagella and an eye-spot; 7, sporophyte with many spores with eye-spots; 8, zoospore with four identical flagella and an eye-spot.

Monostroma nitidum Wittr.

Protologue: Förs. Monostroma: 41, pl. II, fig. 7 (1866).

Family: Monostromaceae

Chromosome number: 2n= unknown

Synonyms

• Monostroma latissimum Wittr. (1866).

Vernacular names

• China: jiao-mo
• Japan: aonori.

Origin and geographic distribution

M. nitidum is widely distributed in the South-East Asian region, particularly in Thailand, Vietnam and the Philippines. It is very common in China and Japan.

Uses

M. nitidum is widely used as human food. Dried fronds are boiled with sugar, soya sauce and several chemicals, and then bottled as "nori-jam". Fresh M. nitidum is used as a vegetable in salads. Both fresh and dried M. nitidum are used in soups. It is also used as animal feed and as medicine (antibacterial, antitumour, lowers blood pressure, reduces blood cholesterol level).

Production and international trade

Approximately 1500 t (dry weight) of M. nitidum are produced annually in Japan for commercial purposes. This mainly consists of what is known as M. latissimum, which is preferred over M. nitidum because of its larger fronds. No data are available on production or phycoculture of M. nitidum in South-East Asia, although it is often mentioned as a commercially used alga.

Properties

Dried M. nitidum from Japan contains per 100 g dry weight (in g): protein 20.0, lipids 1.2, non-fibrous carbohydrates 57.2, fibrous carbohydrates 6.7, and (in mg) K 810, Na 690, Ca 690, P 200, Fe 2.5 and traces of Mn, Se and Mo. It also contains C₆-C₁₂ phenols and 3,4 dihydroxyphenylethylamine dopamine.

It also contains nearly 200 mg per 100 g (dry weight basis) of a compound identified as dimethyl β-propiothetin. This compound has been found to prevent gastric ulcer in guinea pigs as well as to act as a tonic for liver function. Another compound, 3,4 dihydroxyphenylethylamine dopamine liberates dimethyl sulphide when exposed to air. Dimethyl sulphide, which emits a flavour typical of marine algae, is enzymatically formed. M. nitidum contains several kinds of betaine, of which β-homobetaine shows a high activity for lowering the artificially elevated level of plasma cholesterol in rats. The dialyzed residue of hot water-extractable substances from M. nitidum have been demonstrated to increase the life span of male mice implanted with L-1210 leukaemia cells.

Description

• Mature blades (gametophytes) yellowish-green, soft, lubricous, membranous, with crisped margins, 2-6(-15) cm long, one cell thick.
• Cells polygonal, mostly grouped in twos with slightly rounded corners, 12-15 μm in diameter (surface view). Life cycle diplo-haplontic and heteromorphic.
• Gametophytes dioecious; gametes 6-7 μm long, elongated, ovoid, with two identical flagella and eye-spot.
• Zoospores 9-10 μm long, in inconspicuous unicellular sporophytes, with four identical flagella and eye-spot.

Growth and development

Gametes of M. nitidum fuse by isogamy, forming small and unicellular sporophytes. Zoospores germinate and form the much larger, leafy, gametophytic fronds.

Other botanical information

There are not many references to the conspicifity of M. nitidum and M. latissimum, and in several papers they are considered as separate species. Of these, M. latissimum most probably does not occur in South-East Asian waters, although it has been recorded from Palawan (the Philippines).

Ecology

M. nitidum is found growing in brackish water habitats such as estuaries, river mouths and inner bays where it is influenced by freshwater. Thalli are attached to solid substrates. The leafy gametophytic stage appears in autumn and lasts to early summer and is thus highly seasonal in distribution.

Propagation and planting

There are two methods of collecting M. nitidum spores: natural and artificial seeding. In the natural "seed collection" method, zoospores are collected on culture nets submerged in selected collection grounds where natural Monostroma populations appear in early autumn. In recent times, poor collection techniques in Japan, render the water at the bottom of inner bays turbid, and have led to the development of artificial seeding. Here "seed" culture begins by collecting many gametophytes during neap tides in April. The release of gametes may be induced by drying fronds overnight in the dark. A gamete solution is mixed in filtered seawater to form zygotes by isogamy. The zygotes then adhere to plastic settlement boards (20-30 cm long and 10 cm wide with both surfaces roughened on a glass-grinding machine). During summer the zygote settlement boards are maintained in large water tanks. These culture tanks are kept at room temperature and receive natural sunlight of 20-50 μmol photons/m²/s. The seawater used for cultivation is supplemented with a solution containing nutrients and a weed killer. The cultivated zygotes (now sporophytes) gradually increase in size, reaching 50-60 μm after three months. Sporophytes are large enough by the middle of August to be allowed to mature by early September. Maturation of the sporophytes is promoted and synchronized by providing dark conditions for 2-3 weeks. Subsequently the settlement boards are transferred to smaller tanks which are exposed to a higher light intensity (100 μmol photons/m²/s) provided by white fluorescent lamps and are used for the production of concentrated zoospore solutions. The zoospore solution is diluted in a large water tank using filtered seawater. Culture nets are then immersed in the water tank and zoospores attach themselves to them. The culture nets with attached zoospores are transferred to the cultivation grounds the following day.

Phycoculture

M. nitidum is usually cultured in the shallow and calm areas of inner bays or river mouths where they are exposed to the influence of freshwater. One method of cultivation is the pole system, in which culture nets are supported by bamboo poles. The nets are set out horizontally at the level where the algal fronds will be exposed for about 4 hours during low tide. A second culture method is the floating system where nets are kept under the surface of the water. This allows the alga to be grown in deeper parts of the sea. During the germination of zoospores into Monostroma blades the seeding nets are at first spread in sets of about 5 "nursery nets" tied together. Once the fronds attain a length of 1-2 cm, individual nets are transferred to the culture ground for further growth. The growth rate usually increases in winter after transplantation.

Harvesting

M. nitidum nets in Japan are harvested 3-4 times during the culture period. The total production is approximately 6-10 kg per net. A slight decrease in population density is observed immediately after harvesting. However, the cut fronds regenerate quickly and attain a harvestable size again after 3-4 weeks of growth.

Yield

Total yield of M. nitidum is 300-500 g/m² dry weight after 3-4 harvests. The total production of nets which are harvested 3-4 times per year is higher than of nets which are harvested only once.

Handling after harvest

Harvested fronds of M. nitidum are cleaned of contaminating organisms, washed well in seawater and freshwater, and then dried outdoors. Dryers may be used during cloudy and rainy conditions. Dried Monostroma is then boiled and bottled.

Prospects

The increasing demand for green laver and its finished products in Japan augurs well for the M. nitidum industry.

Literature

• Challenger, F., Bywood, R., Thomas, P. & Hayward, B.J., 1957. Studies on the biological methylation. XVII. The natural occurrence and chemical reaction of some thetins. Archives of Biochemistry and Biophysics 69: 514-523.
• Masao, O., 1993. Cultivation of the green algae Monostroma and Enteromorpha "aonori". In: Ohno, M. & Critchley, A.T. (Editors): Seaweed cultivation and marine ranching. Japan International Cooperation Agency, Yokosuka, Japan. pp. 7-15.
• Ohno, M. & Vo Duy Triet, 1997. Artificial seeding of the green seaweed Monostroma for cultivation. Journal of Applied Phycology 9: 417-423.
• Yamamoto, I., 1982. Antitumor activity of crude extracts from edible marine algae against L-1210 leukemia. Botanica Marina 25: 455-457.

Sources of illustration

Hori, T. (Editor), 1994. An illustrated atlas of the life history of algae. Vol. 1. Green algae. Uchida Rokakuho Publishing Company, Tokyo, Japan. Fig. 85 (habit 2, detail gametophyte, gamete, sporophyte, zoospore); Pham-Hoang Hô, 1969. Rong bien Vietnam [Marine algae of South Vietnam]. Ministry of Education and Youth, Trung-tam hoc-lieu xuat-ban. Fig. 4.2, p. 398 (habit 1, cross-section and details of vegetative gametophytic thallus). Redrawn and adapted by P. Verheij-Hayes.

Authors

• G.C. Trono Jr