|The red algal genus Gracilaria is of
considerable economic importance as an agarophyte (seaweed
producing the hydrocolloid, agar). Gracilaria has been harvested from naturally occuring stocks in a number of countries in the
developing world, but because of over-harvesting declining populations, and the need for a
constant and reliable supply, cultivation is an increasingly important source of raw
The cultivation of the agarophytic red alga Gracilaria is now of major importance in several parts of the world, such as Asia, South America and southern Africa. Gracilarioids have been cultivated in the open sea using various ground planting methods such as direct insertion of thalli in soft substrata, or by ropes and/or nets hung horizontally or vertically in the water column. Methods have been investigated using various land-based techniques such as tanks and ponds.
Bottom Stocking has been developed using several techniques to fix Gracilaria.
The Direct Method is one of the first methods which involves the direct insertion of Gracilaria into the sandy or muddy bottom using different types of tools.
The Plastic tube method involves fixing bundles of Gracilaria thalli to plastic tubes filled with sand that anchor the alga to the sea bottom.
Other methods include 10mm mesh nylon laced over rocks to keep the thalli in place, and the use of rubber bands to secure Gracilaria thalli to rocks.
Suspended cultivation has some advantages over bottom stocking, such as surface harvesting, positioning in optimal environment, crop predictablility and control, and epiphyte and grazing control. Two different approaches to suspended cultivation have been developed:
1. one from spores settled on ropes and lines, and
2. from vegetative thalli or cuttings being inserted into or tied to a rope/line.
"Seeding" lines or nets with spores is seen as an attractive method to produce a large biomass from small numbers of plants. Lines can be seeded by placing new lines in dense wild crops or by spore attachment to lines in nursery tanks on land, provided reproductive material is available.
There is a major drawback to this method, apart from it being expensive, this being the fact that spores may not always be available and their initial biomass is very small.
Vegetative propagation involves the insertion of thalli or vegetative cuttings into ropes or lines. The material to which the Gracilaria is attached was initially polypropylene ropes but recently netlon (also reffered to as "superope") has been used. Netlon is considered superior in commercial systems since it is far cheaper and does not cut the Gracilaria as it pulls tighter.
In Namibia, an experimental phase was initiated to investigate the feasibility of the cultivation of Gracilaria gracilis. A similar experiment was run in Saldanha Bay, South Africa, which proved to be ecologically and possibly economically feasible.
The suspended raft consists of rectangular frames of 18mm polpropylene rope, which is 25m long and 5m wide. They are suspended at 0.4m depth, from 25L plastic drums at intervals along the frame. These frames are anchored at four corners and in the middle on both sides, by lateral risers, each attached to 120kg of chain shackled to a 40kg iron block. The frame is tensioned between the anchors and floats. A loop of thin cord is inserted through the lay of the frame rope every 0.25m, to assist with positioning and retention of the seaweed cultivation lines. These lines are tied across the frame, so that 5m of their length can be stocked with Gracilaria.
USES OF GRACILARIA:
Gracilaria is used as a source of food for both humans and shellfish (abalone). It is also used as a raw material for the extraction of the phycocolloid agar. The first source of agar was from Gelidium in Japan. Gracilaria now plays a major role in the production of agar. The value of Gracilaria has increased with demand in the Carribbean for human consumption and the production of thickened milk drinks, and in Hawaii as a luxury "vegetable/salad". In Fiji and often Pacific Islands it is used to make a kind of seafood jelly that is quite popular.
The agar that is produced from Gracilaria and Gelidium has many uses. The most important use of agar is in bacteriological and fungal culture work, because after nutrient materials have been added, even a dilute solution sets to a firm jelly upon which the bacteria or fungi grow. Agar is used in a number of countries by embedding preserved cooked fish in the firm jelly, which protects it from breakage. It also prevents the spoiling of foods, especially in hot countries, where cooking with some agar provides a temporary method of preservation.
Agar is also used in bakery products for it's gelling properties. The gels are able to withstand high temperatures and are used as stabilisers in pies, meringues, pie fillings, icings and toppings. It is used in confectionary for it's rigid gel. Agar is used in industry in the sizing of fabrics, and because of the great demand, only the finest grades of agar are used for the valuable silks, whilst the poorer quality can be used for muslin. The agar that is of poorer quality is used as a coating in the manufacture of paper, and also in the manufacture of waterproof cloth. It is used as a glue, and as a cleaning medium for liquids. Leather manufacturers consider agar to be very important as it helps with the finishing processes, by adding the gloss and stiffness to the final product. Agar is used as a lubricant in the hot drawing of tungsten wire for electrical lamps, as well as in the photographic industry, in the manufacture of plates and films. Agar acts as a thickening agent in the manufacture of ice-creams, malted milks, jelly, candies and pastries.
Agar is used in western countries in the brewing of beer and the manufacture of wines and coffee, where it is used as a clarifying agent. It also finds a use as a pill and suppository excipient, as a base in shoe stains, shaving soaps, cosmetics and hand lotions. In the war-time, agar enabled wounds to be cleansed properly, because agar contains a principle that stops blood clotting.