SEAWEEDS are multicellular algae that are found in marine and brackish water and get attached to any substrate in the water at some stage in their lives. Globally, there are about 10,000 species, 221 of which are useful to man. While 145 species are used in food directly, over 100 find application in the production of phycocolloids (such as alginates, agar and carrageenan), which are also food additives. Each year, some two million tonnes dry weight (t dw), or approximately 13 million tonnes fresh weight, of seaweed, valued at over $6.2 billion, is collected.Caulerpa taxifolia
Of all the seaweeds, Eucheuma is the most cultivated species - about 120,000 t dw annually compared to 15,500 t dw of Gracilaria and 800 t dw of Caulerpa. From the cell walls of Eucheuma is produced carrageenan, a gel-forming polysaccharide, which has a variety of applications, primarily in the food industry. Carrageenans bind with proteins, making them ideal for stabilising milk products and suspending fat globules and flavour particles. When added to hot milk and cooled, carrageenan bonds with the proteins to give milk a creamy thick texture. As it is resistant to high temperatures, carrageenan is used extensively in ultra-high temperature (UHT) processed goods.
A sulphated galactan, carrageenen is commercially available in three forms - lambda, iota and kappa. Lambda carrageenan does not form a gel and is used for purposes of viscosity control such as thickening, bodying and suspending applications as in milkshakes, flavoured milk, syrups and sauces. Iota and kappa form thermoreversible gels and are used in both water and milk gelling systems. Eucheuma contains only iota carrageenan which is easily extracted and used in fast foods and upmarket beverages. To meet the increasing demand, seaweed farming is rising sharply in the countries that produce it, and it is also being introduced into new locations in the tropics, says Dr. W. Lindsey Zemke-White, a marine biologist from Auckland University, New Zealand.
Eucheuma, according to Dr. White, is cultured mainly by the monoline method - fixed and raft - in which cuttings of seaweed are tied at 25-30 cm intervals to nylon ropes that have a clearing of 0.5 m from the ground. Additional rows are added about one metre apart. This is fixed-type monoline farming. Where there is little water movement, the monolines are allowed to float, and are called raft monolines. Usually, the monolines are stretched between two poles (usually bamboo), anchored to the bottom. The seaweed is propagated by the nutrients of the sea water. Plants are harvested when they reach approximately one kilogramme wet weight; harvesting involves the complete removal of the plants. The fixed-type monoline farms are generally located inshore of coral reefs, over sandy substrates, and can cover extensive areas of these reef flats. The raft monoline farms need not be placed over sand and are sometimes located over coral heads.
While the adverse impact of accidental algal introduction is well documented, there is hardly any study on the intentional introduction or farming of seaweeds for culture. Quarantine or safety procedures have also been researched, but they have hardly been implemented when the seaweed is introduced to new locations.
From available research findings, it is clear that the cultivated seaweeds eventually escape from farms and set up free-living populations. The impact of these populations on the local flora and fauna may vary between locations, but there is, according to Dr. White, some evidence from Hawaii that it overgrows and kills endemic corals.
Seaweed cultivation changes the environment in and around the farms primarily in three ways. First, while harvesting the seaweed, farmers remove the macro benthic organisms (which live in the bottom of the sea or lake) and cut or remove the seagrass, both of which are sure to alter the ecosystem. Fewer organisms will result in more non-farmed seaweeds while a lower density of seagrass encourages tubeworms. Second, the seaweed scrapes the surface of the substrate, altering the sediment structure and eliminating the micro-algal mats in coral reef lagoons. This affects the community structure of the organisms living under the algal farms.
Third, the seaweed farms provide a larger habitat for invertebrates and juvenile fish. There is generally a higher diversity index on seaweed farms, but as many of these organisms are harvested along with the seaweed, this may have no net positive effect on the wider community. The rise in juvenile fish may also contribute to the change in community structure of the mieobenthic organisms under the farms, as they eat marine species. While there is some idea about the changes seaweed cultivation can cause in the marine community, more research is needed to understand fully the effect of these changes on the ecosystem.
Some of the seaweed farming practices have negative impact on the local environment. For example, the refuse from the farms, which litter the beach and sea floor, and the tying of raft-anchoring lines to live corals can damage the environment. Some negative impacts arise from the shading of the underlying coral and the microalgae growing in the top layer of the sediment, and also from changes in sedimentation and improper treatment of waste water from carrageenan production facilities.
According to Dr. White, while the exact impact of seaweed farming is unclear, there is enough evidence to show that it impacts the environment negatively. Thus, under the tenets of `precautionary principle', there is a need to undertake a comprehensive impact study of Eucheuma farming before allowing seaweed cultivation in a fragile ecosystem. That no such study was done before allowing Pepsi Foods Ltd. to cultivate the seaweed in Tamil Nadu is cause for concern.