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Aquaculture production of duckweed
The Lemnaceae (duckweed, water lentils) family are the smallest flowering plants. They are free-floating plants with 1 to 3 leaves and a single root (or root-hair) from each frond. Because of their extremely rapid growth, duckweeds can be aggressive invaders of ponds and are often considered a nuisance. They grow in dense colonies in quiet water, best if undisturbed by wave action. They require nutrient-rich water, high in phosphorus and nitrogen, and are therefore often found in areas of agricultural run-off. Various species are known and grow in different climates throughout the world. Because of the very high productivity per surface area, duckweed holds great potential for future global villages. This tiny aquatic plant has tremendous potential for cleaning up pollution, combating global warming and feeding the world.
Duckweed - Not just for Ducks: Research from the Tropical Ecological Farm, College of Agriculture and Forestry -CAF, Thu Duc, Ho Chi Min City, Vietnam:
"Duckweed is probably the fastest growing multi-cellular plant. It grows naturally on waste water and can double its weight in 24 hours. It is unique amongst plants in that its protein content can be manipulated according the nitrogen content of the water in which it is growing. This is important because it integrates with the biodigester. It is the ideal water plant to introduce into an integrated farming system because it can use the nitrogen in the effluent coming from the biodigester to enrich its protein content to a level only slightly lower than Soya Bean, approaching 35%. In terms of protein production, grown under ideal conditions in can produce 10 tonnes of protein per hectare per year. This compares with Soya bean which produces less than 1 tonne per year.
Duckweed is good for the environment because it doesn’t require artificial fertilizers, on the contrary it cleans up waste by removing organic and inorganic nitrogen coming from decomposition of organic matter, contributing to the fight against eutrophication. It doesn’t need fungicides and has no significant natural pests.
Duckweed can be eaten by chickens, ducks and pigs and can supply all of the protein needs for locally adapted breeds."
Duckweed as Food
Depending on strain and growing conditions, duckweeds can have very high protein content of up to 50% of dry mass. High levels of vitamins are also present. The taste is remotely similar to spinach. Duckweeds have historically played a role in some east Asian cuisines (Wolffia genus). Because of rapid growth and ease of cultivation, duckweeds for consumption by animals and humans are now getting more attention. Current uses as animal fodder are mostly as fish food (carp, tilapia) and bird food (chicken, duck). Possible uses as part of a human diet are still vastly under-explored.
Duckweed for Bio-energy
Duckweed produces biomass faster than any other flowering plant. It has clear potential as an alternative for biofuel production.
Duckweed as Biosensors
Duckweeds are used for the detection of heavy metals and organic contaminants. A variety parameters can then be measured to assess the health of the plants: growth rate, leaf size and color, root size etc.
Duckweed for Contaminant Removal
Duckweed is very efficient at cleaning water. First it will remove all macronutrients. Then it will remove micronutrients. Finally it will remove all metal ions, including toxics and radioactives. As such, it can be used to 'polish' industrial effluent, pre-treat sewage, denitrify effluent from other aquaculture activites and other nitrogen drainange situations (stock lots, etc.). Article below on Duckweed Aquaculture addresses this topic, and there is little doubt more academic studies of duckweed as a hyperaccumulator will be forthcoming.
Duckweed for Biomining
Duckweeds are excellent concentrators of nitrogen and phosphates. They could be used for bio-mining, given their ability to pull minerals out of solution down to mere trace concentrations. The harvested duckweeds can then enter into a nutrient cycle which may include animal food, biogas digester etc. This way, phosphorus and other nutrients could be bio-accumulated from muddy water even in regions with generally phosphorus-deficient soils. However, this is still in the idea stage, and issues of undesired bio-accumulation (e.g. aluminum) will have to be solved.
A full layer of duckweed will block out most sunlight from deeper levels of a pond. Therefore, algal blooms are eliminated - a desired effect. However, the lack of sunlight at those deeper levels may also lead to low-oxygen conditions. As a result, anaerobic bacteria may proliferate, making the plants unsuitable as food for humans. An aeration device such as airstone may then be required. It is important not to agitate the surface, as duckweeds grow best on very quiet water.
Ideas and applications
- duckweed in salad or soup, on a sandwich or as a component of vegetable spread
- as a substitute for: lettuce, spinach, water cress, ... ?)
- food for fish (e.g. tilapia), chicken, pigs,
[insert here: information on new breeds, optimized for specific uses, higher yields, more lipids, different taste, more specific bio-accumulation etc.].
The duckweed genome is expected to be fully sequenced by the end of 2009. This should greatly improve our understanding of duckweed biology and will likely accelerate genetic research on these plants.
Cross, J.W. (2006). The Charms of Duckweed. 
July 2008 article on Treehugger 
Very comprehensive manual on duckweed aquaculture 
Duckweed as a Primary Feedstock for Aquaculture 
Duckweed as a feed supplement for livestock 
Older patent with many details on duckweed for human consumption 
Integrated Tilapia & Duckweed Farming System 
Manual for the use of biodigester effluent and ponds for duckweed production (from Vietnam)