Lemna
Encyclopedia
Lemna is a genus of free-floating aquatic plant
s from the duckweed family. These rapidly-growing plants have found uses as a model system
for studies in community ecology, basic plant biology, in ecotoxicology
, in production of biopharmaceutical
s, and as a source of animal feeds for agriculture
and aquaculture
.
) consider the duckweeds members of the Araceae
.
Lemna species grow as simple free-floating thalli
on or just beneath the water
surface. Most are small, not exceeding 5 mm in length, except Lemna trisulca which is elongated and has a branched structure. Lemna thalli have a single root, which distinguishes them from related genera
Spirodela and Landoltia
The plants grow mainly by vegetative reproduction
: two daughter plants bud off from the adult plant. This form of growth allows very rapid colonisation of new water. Duckweeds are flowering plant
s, and nearly all of them are known to reproduce sexually, flower
ing and producing seed
under appropriate conditions. Certain duckweeds (e.g. L. gibba) are long day plant
s, while others (e.g. L. minor) are short day plant
s.
When Lemna invades a waterway, it can be removed mechanically, by the addition of herbivorous fish (e.g. grass carp
) or treated with a herbicide
.
The rapid growth of duckweeds finds application in bioremediation
of polluted waters and as test organisms for environmental studies. It is also being used as an expression system
for economical production of complex biopharmaceutical
s.
Duckweed meal (dried duckweed) is a good cattle feed. It contains 25-45% proteins (depending on the growth conditions), 4.4% fat, and 8-10% fibre, measured by dry weight.
interest. Expression constructs were engineered to cause Lemna to secrete the transformed proteins into the growth medium at high yield. Since the Lemna is grown on a simple medium, this substantially reduces the burden of protein purification
in preparing such proteins for medical use, promising substantial reductions in manufacturing costs. In addition, the host Lemna can be engineered to cause secretion of proteins with human patterns of glycosylation
, an improvement over conventional plant gene-expression systems. Several such products are being developed, including monoclonal antibodies.
range is 5 to 9, but better growth is obtained in the pH range of 6.5 to 7.5. A minimum water depth of 1 ft is desirable to prevent excessive temperature swings. High nitrogen levels, for example 20 mM urea, have provided a protein content in the range of 45% by weight. The water may typically contain 60 mg/L of soluble nitrogen
and 1 mg/L of phosphorus
. Fertiliser is required on a daily basis for optimal growth.
Duckweed can be farmed organically, with nutrients being supplied from a variety of sources, for example cattle dung, pig waste, biogas plant slurry, or other organic matter in slurry form. Because of the rapid growth of duckweed, daily harvesting is necessary to achieve optimal yields. Harvesting is done such that less than a kilogram per square metre of duckweed remains. Under optimal conditions, a duckweed farm can produce 10 to 30 tons of dried duckweed per hectare per year.
Aquatic plant
Aquatic plants are plants that have adapted to living in aquatic environments. They are also referred to as hydrophytes or aquatic macrophytes. These plants require special adaptations for living submerged in water, or at the water's surface. Aquatic plants can only grow in water or in soil that is...
s from the duckweed family. These rapidly-growing plants have found uses as a model system
Model organism
A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms. Model organisms are in vivo models and are widely used to...
for studies in community ecology, basic plant biology, in ecotoxicology
Ecotoxicology
Ecotoxicology is the study of the effects of toxic chemicals on biological organisms, especially at the population, community, ecosystem level. Ecotoxicology is a multidisciplinary field, which integrates toxicology and ecology....
, in production of biopharmaceutical
Biopharmaceutical
Biopharmaceuticals are medical drugs produced using biotechnology. They include proteins , nucleic acids and living microorganisms like virus and bacteria where the virulence of viruses and bacteria is reduced by the process of attenuation, they can be used for therapeutic or in vivo diagnostic...
s, and as a source of animal feeds for agriculture
Agriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...
and aquaculture
Aquaculture
Aquaculture, also known as aquafarming, is the farming of aquatic organisms such as fish, crustaceans, molluscs and aquatic plants. Aquaculture involves cultivating freshwater and saltwater populations under controlled conditions, and can be contrasted with commercial fishing, which is the...
.
Taxonomy and growth habits
The duckweeds have been classified as a separate family, the Lemnaceae, but some researchers (the AGP IIAngiosperm Phylogeny Group
The Angiosperm Phylogeny Group, or APG, refers to an informal international group of systematic botanists who came together to try to establish a consensus on the taxonomy of flowering plants that would reflect new knowledge about plant relationships discovered through phylogenetic studies., three...
) consider the duckweeds members of the Araceae
Araceae
Araceae are a family of monocotyledonous flowering plants in which flowers are borne on a type of inflorescence called a spadix. The spadix is usually accompanied by, and sometimes partially enclosed in, a spathe or leaf-like bract. Also known as the Arum family, members are often colloquially...
.
Lemna species grow as simple free-floating thalli
Thallus (tissue)
Thallus, from Latinized Greek θαλλός , meaning a green shoot or twig, is the undifferentiated vegetative tissue of some organisms in diverse groups such as algae, fungus, some liverworts, lichens, and the Myxogastria. Many of these organisms were previously known as the thallophytes, a polyphyletic...
on or just beneath the water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
surface. Most are small, not exceeding 5 mm in length, except Lemna trisulca which is elongated and has a branched structure. Lemna thalli have a single root, which distinguishes them from related genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Spirodela and Landoltia
The plants grow mainly by vegetative reproduction
Vegetative reproduction
Vegetative reproduction is a form of asexual reproduction in plants. It is a process by which new individuals arise without production of seeds or spores...
: two daughter plants bud off from the adult plant. This form of growth allows very rapid colonisation of new water. Duckweeds are flowering plant
Flowering plant
The flowering plants , also known as Angiospermae or Magnoliophyta, are the most diverse group of land plants. Angiosperms are seed-producing plants like the gymnosperms and can be distinguished from the gymnosperms by a series of synapomorphies...
s, and nearly all of them are known to reproduce sexually, flower
Flower
A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...
ing and producing seed
Seed
A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant...
under appropriate conditions. Certain duckweeds (e.g. L. gibba) are long day plant
Photoperiodism
Photoperiodism is the physiological reaction of organisms to the length of day or night. It occurs in plants and animals.Photoperiodism can also be defined as the developmental responses of plants to the relative lengths of the light and dark periods...
s, while others (e.g. L. minor) are short day plant
Photoperiodism
Photoperiodism is the physiological reaction of organisms to the length of day or night. It occurs in plants and animals.Photoperiodism can also be defined as the developmental responses of plants to the relative lengths of the light and dark periods...
s.
When Lemna invades a waterway, it can be removed mechanically, by the addition of herbivorous fish (e.g. grass carp
Grass carp
The grass carp is a herbivorous, freshwater fish species of family Cyprinidae, and the only species of the genus Ctenopharyngodon. It is cultivated in China for food, but was introduced in Europe and the United States for aquatic weed control...
) or treated with a herbicide
Herbicide
Herbicides, also commonly known as weedkillers, are pesticides used to kill unwanted plants. Selective herbicides kill specific targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often synthetic "imitations" of plant...
.
The rapid growth of duckweeds finds application in bioremediation
Bioremediation
Bioremediation is the use of microorganism metabolism to remove pollutants. Technologies can be generally classified as in situ or ex situ. In situ bioremediation involves treating the contaminated material at the site, while ex situ involves the removal of the contaminated material to be treated...
of polluted waters and as test organisms for environmental studies. It is also being used as an expression system
Gene expression
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...
for economical production of complex biopharmaceutical
Biopharmaceutical
Biopharmaceuticals are medical drugs produced using biotechnology. They include proteins , nucleic acids and living microorganisms like virus and bacteria where the virulence of viruses and bacteria is reduced by the process of attenuation, they can be used for therapeutic or in vivo diagnostic...
s.
Duckweed meal (dried duckweed) is a good cattle feed. It contains 25-45% proteins (depending on the growth conditions), 4.4% fat, and 8-10% fibre, measured by dry weight.
Assessing the toxicity of chemicals with Lemna
Organisation for Economic Cooperation and Development and U.S. Environmental Protection Agency (US EPA) guidelines describe toxicity testing using Lemna gibba or Lemna minor as test organisms. Both of these species have been studied extensively for use in phytotoxicity tests. Genetic variability in responses to toxicants can occur in Lemna, and there are insufficient data to recommend a specific clone for testing. The US EPA test uses aseptic technique. The OECD test is not conducted axenically, but steps are taken at stages during the test procedure to keep contamination by other organisms to a minimum. Depending on the objectives of the test and the regulatory requirements, testing may be performed with renewal (semi-static and flow-through) or without renewal (static) of the test solution. Renewal is useful for substances that are rapidly lost from solution as a result of volatilisation, photodegradation, precipitation or biodegradation.Production of biopharmaceuticals
Lemna has been transformed by molecular biologists to express proteins of pharmaceuticalBiopharmaceutical
Biopharmaceuticals are medical drugs produced using biotechnology. They include proteins , nucleic acids and living microorganisms like virus and bacteria where the virulence of viruses and bacteria is reduced by the process of attenuation, they can be used for therapeutic or in vivo diagnostic...
interest. Expression constructs were engineered to cause Lemna to secrete the transformed proteins into the growth medium at high yield. Since the Lemna is grown on a simple medium, this substantially reduces the burden of protein purification
Protein purification
Protein purification is a series of processes intended to isolate a single type of protein from a complex mixture. Protein purification is vital for the characterization of the function, structure and interactions of the protein of interest. The starting material is usually a biological tissue or...
in preparing such proteins for medical use, promising substantial reductions in manufacturing costs. In addition, the host Lemna can be engineered to cause secretion of proteins with human patterns of glycosylation
Glycosylation
Glycosylation is the reaction in which a carbohydrate, i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule . In biology glycosylation refers to the enzymatic process that attaches glycans to proteins, lipids, or other organic molecules...
, an improvement over conventional plant gene-expression systems. Several such products are being developed, including monoclonal antibodies.
Duckweed farming
High yields of duckweed with a high protein content can be achieved by careful control of growth conditions. Although duckweed can tolerate temperatures ranging from 6 to 33 °C, the optimal growth range is 20 to 28 °C. The acceptable pHPH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
range is 5 to 9, but better growth is obtained in the pH range of 6.5 to 7.5. A minimum water depth of 1 ft is desirable to prevent excessive temperature swings. High nitrogen levels, for example 20 mM urea, have provided a protein content in the range of 45% by weight. The water may typically contain 60 mg/L of soluble nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
and 1 mg/L of phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
. Fertiliser is required on a daily basis for optimal growth.
Duckweed can be farmed organically, with nutrients being supplied from a variety of sources, for example cattle dung, pig waste, biogas plant slurry, or other organic matter in slurry form. Because of the rapid growth of duckweed, daily harvesting is necessary to achieve optimal yields. Harvesting is done such that less than a kilogram per square metre of duckweed remains. Under optimal conditions, a duckweed farm can produce 10 to 30 tons of dried duckweed per hectare per year.
General readings
- Cross, J.W. (2006). The Charms of Duckweed.
- Landolt, E. (1986) Biosystematic investigations in the family of duckweeds (Lemnaceae). Vol. 2. The family of Lemnaceae - A monographic study. Part 1 of the monograph: Morphology; karyology; ecology; geographic distribution; systematic position; nomenclature; descriptions. Veröff. Geobot. Inst., Stiftung Rübel, ETH, Zurich.