''Spirodela polyrhiza'' ( ''S. polyrrhiza'') is a species of
duckweed
Lemnoideae is a subfamily of flowering aquatic plants, known as duckweeds, water lentils, or water lenses. They float on or just beneath the surface of still or slow-moving bodies of fresh water and wetlands. Also known as bayroot, they arose fr ...
known by the common names common duckmeat, greater duckweed, great duckmeat, common duckweed, and duckmeat. It can be found nearly worldwide in many types of
freshwater
Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include non-salty mi ...
habitat.
Description
''Spirodela polyrhiza'' is a
perennial
In horticulture, the term perennial ('' per-'' + '' -ennial'', "through the year") is used to differentiate a plant from shorter-lived annuals and biennials. It has thus been defined as a plant that lives more than 2 years. The term is also ...
aquatic plant usually growing in dense colonies, forming a mat on the water surface. Each plant is a smooth, round, flat disc 0.5 to 1.0 cm wide. Its upper surface is mostly green, sometimes red, while the lower surface is dark red.
It produces several minute roots and a pouch containing male and female flowers. The top part dies in the fall and the plant often overwinters as a
turion. The turion sinks to the bottom of the water body and stays in a
dormant phase, until water temperature reaches 15 °C. The turions then germinate on the bottom of the water body and start a new life cycle.
As this species lives in ponds and slow-moving water bodies, differs developmentally from terrestrial plants in
morphology
Morphology, from the Greek and meaning "study of shape", may refer to:
Disciplines
*Morphology (archaeology), study of the shapes or forms of artifacts
*Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...
and
physiology
Physiology (; ) is the science, scientific study of function (biology), functions and mechanism (biology), mechanisms in a life, living system. As a branches of science, subdiscipline of biology, physiology focuses on how organisms, organ syst ...
. It undergoes mainly
vegetative growth in spring and summer, forming new
frond
A frond is a large, divided leaf. In both common usage and botanical nomenclature, the leaves of ferns are referred to as fronds and some botanists restrict the term to this group. Other botanists allow the term frond to also apply to the lar ...
s. ''Spirodela polyrhiza'' rarely flowers. In fall and winter it switches into a
dormant phase represented by the
turions due to nutrition starvation and freezing temperatures.
Because of its fast growth, direct contact with media and small genome size (~150 Mb), ''S. polyrhiza'' is an ideal system for biofuels, bioremediation, and carbon cycling.
A comprehensive genomic study of ''S. polyrhiza'' was published in February 2014. The results provide insights into how this organism is adapted to rapid growth and an aquatic lifestyle.
Turion induction by abscisic acid
Turions were induced by the plant hormone
abscisic acid
Abscisic acid (ABA or abscisin II) is a plant hormone. ABA functions in many plant developmental processes, including seed and bud dormancy, the control of organ size and stomatal closure. It is especially important for plants in the response to ...
(ABA) in the lab. Researchers reported that turions were rich in
anthocyanin
Anthocyanins (), also called anthocyans, are solubility, water-soluble vacuole, vacuolar pigments that, depending on their pH, may appear red, purple, blue, or black. In 1835, the German pharmacist Ludwig Clamor Marquart named a chemical compou ...
pigmentation and had a density that submerged them in liquid media.
Transmission electron microscopy
Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a g ...
of turions showed in comparison to fronds shrunken
vacuole
A vacuole () is a membrane-bound organelle which is present in Plant cell, plant and Fungus, fungal Cell (biology), cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water ...
s, smaller
intercellular space, and abundant
starch granules surrounded by
thylakoid
Thylakoids are membrane-bound compartments inside chloroplasts and cyanobacterium, cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a #Membrane, thylakoid membrane surrounding a #Lumen, ...
membranes. Turions accumulated more than 60% starch in dry mass after two weeks of ABA treatment.
Distribution
''Spirodela polyrhiza'' is found worldwide, namely in
North America
North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...
,
Asia
Asia ( , ) is the largest continent in the world by both land area and population. It covers an area of more than 44 million square kilometres, about 30% of Earth's total land area and 8% of Earth's total surface area. The continent, which ...
,
more rarely in
Central and
South America
South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...
, but also in
Central Europe
Central Europe is a geographical region of Europe between Eastern Europe, Eastern, Southern Europe, Southern, Western Europe, Western and Northern Europe, Northern Europe. Central Europe is known for its cultural diversity; however, countries in ...
.
It grows in
tropical
The tropics are the regions of Earth surrounding the equator, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's ax ...
and
temperate
In geography, the temperate climates of Earth occur in the middle latitudes (approximately 23.5° to 66.5° N/S of the Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ran ...
climates.
It is not prevalent in
New Zealand
New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...
and only rarely in
Australia
Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
.
Cultivation
Large scale cultivation is done in outdoor water tanks, mostly in connection with
wastewater
Wastewater (or waste water) is water generated after the use of freshwater, raw water, drinking water or saline water in a variety of deliberate applications or processes. Another definition of wastewater is "Used water from any combination of do ...
treatment. Tanks are fed with wastewater and the floating duckweed is harvested from the surface. It is then further used as a
biofuel
Biofuel is a fuel that is produced over a short time span from Biomass (energy), biomass, rather than by the very slow natural processes involved in the formation of fossil fuels such as oil. Biofuel can be produced from plants or from agricu ...
from
industrial wastewater
Industrial wastewater treatment describes the processes used for treating wastewater that is produced by industries as an undesirable by-product. After treatment, the treated industrial wastewater (or effluent) may be reused or released to a s ...
or as animal feed from
agricultural wastewater treatment
Agricultural wastewater treatment is a farm management agenda for controlling pollution from confined animal operations and from surface runoff that may be contaminated by chemicals in fertilizer, pesticides, animal slurry, crop residues or ...
facilities.
Use
''Spirodela polyrhiza'' can be used for
bioremediation
Bioremediation broadly refers to any process wherein a biological system (typically bacteria, microalgae, fungi in mycoremediation, and plants in phytoremediation), living or dead, is employed for removing environmental pollutants from air, wate ...
, removing
toxic
Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subst ...
substances from aquatic environment as well as cleaning
eutrophic
Eutrophication is a general term describing a process in which nutrients accumulate in a body of water, resulting in an increased growth of organisms that may deplete the oxygen in the water; ie. the process of too many plants growing on the s ...
waters, especially in
wastewater
Wastewater (or waste water) is water generated after the use of freshwater, raw water, drinking water or saline water in a variety of deliberate applications or processes. Another definition of wastewater is "Used water from any combination of do ...
treatment plants. Its uses as
biofuel
Biofuel is a fuel that is produced over a short time span from Biomass (energy), biomass, rather than by the very slow natural processes involved in the formation of fossil fuels such as oil. Biofuel can be produced from plants or from agricu ...
and animal feed are also gaining importance. It is hardly used for human nutrition.
Bioremediation
Because of its capability to
hyperaccumulate heavy metals
upright=1.2, Crystals of lead.html" ;"title="osmium, a heavy metal nearly twice as dense as lead">osmium, a heavy metal nearly twice as dense as lead
Heavy metals is a controversial and ambiguous term for metallic elements with relatively h ...
and its high uptake of nutrients from the water, ''S. polyrhiza'' is used for bioremediation. The main pollutants it can be used to remediate are
arsenic
Arsenic is a chemical element; it has Symbol (chemistry), symbol As and atomic number 33. It is a metalloid and one of the pnictogens, and therefore shares many properties with its group 15 neighbors phosphorus and antimony. Arsenic is not ...
(As) and
mercury (Hg)
and common wastewater nutrients, like
sulphate
The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ...
(SO
42-),
phosphate
Phosphates are the naturally occurring form of the element phosphorus.
In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthop ...
(PO
43-) and
nitrate
Nitrate is a polyatomic ion with the chemical formula . salt (chemistry), Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are solubility, soluble in wa ...
(NO
3−).
Arsenic
Greater duckweed showed accumulation of arsenic in laboratory tests. Arsenic uptake was found to be negatively correlated with phosphate and positively correlated with
iron
Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...
uptake. This indicates that phosphate and arsenic compete for uptake by ''S. polyrhiza'', while arsenic's absorption is facilitated by iron oxides, because it shows an affinity to the root surface of ''S. polyrhiza'', where it is taken up. Greater duckweed is thought to
detoxify the arsenic by reducing As (V) to the less toxic As (III). Difficulties arise with the management of the plants with high As contents. One possible use of the biomass containing As is production of charcoal and gas as a byproduct, which can be used as a fuel. The problems with this approach are low charcoal quality and high investments. Direct burning or burning of the coal is thought to release arsenic into the air, which would
pollute the environment. Other options for
fuel
A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work (physics), work. The concept was originally applied solely to those materials capable of releasing chem ...
production would be
hydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...
and
fermentation
Fermentation is a type of anaerobic metabolism which harnesses the redox potential of the reactants to make adenosine triphosphate (ATP) and organic end products. Organic molecules, such as glucose or other sugars, are catabolized and reduce ...
, which are economically not feasible. The
biomass
Biomass is a term used in several contexts: in the context of ecology it means living organisms, and in the context of bioenergy it means matter from recently living (but now dead) organisms. In the latter context, there are variations in how ...
would have to be treated with strong acids and heat, which are both capital intensive.
Briquetting is considered one of the best options, where the plants are dried and pressed into pellets of briquets. This raises the question of whether the arsenic is released back into the environment during the burning process. The production of
biogas
Biogas is a gaseous renewable energy source produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste, Wastewater treatment, wastewater, and food waste. Biogas is produced by anaerobic ...
is also considered, but again, the redistribution of the As has to be avoided .
Mercury
''Spirodela polyrhiza'' was found to be an efficient
bio accumulator of
mercuric chloride
Mercury(II) chloride (mercury bichloride, mercury dichloride, mercuric chloride), historically also sulema or corrosive sublimate, is the inorganic chemical compound of mercury and chlorine with the formula HgCl2, used as a laboratory reagent. ...
(HgCl
2) in laboratory settings. Its plant biomass showed a 1000 times higher mercuric chloride concentration than its aquatic environment. ''Spirodela polyrhiza'' showed the highest accumulation factor compared to ''
Lemna gibba'' and ''
L. minor'', which were also investigated.
Urban wastewater treatment
Greater duckweed has been used to remove common
pollutant
A pollutant or novel entity is a substance or energy introduced into the environment that has undesired effect, or adversely affects the usefulness of a resource. These can be both naturally forming (i.e. minerals or extracted compounds like oi ...
s from wastewater. In a laboratory setting, ''S. polyrhiza'' showed a maximum of 90% removal efficiency of nitrate, 99.6% of phosphate and 69.8% of sulphate. The efficiency for all three pollutants combined was 85.6%, which makes it an environmentally and economically viable
bioremediatory for wastewater treatment.
Biofuel
Due to space-efficient starch production and good growth in animal wastewater, ''S. polyrhiza'' has great potential in
bioethanol
Ethanol fuel is fuel containing ethyl alcohol, the same type of alcohol as found in alcoholic beverages. It is most often used as a motor fuel, mainly as a biofuel additive for gasoline.
Several common ethanol fuel mixtures are in use a ...
production. Despite environmental problems associated with production and competition from human and animal feed, corn is the main raw material for bioethanol. ''Spirodela polyrhiza'' could produce up to 50% more bioethanol on the same area. At the same time the production of bioethanol from ''S. polyrhiza'' is not in competition with human food. The production of bioethanol from ''S. polyrhiza'' is still in the development phase.
Animal feed
In small-scale agriculture ''S. polyrhiza'' is used as fish or poultry feed. Due to its fast growth and high protein content, it is an interesting feedstuff. Because of sanitary problems and the risk of heavy metal accumulation, it is not yet used for feeding in larger animal husbandry systems.
For
rainbow trout
The rainbow trout (''Oncorhynchus mykiss'') is a species of trout native to cold-water tributary, tributaries of the Pacific Ocean in North America and Asia. The steelhead (sometimes called steelhead trout) is an Fish migration#Classification, ...
, poorer growth rates were found when ''S. polyrhiza'' was added to the feed. For tilapia (
''Oreochromis niloticus'' L.), greater weight gains were found when 30% of the
fish meal in the feed was replaced with ''S. polyrhiza''. A review has also shown that duckweed can be used in cattle, pig and poultry diets. However, the problems of heavy metals and pathogen contamination occur.
Human nutrition
Although other duckweed species, such as ''
Wolffia arrhiza'', are consumed by people in rural areas, ''S.polyrhiza'' is not cultivated for human consumption.
This is because of high concerns about heavy metal accumulation and possible
contamination
Contamination is the presence of a constituent, impurity, or some other undesirable element that renders something unsuitable, unfit or harmful for the physical body, natural environment, workplace, etc.
Types of contamination
Within the scien ...
with ''
Escherichia coli
''Escherichia coli'' ( )Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. is a gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Escherichia'' that is commonly fo ...
'' or ''
Clostridium botulinum
''Clostridium botulinum'' is a Gram-positive bacteria, gram-positive, Bacillus (shape), rod-shaped, Anaerobic organism, anaerobic, endospore, spore-forming, Motility, motile bacterium with the ability to produce botulinum toxin, which is a neurot ...
''.
In contrast to ''
W. arrhiza'', ''S. polyrhiza'' contains, like most duckweed species,
calcium oxalate crystals which are known to cause
kidney stones
Kidney stone disease (known as nephrolithiasis, renal calculus disease, or urolithiasis) is a crystallopathy and occurs when there are too many minerals in the urine and not enough liquid or hydration. This imbalance causes tiny pieces of cr ...
.
References
External links
Spirodela polyrhiza genomicsJepson Manual TreatmentFlora of North AmericaWashington Burke Museum
{{Taxonbar, from=Q154500
Lemnoideae
Plant models
Freshwater plants