Duckweed nutritional composition.

Mineral Composition
Effects of the Water on Mineral Composition
Organic Composition
Amino Acid Composition
Calcium Oxalate
Human Nutrition

This page provides some general information about the nutritional composition of duckweeds, much of it from the extensive data collected by Landolt and Kandeler, Biosystematic investigations in the family of duckweeds (Lemnaceae) vol 4, 1987 (Phytochemistry, physiology application, bibliography), vol. 2 of The Family of Lemnaceae- a monographic study.  Information about the suitability of duckweeds for human or animal nutrition is also available.

This page is will be expanded to include additional information, particularly on the amino acid and saccharide composition of duckweeds.

Mineral Composition

Water:  Duckweeds are reported to vary between 86% and 97% water (Landolt and Kandeler, 1987, p 149), not surprising for an aquatic plant.  In my own studies, Lemna gibba grown on E-Medium typically is 93% water by weight.

Mineral elements:

The table below reproduces the collected data of Landolt and Kandeler (1987). He also indicated (1987, p 11) that the K/Ca ratio varies from  1 to 2, the Ca/Mg ration from 0.05 to 20.  this is not surprising, because duckweeds store varying amounts of calcium as calcium oxalate crystals in the vacuole, as demonstrated by the work of V.R. Francesci.  The C/N ratio varies more modestly, in the range of 6.7 to 8.6.
 
Variation of content of elements,  % of dry weight
Ag
Al
As
B
Ba
Br
C
Ca
Cd
Ce
Cl
Co
Cr
Cs
Cu
F
Fe
Ga
H
Hg
J
K
La
Li
0.3 — 50 x10-6
0.000—11.4
0.2—23.5 x10-3
0.02—3.25
0.03—0.11
0.25—0.65 x10-2
30.5—43.7
0.18—4.5
<0.1x10-4 — 6.7
0.2 x 10-3
0.08—4.29
0.9 x 10-4 —1.1
0.3—17.8 x 10-3
0.4—50 x 10-3
0.2 X 10-3 - 3.2 
0.2 X 10-3
0.007—3.2
0.9 X 10-4
5.4
0.04—18 x 10-4
0.4—25 x l0-4
0.03—7.0
0.9 x 10-4
0.8—6 x 10-3
Mg
Mn
Mo
N
Na
Nb
Ni
P
Pb
Pr
Ra
Rb
S
Sb
Se
Si
Sn
Sr
Ti
V
Y
Zn
Zr
0.04—2.8
0.003—6.4
0.2—0.4 x10-3
0.8—7.8
0.03—1.3
0.2 x 10-3
0.7 x 10-4 — 0.2
0.03—2.8
0.2x10-4 — 0.02
0.4 x 10-4
traces
0.0054
0.33—7.0
0.0015—0.012
0.0018—0.012 
0.41-5.35 
0.2 - 3.6 x 10-2 
0.008 - 0.11 
0.0018 - 0.32
0.3—10 x 10-3
0.4 x 10-4
0.004—0.14
0.9 x 10-4
From Landolt and Kandeler, 1987 p. 10, according to many authors, see list in the text.
 

Effects of the Water (Medium) on Mineral Composition.

Elias Landolt (1986, p 149) did extensive investigations of the effects of medium on duckweed growth.  The following paragraph discusses his findings.  For more information about the range of natural waters supporting duckweed growth [ link ].
 
4.2.4. Chemical composition of the water

4.2.4.1. General remarks

The occurrence and composition of species of Lemnaceae in different lakes and ponds of the same region (with the same climate) may vary considerably. Though many differences in the composition of the species are merely accidental as was shown in WOLEK (1983), the chemical composition of the water is responsible in many cases. In addition, competition by other water plants, presence or absence of fish and other animals feeding on duckweed, occasional drying out, and use of herbicides play an important role. In spite of many field investigations with water analysis and extensive cultivation experiments, little is known about the optimal chemical composition which selects different species in mature.
This may be due to the following facts:

- the nutrient content of the water changes during the year; to be able to compare the different locations, it is necessary to make several measurements during the course of the year;

- the nutrients of the ecosystem are only partly in the water; many nutrients may be stored in the biomass of a thick Lemnaceae cover;

- some of the nutrients in the water (e.g. Fe, Mn) may not be accessible for the Lemnaceae due to precipitation at a pH which is too high, and due to the absence of chelating organic substances;

- dust and clay particles in the water may contain some nutrients accessible for Lemnaceae (McLAY 1973, MEALY and McCOLL 1974b); these particles are filtered off before analysing the water and are therefore not measured.
 

Organic Composition
 
 
Organic composition in the Lemnaceae, % of dry weight
protein
lipid
crude fiber
carbohydrate
ash
 6.8 — 45.0
 1.8 —  9.2
 5.7 — 16.2
14.1 — 43.6
12.0 — 27.6
From data collected by Landolt and Kandeler (1987, p 20)

Leaf Protein Concentrate

Leaf protein concentrates (LPC) and the residual pulp fibers were extracted and analysed by E.A. Faskin, Federal University of Technology, Akure, Nigeria.  Duckweed LPC was 64% protein, while the residual pulp fiber contained 20% crude protein.  The LPC from duckweed was low in lipid and fiber content.  Generally, the content of phytin, tannin and other extractives was lower than in the original plant.

Reference:

Faskin, E.A. (1999) "Nutrient quality of leaf protein concentrates produced from water fern (Azolla africana Desv) and duckweed (Spirodela polyrrhiza L. Schleiden)."  Bioresource Technology. 69(2):185-187.
Amino Acid Composition

The protein content of duckweeds is one of the highest in the plant kingdom, but it is dependent on growth conditions.  Typically duckweeds are rich in leucine, threonine, valine, isoleucine and phenylalanine.  They tend to be low in cysteine, methionine, and tyrosine.

Calcium Oxalate

Calcium oxalate is not a nutrient (nor a beneficial source of calcium), and it can be toxic in large doses.  Duckweeds can contain up to 2 — 4 percent oxalic acid equivalents by weight.  However, oxalate also is found in a great many leafy and very nutritious vegetables, including spinach, swiss chard and others.  In these edible vegetables, calcium oxalate is found in at levels up to 0.5 — 1 percent.  So, minimizing oxalate has the potential to make duckweeds more nutritious and digestible.

However, published reports of calcium oxalate levels in duckweeds are likely to be misleading. The late Vincent Franceschi (Washington State University, see reference below) demonstrated that the calcium oxalate content of Lemna minor depends greatly on the calcium content of the water on which they are growing.  Elevated calcium in the water favors formation of calcium oxalate crystals, and their content can be lowered by growth on low-calcium medium.  It seems likely that placing duckweed on soft water for a reasonably short period could lower oxalate content significantly in a practical setting.

The chemical and physical form of oxalate differs in the various duckweeds. Lemna and Spirodela accumulate calcium oxalate crystals.  These crystals (raphides and druses) are easy to see with the microscope, especially under polarized light, and are found in certain specialized cells called idioblasts. Wolffia and Wolffiella form oxalic acid, but do not exhibit calcium oxalate crystals.  Raphide crystals occur as bunches of long-thin crystals, while druses are shaped like a morning star.

Above:  Section of a frond of Spirodela (Landoltia) punctata from Landolt (1986).
Dr = druses, Pi = pigment cells, Ra = raphides

Oxalate in plants is made in the idioblast cells

The metabolic precursor of oxalate is L-ascorbic acid (Vitamin C).  Recent research in the duckweed relative Pistia stratiotes indicates that L-ascorbate and oxalate are synthesized within the crystal idioblast cells (Kostman, 2001).  To do obtain these results, developing crystal idioblasts were isolated and then incubated with various 14C-labeled compounds and then examined by micro-autoradiography for incorporation of 14C into calcium oxalate crystals.  [14C]oxalate gave heavy labeling of crystals, indicating the isolated idioblasts are functional in crystal formation.  Incubation with [1-14C]ascorbate also gave heavy labeling of crystals, whereas [6-14C]ascorbate gave no labeling. Labeled precursors of ascorbate (L-[1-14C]galactose; D-[1-14C]mannose) also resulted in crystal labeling, as did the ascorbic acid analog, D-[1-14C]erythorbic acid.

The pathway of biosynthesis is proposed as:

 D-mannose -> L-galactose -> ascorbic acid -> oxalic acid

These results show that the crystal idioblasts themselves synthesize the oxalate used for crystal formation.  The oxalate comes from the number 1 and 2 carbons of ascorbate.
 

References:

Franceschi, V.R. (1989) "Calcium oxalate formation is a rapid and reversible process in Lemna minor L." Protoplasma 148 (2/3):130-137.

Kostman, T.A., Tarlyn, N.M., Loewus, F.A., Franceschi, V.R.. (2001) Biosynthesis of L-ascorbic acid and conversion of carbons 1 and 2 of L-ascorbic acid to oxalic acid occurs within individual calcium oxalate crystal idioblasts. Plant Physiol, February 2001, 125(2):634-640 .

Landolt, E. (1986) "Biosystematic investigations in the family of duckweeds (Lemnaceae).  Veroff. Geobot. Inst. ETH, Zurich. vol. 1, pp 61-64.

Landolt, E. and Kandeler, R. (1987) "Biosystematic investigations in the family of duckweeds (Lemnaceae).  Veroff. Geobot. Inst. ETH, Zurich. vol. 2, pp 42-43.


Duckweeds in Human Nutrition

For people to eat duckweed, it would need to be grown under sanitary conditions.  In addition, it may be desirable to pay attention to the calcium content (see above).  Evidence is now emerging that the absorption of dietary oxalate makes a major contribution to urinary oxalate excretion, particularly in stone formers.    There is a patent on a method to select duckweeds for human consumption.


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Revised:  December 31, 2011