Lemna For Fish

Bioavailability and assimilation of Lemna is greatly influenced by the presence of anti-nutrients, such as pectins, tannins, phytic acid and oxalic acid. Lemna minor has been shown to contain pectin and arabinogalactan.[1] Fermentation (using Bacillus sp.) may somewhat reduce fibre content, drastically reduce tannin and phytic acid contents, and markedly increase nutrient bioavailability. (30% vs 10% maximal diet incorporation) [2]

Feed for Tilapia

Wolffia arrhiza © Christian Fischer

Tilapia prefer to eat Equisetum sp. (horsetail aka snake grass, containing thiaminase and nicotine) over Lemna and Spirodela spp subsequently.[3] Tilapia prefer Wolffia over Lemna.[4] In terms of growth rate and flesh protein content, Lemna sp. as feed give better results than Hydrilla sp. and Chara sp.[5] Lemna minor gave better growth results (both somatic and reproductive) than Eichhornia crassipes (Water hyacinth) and Azolla pinnata (Azolla). [6][7] Lemna are a more suitable feed for Tilapia (Oreochromis niloticus) than Spirodella. The optimal daily feeding rates of Lemna perpusilla were 3 to 5% of the total fish body weight on a duckweed-dry-weight basis for fish of 25 to 105 g in weight.[8] Fish feed supplemented with duckweed gave much better growth rates than fish feed alone, and than fish feed supplemented with taro.[9]

In tilapia fed Lemna gibba alone had a poor growth rate with 65% of the duckweed assimilated, and 26% converted to fish. Fed a mixed diet of duckweed and fish pellets, the growth rate doubled (similar to pellets alone), with 70% assimilated, but only 21% converted to fish.[10] Comparing a control diet (fishmeal, wheat, corn, oil) to that diet with 20% fresh duckweed, there was no significant difference in growth rate. When the diet contained dry duckweed or 40% duckweed, the growth rate was lower. Tilapia fed the diet with duckweed contained more protein and phosphorus and less fat.[11] In tilapia fed 15% duckweed + 85% fishmeal, growth rates was lower and nitrogen retention was similar in comparison to a 100% fishmeal diet.[12] In tilapia fingerlings, duckweed could replace commercial feed for up to 50% without a reduction in final weight.[13] In (red) tilapia fingerlings, a diet with 25% duckweed (Lemna obscura) replacement gave better growth rates than the control diet free of Lemna.[14] In tilapia fingerlings a diet with 10% duckweed (Lemna paucicostata) gave better growth rates than a standard diet (KLRI/40/6) free from Lemna.[15][]

Yield of (GIFT) tilapia fed phytoplankton + duckweed (Lemna minor) was signaficantly higher than of tilapia feeding on phytoplankton alone.[16]

The solid excrements of tilapia are poor in free NH3 and ammonium compounds and prolonged digestion at 38°C is necessary before they can be used for duckweed growth.[17]

Wolffia Digestibility of fresh Wolffia by Nile tilapia fingerlings (2.5 g) and Nile tilapia adults (40-50 g) was not significantly different. Incorporating more than 15% Wolffia in the diet results in decreased growth rates.[18] (similar for quails [19]; 4 to 12 g. of Wolffia arrhiza may survive on 1L quail effluent / 300 days[20]) The growth rate of Tilapia fed 15% Wolffia was higher than those fed 30% and 45% Wolffia. There was no significant difference with controls (0% Wolffia).[21] When Wolffia arrhiza needs to compete with Microcystis aeruginosa (not toxic to tilapia[22]), the supplementation of 4.8 mM KCl gives W. arrhiza the upper hand.[23]

Feed for Carp

When Lemna was fed to grass carp (Ctenopharyngodon idella), digestibility of dry matter was 65% (80% for crude protein, 61% for gross energy).[24] Growth rates of common carp (Cyprinus carpio) fed fishmeal or 15% duckweed + 85% fishmeal was not significantly different. Higher rates of duckweed (Lemna minor) incorporation resulted in lower growth rates.[25]

Oxalic acid

Spirodella and Lemna genera may contain large amounts of oxalic acid. Nutrient assimilation potential is greatly reduced by oxalic acid. Plants are highly tolerant of oxalic acid and oxalates. Animals are not. Oxalic acid may combine with several minerals (eg calcium, iron) to form oxalates. Oxalate may form crystals. Calcium-oxalate, which is somewhat soluble (up to only 6 mg/L at 18 C). Sodium and potassium oxalates are soluble. Oxalate accumulation is influenced by:

• Glyoxylate is a major precursor of oxalate (induced by glycolate-oxidase (as in L. minor[26]) and lactate dehydrogenase). Activity of glycolate-oxidase is doubled in extracts of tissue grown with an ammonium nitrogen source as compared with the enzyme from Lemna grown on nitrate.[27] Glycolate is mostly converted to glycine and serine instead of oxalate [28], as L. minor lacks the machinery for converting glycolate to oxalate.[29]
• C2/C3 cleavage of ascorbic acid may also yield oxalate in oxalate accumulators. In Lemna gibba 10% of ascorbic acid is converted to oxalic acid.[30]
• D-glucosone may yield ascorbic acid and oxalate in Lemna minor.[31]
• nitrogen source (ammonium/nitrate; Oxalic acid was 40–50% lower in the leaves (of Purslane) grown in solutions containing ammonium compared to the leaves grown with no ammonium [32]. More crystals are formed by L. minor grown on nitrogen from ammonium than by plants grown on nitrogen from nitrate.[33]
• inorganic ion availability (oxalate plays a role in ion regulation and osmoregulation; free iron is toxic to cells, and oxalic acid and oxalate are closely linked to calcium metabolism in cells). Lemna minor rapidly forms, but also readily dissolves crystals (soluble oxalates remain constant), depending on calcium concentration in the growth medium (threshold is 0.5 mM Ca).[34]

Oxalate primarily accumulates as:

• soluble oxalate
• insoluble calcium oxalate

The greatest oxalate accumulators (>5% of dry weight) are flowering plants such as Caryophyllaceae (many ornamental plants), Chenopodiaceae (the Goosefoot family) and Polygonaceae (the Knotweed family).[[35]] Crystal formation (as opposed to oxalate formation) in L. minor is not tightly coupled to nitrogen assimilation or absolute calcium concentration. Dark-grown plants form almost four times as many crystal cells (idiobblasts) as do light-grown plants.[36] Low oxalate production in Lemna plants grown in the dark and supplied with organic nutrients correlates with lower glycolate oxidase activity.[37]

Nutrient contents

Lemna paucicostata dry mass protein contents may range from 26% to 45%. Its amino acid profile favourably compares with blood, soybean and cottonseed meals. Mineral levels are high but not toxic. Nitrogen levels are comparable to those in fertilizers.[38] The feed provided (incl. Lemna) neither has any influence on the accumulation of omega-3, nor on the omega-3:omega-6 ratio in Tilapia. [39]

Nutrient loading

Lemna gibba yields no difference in protein content (in dry matter) when grown over a wide range of nutrient levels (52 to 176 mg N/l).[40] Lemna minor responds indifferently to nutrient loading. Lemna minuta responds opportunistically to high nutrient availability. As a result, the L. minuta is dominant (60% in dry biomass) in high nutrient availability but loses (< 50%) to L. minor at low nutrient availability.[41] Compared to other Lemna, Lemna minuta has the lowest nitrogen requirement for growth.(Minimum = 0.0016 mM/L and maximum is 30 mM/L) Most species appear to exhibit optimum growth between 20°C to 30°C. (Landolt 1986)[42] Best growth is achieved where total nitrogen concentrations range from 10 to 40 mg/L.[43]. Optimal growth occurs around neutral pH for both Spirodella and Lemna species. Duckweed is able to accumulate up to 1.5% of its weight as phosphorus in nutrient rich waters.[44]