Lemna For Fish
Contents
Feed for Tilapia
Lemnacea generally grow optimally in water with a temperature of 17.5 to 30 ºC. Raising Nile Tilapia in fishponds fed on fresh duckweed, the toxic level of un-ionised ammonia nitrogen and its negative effect on the growth performance lies between 0.07 and 0.14 mg/L.[1] Tilapia prefer to eat Equisetum sp. (horsetail aka snake grass, containing thiaminase and nicotine) over Lemna and Spirodela spp subsequently.[2] Tilapia prefer Wolffia over Lemna.[3] In terms of growth rate and flesh protein content, Lemna sp. as feed give better results than Hydrilla sp. and Chara sp.[4] Lemna minor gave better growth results (both somatic and reproductive) than Eichhornia crassipes (Water hyacinth) and Azolla pinnata (Azolla). [5][6] 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.[7] Fish feed supplemented with duckweed gave much better growth rates than fish feed alone, and than fish feed supplemented with taro.[8]
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.[9] 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.[10] In tilapia fed 15% duckweed + 85% fishmeal, growth rates was lower and nitrogen retention was similar in comparison to a 100% fishmeal diet.[11] In tilapia fingerlings, duckweed could replace commercial feed for up to 50% without a reduction in final weight.[12] In (red) tilapia fingerlings, a diet with 25% duckweed (Lemna obscura) replacement gave better growth rates than the control diet free of Lemna.[13] In tilapia fingerlings a diet with 10% duckweed (Lemna paucicostata) gave better growth rates than a standard diet (KLRI/40/6) free from Lemna.[14][]
Yield of (GIFT) tilapia fed phytoplankton + duckweed (Lemna minor) was signaficantly higher than of tilapia feeding on phytoplankton alone.[15]
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.[16]
Wolffia
Wolffia arrhiza is rootless, less than 2 mm long and is native to Europe, Africa and Asia. It may grow in pH ranging from 5 to 10, with an optimum of 7 to 8 pH.[17] 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 (dried meal) 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] Wolffia globosa does not survive cold weather.[24]
Lemna
Lemna minor may be 1 to 8 mm long. Lemna minuta is maximally 3 mm long. It is present in the Americas, Europe, Japan. Lemna minuta (aka Lemna valdiviana var. minim and Lemna miniscula) is often confused with Lemna minor. L. minuta has just one vein in the frond, whereas L. minor has 3.[25] Lemna minuta propagate almost exclusively by vegetative reproduction (budding). Flowering is very rare.[26] The plant mass can double in size every two or three days in optimum conditions. In full sunlight growing in high abundance, the fronds reach no more than 1-2 mm with solitary fronds or clusters of two; in shady conditions, the fronds can reach 4 mm long and can be clustered in four segments. In the shade form, L. minuta mas also be confused with L. valdiviana. The nerve of L. valdiviana extends more than half the distance from the node to apex, compared to about half in L. minuta.[27] Lemnaceae are very sensitive to herbicides[28]
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).[29] 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.[30]
Anti-nutrients
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.[31] 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) [32]
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[33]) 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.[34] Glycolate is mostly converted to glycine and serine instead of oxalate [35], as L. minor lacks the machinery for converting glycolate to oxalate.[36]
- 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.[37]
- D-glucosone may yield ascorbic acid and oxalate in Lemna minor.[38]
- 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 [39]. More crystals are formed by L. minor grown on nitrogen from ammonium than by plants grown on nitrogen from nitrate.[40]
- 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).[41]
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).[[42]] 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.[43] Low oxalate production in Lemna plants grown in the dark and supplied with organic nutrients correlates with lower glycolate oxidase activity.[44]
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.[45] 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. [46]
Duckweed starch concentrations increase continuously (with low lignification ratios and high flavonoid biosynthesis) under nutrient starvation.[47] Phosphorylation of starch is required for its degradation. Illuminated cells incorporate phosphorus into starch.[48] Phosphorus deficiency decreases growth, increases starch and anthocyanin concentrations in Spirodela oligorrhiza, and does not affect nitrogen metabolism and protein contents.[49] Degradation of storage starch in Spirodela polyrhiz, is induced by light.[50] Spirodela oligorrhiza continue to increase in frond number for 2 to 3 days after transfer to darkness, after which growth ceases but starch accumulation progresses.[51] Non-growing Spirodela oligorrhiza left in the dark for 6 days may contain 3-fold more starch.[52] Light exposure induces, and nitrate (not ammonium) is required for germination in Spirodela polyrhiza, breaking down starch reserves. Starch will only be broken down when sufficient nitrogen (from nitrate) is present to ensure appropriate utilization of the released carbohydrate.[53] Long-term exposure to UV-B reduces duckweed (Spirodela polyrhiza) productivity along with depletion in protein, pigments and biomass.[54] In Lemna major UV-B exposure increases peroxidase activity, dose-dependently decreases chlorophyll and starch with drastic depletion in protein and free sugar content.[55]
Lemna minor contains 51% carbohydrates of dry matter while starch accounts for 20%. The cell wall is rich in cellulose and also contains 20.3% pectin comprising galacturonan, xylogalacturonan, rhamnogalacturonan; 3.5% hemicellulose comprising xyloglucan and xylan, and 0.03% phenolics. In addition, essential fatty acids (0.6%, α-linolenic and linoleic/linoelaidic acid) and p-coumaric acid (0.015%) respectively are the most abundant fatty acids and phenolics in whole duckweed.[56]
Wolffia arrhiza (dry weight) may contain up to 40% protein, up to 40% starch and up to 2% phosphorus.[57] may contain 44% carbohydrates, 20% protein and 5% fat.[58] By dry weight this may be 40-50% protein and 40-50% starch [59], or 20.4% in low nutrient conditions.[60] Wolffia arrhiza and Lemna also contain apiose (a sugar).[61]
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).[62] 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.[63] 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)[64] Best growth is achieved where total nitrogen concentrations range from 10 to 40 mg/L.[65]. 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.[66] Unlike copper, zinc, manganese and boron, iron stimulates growth and flowering.[67] Duckweed consumes dissolved oxygen as the result of mineralization, and Wolffia more so than Lemna.[68] Carbon isotopes are fractionated in duckweeds by both the central photosynthetic enzyme (ribulose-1,5-biphosphate carboxylase) and by glycine decarboxylase.[69]
Cultivation
In tropical conditions, increased water depth may prevent heat stress. The duckweed should also be dipped below the water surface once a day. The banks/sides need to be vertical, to prevent the plants from getting stranded, dry out and die. In large ponds, one may create 5 by 5 meter sections by applying a grid of bamboo sticks. Use multiple water inlets to prevent clogging and to spread nutrient inflow. Every day, 10 to 35% of the duckweed may be harvested.[70]
Productivity
The optimum pH for growth of Lemna and Wolffia species ranges from 7 to 8.[71] Submerged growth increases the productivity unit volume of Lemna minor and Wolffia sp.[72]
