Lemna For Fish

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[1]) 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.[2] Glycolate is mostly converted to glycine and serine instead of oxalate [3], as L. minor lacks the machinery for converting glycolate to oxalate.[4]
• 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.[5]
• D-glucosone may yield ascorbic acid and oxalate in Lemna minor.[6]
• 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 [7]. More crystal are formed by L. minor grown on nitrogen from ammonium than by plants grown on nitrogen from nitrate.[8]
• 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).[9]

Oxalate primarily accumulates as:

• soluble oxalate
• insoluble calcium oxalate

The greatest oxalate accumulators (>5% of dry weight) are members of Caryophillaceae, Chenopodiaceae and Polygonaceae.[[10]] 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.[11] Low oxalate production in Lemna plants grown in the dark and supplied with organic nutrients correlates with lower glycolate oxidase activity.[12]

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).[13] 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.[14] 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)[15] Best growth is achieved where total nitrogen concentrations range from 10 to 40 mg/L.[16]. 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.[17]