Kumar et al. 2022 — Heavy metals and minerals in 12 freshwater macrophytes as feed ingredients
Kumar and colleagues analyzed the mineral composition of twelve freshwater macrophyte species (duckweeds, watercress, waterthyme, water spinach, water clover, water cabbage, water fern, and others) grown under controlled outdoor culture conditions in India, using ICP-MS (Agilent 7900). Among the heavy metals of concern, Pb was detected in all twelve species; Cd was highest in Hydrilla verticillata; Sn, Ni, and Cr were also measured. All Cd, Cu, Pb, and Sn values were reported as within WHO/FAO permissible limits (Cd: 0.2 mg/kg wet wt; Pb: 0.3 mg/kg wet wt; Sn: 250 mg/kg wet wt), noting that the study measured dry weight and the comparison requires adjustment for approximately 90% moisture. Several macrophytes are consumed by humans in South and Southeast Asia; the majority are candidates for use as animal feed ingredients.
Key numbers
Species studied (n=12): Azolla microphylla, A. pinnata, Enhydra fluctuans, Hydrilla verticillata, Ipomoea aquatica, Lemna minor, Marsilea quadrifolia, Pistia stratiotes, Salvinia molesta, S. natans, Spirodela polyrhiza, Wolffia globosa. Analytical method: ICP-MS (Agilent 7900). Sample prep: microwave digestion with suprapure HNO3. All values reported on dry weight basis; fresh weight comparison requires ~90% moisture correction. Heavy metals (trace and ultra-trace): Cd highest in H. verticillata; Pb present in all species (figure data, not individually tabulated in retrieved text); Sn highest in A. microphylla; Ni highest in S. polyrhiza; Cr highest in P. stratiotes, A. microphylla, and A. pinnata; Co highest in H. verticillata. All values reported within WHO/FAO permissible limits on a wet-weight-equivalent basis. Al: highest levels in P. stratiotes, second in A. pinnata, third in A. microphylla (figure data only). Fatty acid profiles also measured; dominated by linoleic acid (n-6) and alpha-linolenic acid (n-3).
Comparison to WHO/FAO permissible limits (wet weight): Cd ≤ 0.2 mg/kg, Cu ≤ 73.3 mg/kg, Pb ≤ 0.3 mg/kg, Sn ≤ 250 mg/kg, Zn ≤ 99.4 mg/kg.
Methods (brief)
ICP-MS quantification of macro-, trace-, and ultra-trace minerals. Microwave digestion (Anton Paar Multiwave PRO) with 8 mL suprapure 69% HNO3. Standards from Agilent Technologies; blanks with Milli-Q water/1% HNO3. Three replicates per species. Results expressed in µg/g (dry weight). Fish, poultry, and livestock dietary mineral requirements reviewed for context.
Limitation: Values are from controlled culture with organic manure (cattle, poultry droppings, mustard oil cake); concentrations may differ from macrophytes in natural waterways or those grown in contaminated water. Study is single-site (India); no inter-site or inter-season data reported.
Implications
Certification: Macrophytes used in aquaculture feed contribute to heavy metal loads in farmed fish and shellfish; relevant to supply-chain context for aquaculture-origin products. Species such as I. aquatica, E. fluctuans, and M. quadrifolia are directly consumed by humans; their Pb and Cd levels are relevant to the food-safety register.
Courses: Useful example of accumulation of Cd, Pb, and other metals in aquatic plants that enter the human food chain via both direct consumption and animal feed pathways.
App: Data support the contamination profile for aquatic-plant-derived feed ingredients. Values are below WHO/FAO limits but the dry-weight/wet-weight adjustment must be clearly communicated.