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The bioaccumulation and toxicity of heavy metals were reviewed with special reference to microalgae, the key component of the food web in aquatic ecosystems. Heavy metals enter algal cells either by means of active transport or by endocytosis through chelating proteins and affect various physiological and biochemical processes of the algae. The toxicity primarily results from their binding to the sulphydryl groups in proteins or disrupting protein structure or displacing essential elements. Metals can break the oxidative balance of the algae, inducing antioxidant enzymes, such as superoxide dismutase (SOD), glutathione peroxidase (GPX) and ascorbate peroxidase (APX). The amount of oxidized proteins and lipids in the algal cells thus indicates the severity of the stress. Algal tolerance to heavy metal is highly dependent upon the defense response against the probable oxidative damages. Production of binding factors and proteins, exclusion of metals from cells by ion-selective transporters and excretion or compartmentalization have been suggested with regard to reducing heavy metal toxicity. However, a comprehensive description on the mechanisms underlining metal toxicity of microalgae and gaining tolerance is yet to be elaborated.

Aluminum (Al), the third most abundant element overall, after silicon and oxygen, is found virtually in all food, air, soil and water. Under acidic conditions, Al is solubilized into [Al(H 2 O) 6 ] 3+ , which is highly toxic to many plant species. Present paper reviewed Al toxicity and tolerance mechanism in agricultural plants with special reference to cereals and legumes. Even at micromolar concentrations, cell division in the root tip meristem in sensitive plants is quickly inhibited by Al 3+ . Expressing the species-dependent manner of response, growth enhancement at low concentrations of Al 3+ is also reported from some plants such as soybean. Plasma membrane can be identified as the primary target of Al toxicity, where production of higher reactive oxygen species and higher fatty acid peroxidation has been observed due to alteration of plasma membrane integrity. Though, toxicity and the mechanisms attributed to Al-resistance are extremely complex phenomena, exclusion is widely accepted as the key mechanism involved in detoxifying Al 3+ . Exudation of chelating ligands, formation of pH barrier at the rhizosphere or at root apoplasm, cell wall immobilization, selective permeability of the plasma membrane, and Al efflux have been proposed as the possible mechanisms for Al exclusion. Al-induced exudation in cereals and legumes is dominated by citrate, malate, and oxalate in varying degrees depending on the species and/or cultivar. Apart from sensitive cultivars, moderately tolerant or tolerant cultivars can also be distinguished from various kinds of cereals and legumes. However, reliable techniques for screening such resistant genotypes have not been developed for any economically important crops.

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Salacia reticulata is a large woody climbing shrub naturally found in Sri Lanka and Southern region of India. It is widely used in treating diabetes, a chronic disorder in metabolism of carbohydrates, proteins and fat due to absolute or relative deficiency of insulin secretion with/without varying degree of insulin resistance. The decoction of S. reticulata roots is also used in the treatment of gonorrhea, rheumatism, skin diseases, haemorrhoids, itching and swelling, asthma, thirst, amenorrhea and dysmenorrheal. Presence of mangiferin (a xanthone from the roots), kotalanol and salacinol (from the roots and stems) have been identified as the antidiabetic principles of S. reticulata. Chemical constituents such as 1,3diketones, dulcitol and leucopelargonidin, iguesterin, epicatechin, phlobatannin and glycosidal tannins, triterpenes, and 30-hydroxy-20(30) dihydroisoiguesterin, hydroxyferruginol, lambertic acid, kotalagenin 16-acetate, 26-hydroxy-1,3-friedelanedione, maytenfolic acid have also been detected in the roots of S. reticulata. The antidiabetic property of Salacia is basically attributed to the inhibitory activity of intestinal enzymes (α-glucosidase and α-amylase). Inhibition of intestinal enzymes delays glucose absorption into the blood and suppresses postprandial hyperglycemia, resulting in improved glycemic control. Furthermore, mangiferin has been reported to inhibit aldose reductase activity delaying the onset or progression of diabetic complications. Though diabetes has now become an epidemic affecting millions of people worldwide, neither insulin nor other modern pharmaceuticals has been shown to modify the course of diabetic complications mainly due to the multifactorial basis that involves both genetic and environmental risk factors. Therefore, effort is being devoted to find new therapeutics aimed at multiple targets, which has become a new paradigm in drug discovery. In this context, the discoveries on S. reticulata have lead to increase the consumption of the species across the world and it has now become a subject of broad studies for diabetes management.

BACKGROUND:Certain crop-based waste materials have been recognized as cost-effective and highly efficient adsorbents for removal and recovery of different kind of heavy metals from aqueous solutions. The ability is strongly attributed to the carboxyl functional group of some pectin substances such as galacturonic acid often found in fruit peels. The present manuscript was aimed at assessing the potential applicability of banana peel for metal removal from contaminated waters. METHODS AND RESULTS:As revealed by laboratory investigations, banana peel contains pectin (10-21%), lignin (6-12%), cellulose (7.6-9.6%), and hemicelluloses (6.4-9.4%). The pectin extraction is reported to have glucose, galactose, arabinose, rhamnose, xylose, and galactouroninc acid. Several studies conducted under different conditions proved that banana peel is capable of adsorbing 5. 71, 2.55, 28.00, 6.88, 7.97, and 5.80 mg/g of Cd

A laboratory experiment was conducted to assess the bioaccumulation of Pb 2+ and its effects on growth, morphology and pigment contents of Spirulina (Arthrospira) platensis. The specimen cultured in Zarrouk liquid medium was treated with various initial metal concentrations (0, 5, 10, 30, 50 and 100 μg mL -1 ). The growth of S. platensis was adversely affected by Pb 2+ at high concentrations (30, 50 and 100 μg mL -1 ). However, at low concentrations (5 μg mL -1 ), Pb 2+ could stimulate its growth slightly. The pigment contents (chlorophyll α and β carotene) were decreased in a dose-dependent manner. The highest reductions (67% and 53% respectively in chlorophyll α and β carotene) were observed in 100 μg mL -1 treatment group. The LC 50 (96 h) of Pb 2+ was measured as 75.34 μg mL -1 . Apart from a few cases of filament breakages at elevated concentrations (50 and 100 μg mL -1 ), morphological abnormalities are not specific. Metal bioaccumulation increased with Pb 2+ concentrations, but decreased with exposure time. The maximum accumulated amount was 188 mg g -1 dry weight. The bioconcentration factor (BCF) reached to a peak at day 2, followed by a gradual reduction for all the exposure concentrations. S. platensis is able to tolerate considerably high Pb 2+ concentrations. Consequently it can be used as a potential species to remove heavy metal from contaminated waters.

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