| Being at high position in the food chain of "soil-plant-edible fungi-human being" and plant residues as culture mediums for artificial cultivation, edible fungi usually contain higher heavy metals compred to foods made from plants such as vegetables and grains, even those from animals. Information on heavy metals in edible fungi are quite scare, resulting in the hygienic criteria of Cd was not listed in the "hygienic stands for edible fungi" of China (GB7096-2004). High concentrations of heavy metals in some edible fungi for export are a serious problem nowadays. Large amount of commercial edible fungi is produced in Chongqing, the biggest industrial city in Southwest China. To study heavy metals in the daily-consumed edible fungi in Chongqing could provide primary information on heavy metal pollution in edible fungi in tanr areas. Mycelium is the base organs of edible fungi. The experiments were thus set up to study Hg absorption characters of edible fungus mycelia in liquid mediums, aiming at reducing heavy metals in sporophores of edible fungi. Also, the contents of Pb, Cd, Hg and As in daily-consumed edible fungi produced in Chongqing were determined.1. Heavy metals in edible fungi produced in ChongqingThe samples of edible fungi were collected from different areas in Chongqing municipality and analyzed for heavy metals such as Pb, Cd, Hg and As. The mean contents showed in the sequence: Pb > Cd > Hg > As, lower than the hygienic criteria of China, suggesting that the edible fungi produced in Chongqing were safe for human health in general. However, the concentrations of Pb in 12% of samples and Hg in 19% of samples were over than the rhygienic criteria, respectively. The concentrations of As were much lower than the hygienic criteria. Moreover, there existed the tendency of city areas > suburb areas far suburb areas in term of variation coeffiencies of heavy metal concentrations in edible fungi.In detail, the contents of heavy metals in edible fungi changed in the sequence: Agaricus bisporus (Lange) sing. (X|- = 0.83 mg · kg-1· dw) > Coprinus comatus (Fr.) S. F. Gray ( X|- = 0.56 mg · kg1- dw> Lentinus edodes (Berk.) Peg lerl (X|- = 0.52 mg · kg-1· dw)> Pholiota maneko {X|- = 0.51 mg · kg-1· dw) > P. ostreatus (X|- = 0.42 mg · kg-1dw) > Flammubine velutipes (Curt, ex Fr.) sing. (X = 0.39mg · kg-1· dw). Most of them were lower than the hygienic criteria. Variation coefficients of heavy metal concentrations in edible fungi were quite high with the tendency of P. ostreatus (C.V= 268%)> Flammubine velutipes (Curt, ex Fr.) sing. (C.V = 261%)>Coprinus comatus (Fr.) S. F. Gray (.C.V = 228%) > Pholiota maneko (C.V = 180%) > Agaricus bisporus (Lange) sing. (C. V. = 150%) > Lentinus edodes (Berk.) Peg lerl (C. V. = 137%). It is necessary to point out that variation coefficients of heavy metal concentrations in edible fungi were quite large, especially those of the P. ostreatus, Flammubine velutipes (Curt, ex Fr.) sing and Coprinus comatus (Fr.) S. F. Gray which were over than 200%. The results suggest that accumulation of heavy metals in the edible fungi is not only related with the cultivars, but also with the environment backgrounds, cultivation methods and culture mediums and so on. It is possible to explain the variable concentrations of heavy meats in same edible fungi but produced in different areas of Chongqing.The mean contents of different heavy metals in edible fungi changed in the sequence: Pb (X = 1.41 mg · kg-1· dw) > Cd (X = 0.40 mg · kg-1· dw) > Hg (X = 0.13 mg · kg-1· dw) > As (X = 0.04 mg · kg-1· dw. The vaiations changed from 0 17.28 mg · kg-1· dw(Pb) , 04.31 mg · kg-1· dw(Cd), 02.030 mg · kg-1· dw(Hg), 0-0.37 mg · kg-1· dw(As). It supposed that the phenomenon is related with the environmental backgrounds of heavy metals. The variation coefficients of heavy metals were Pb(C.V = 276%) > Hg (C. V. = 202%) > Cd (C. V. = 185%) > As (C. V. = 126%). All of them were over than 100%, suggesting that the concentrations of heavy metals in edible fungi varied with the environments and types of metals. Large variation coefficients of heavy metal concentrations in edible fungi also suggest the effects of the environments, edible fungus species and cultivation on the contents of heavy metals in the fungi. It is thus possible to decrease them by selection of good varieties and improvement of cultivation practise in unpolluted environments.2. Hg++ absorption by edible fungi.Two market-common edible fungi Pleurotusostreatus and Pleurotus pulmonarius were isolated from the market-sold sporophores in Chongqing and then cultured in Pachlewski liquid medium for 2 weeks to investigate Hg++ absorption. Mycelia were placed into the mediums containing 2 mg Hg**· kg-1 with and without EDTA and CaH2PO4 added in different concentrations and pH adjustedin variable gradients, respectively. Hg++ concentrations were determined at certain time intervals (5, 10, 20, 40, 60, 120, 180min). Hg++ concentrations in solutions decreased as the time prolonged due to absorption and showed exponential decay curves, suggesting that Hg++ absortion by mycelia followed active influx process. After 180 min of mycelia placed, Hg++ concentrations in solution were constant (Cmin) even though mycelia were different in dry weight in every flask, suggesting Hg influx and efflux reached balance at those concentrations. Pleurotusostreatus showed the significant difference with EDTA and CaH2PO4 added in terms of Cmin0.1 mM and 0.5mM EDTA were added respectively in Hg absorption solutions with 1 mg Hg · L-1 or 2 mg Hg · L-1) to investigate the effect of EDTA on Hg++ absorption. EDTA addition resulted in a significant increase in Cmin for Hg absorption atl80 min compared two the control without EDAT added. Furthermore, the amount of Hg** absorbed by Pleurotusostreatus mycelia was reduced by EDTA addition. The reduction was more obvious by adding 0.5 mM EDTA compared to 0.1 mM EDAT.0.1 mM CaH2PO4 and 0.5mM CaH2PO4 were added respectively into Hg absorption solutions with 2mg Hg ·L-1. After 180 min, the Hg concentrations in solutions suggest Hg absorption promoted by lower CaH2PO4 and redueced by higher one.Medium containing 1 mg·kg-1 and 4 mg·kg-1 Hg++ were applied to investigate Hg++ absorption by the Pleurotusostreatus mycelium. The choice of Hg++ concentrations were based on the former successful experiments: (1) The concentration of Hg++ at 180min was all below 1 mg·kg-1, so experiment at 1 mg·kg-1 Hg++ condition may tell us if the mycelium would stop absorbing Hg++ in condition of 1mg·kg-1 Hg++. However, the results showed significant decrease of Hg++ concentration, suggesting the reduction of Hg++ not only related with the absorption by mycelium, but also related with the attachment of Hg++ on surface of mycelium. The very high S.D. (0.12) of Hg++ concentration at 180min comparing with the former experiment with 2 mg·kg-1 Hg++ also gave positive information. (2) Although the former experiments had a satisfaction result, they were not accordance with Michaelis-Menten equation. The analysis showed that in condition of 2 mg·kg-1 Hg++ absorption of mycelium might not reach the fastest speed, so we expected that 4 mg·kg-1 Hg++ would work. But the results didn't show any accordance as we wished. In the condition of 4 mg·kg-1 Hg++, concentrations at 180min are varied with the dry weight of mycelium, suggesting the Hg++ concentration in the mycelium had reached saturation. The mean contents of Hg++ at 180min were as follow: CK=1.6938 mg·kg-1, CaH2PO4 treated= 1.4882 mg·kg-1, EDTA treated= 0.8915 mg·kg-1, respectively. It is much lower than the results in literature. This information led to a... |
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