Effect Of La And Ce On Cd Absorption And Some Physicochemical Indexes Of Crassostrea Gigas And Their Accumulation Pattern

With the rapid progression of industrialization in both China and the world, theheavy metal （especially Cd） pollution issue of seawater has turned to be a worlwideconcern. Marine bivalves are filter-feeding molluscs and can accumulate extremelyhigh levels of heavy metals. Heavy metal pollution greatly threatens the safety ofmarine bivalve-derived foods. The selection of unpolluted rearing sea area is thepreferred way for avoding heavy metal pollution in bivalves, but it is still meaningfulto develop alternative solutions. Some researches have attempted to elimiate Cd frompolluted bivalves, the current research aims to solve the problem by inhibiting Cdaccumulation in bivalves. Marine bivalves absorb cadmium through the Ca²⁺channeland inhibitors of the channel are theoritically able to inhibit Cd accmuluation. Manyrare earth elements have radiuses similar to that of Ca²⁺and are potential Cdaccumuliation inhibitors. The current research explores the possiblity of applying Laand Ce, which have been approved by the Ministry of Agriculture for addition tolivestock and fish feeds, in inhbiting Cd accumulation in Crassostrea gigas. Theresults are as follows:1. The Cd pollution issue of marine bivales sold in Qingdao markets is serious.The monthly varition of Cd and Cu contents in five common bivalves sold in aQingdao market, includign Crassostrea gigas, Mytilus edulis, Scapharca subcrenata,Ruditapes philippinarum, and Chlamys ferrari, were followed for one year. It wasfound that the Cd contents of all the bivalves except Ruditapes philippinarumexceeded the permitted value of the national standard. Chlamys ferrari andCrassostrea gigas had the highest Cd contents and were within the ranges7.1¹6.3mg/kg·dw and4¹2.7mg/kg·dw respectively. Besides, the Cu contents ofCrassostrea gigas also exceeded the permitted values in some months. Crassostreagigas was selected as the model bivale in subsequent researches.2. Increased Ca concentration in seawater cannot inhibit the absorption of Cd byCrassostrea gigas. Though elevated seawater Ca concentration of0.525g/L,0.7g/L,and0.875g/L increased the96h LD50of Cd from17.271mg/L to24.988mg/L,43.932mg/L, and41.795mg/L respectively and decreased the oxidative damageinduced by Cd in oyster gill, it could not significantly inhibit the accumulation of Cdin Crassostrea gigas.3. Both La and Ce significantly inhibit the accumulation of Cd in Crassostrea gigas. On day12, the inhibition rates of Cd accumulation by5μmol/L and10μmol/LLa（NO₃）₃reached36%and44%respectively and that of25μmol/L Ce（NO₃）₃reached41%. Organ-level experiments revealed that the two rare earth metals elongated thetime for saturated absorption of Cd in the gill and thus reduced the transfer of Cd fromgill to digestive gland.4. La and Ce are accumulated in high levels in Crassostrea gigas. After exposureto1,5, and10μmol/L La（NO₃）₃or Ce（NO₃）₃for24days, the contents of La reached79.75,199.70, and425.82mg/kg·dw respectively and that of Ce reached42.07,158.23, and236.28mg/kg·dw respectively. Distribution in organs revealed that gillhad the highest La and Ce contents, followed by digestive gland, remainder, andmuscle. Gill was the main organ for La and Ce storage and the percentages of the twometals were64%and51%on day24, followed by digestive gland with percentages of13%and25%respectively. Subcellular research indicated that both La and Ce boundmainly with MTLP （metallothionein-like protein） and their percentages were38.2%and35.4%respectively. Organelles were the second most important fractions for Laand Ce sequestration and the percentages were25.7%and29.7%respectively.Depuration experiments showed that no significant reduction of La and Ce contentswas observed after one month, indicating slow elimination of the two elements fromCrassostrea gigas.5. The acute toxicity of La or Ce to Crassostrea gigas is low. After exposure to75μmol/L （the highest concentration available in seawater） La（NO₃）₃or Ce（NO₃）₃for96h, no mortality was observed. La and Ce did not induce oxidative damage aftershort-term exposure, but long-term exposure resulted in elevated MDA levels in boththe gill and digestive gland. Besides, Ce suppressed the expression of p-gp and MTgenes, indicating difference behavior from heavy metals.6. Exposure to La or Ce does not cause changes in muscle index or muscletexture, but influences the contents of protein, lipid, Zn, and Cu. La（NO₃）₃in1μmol/Lincreased the protein contents in Crassostrea gigas, but1and5μmol/L Ce（NO₃）₃reduced the index. Besides,1μmol/L La（NO₃）₃reduced the lipid content, but10μmol/L Ce（NO₃）₃increased the lipid content. Exposure to La and Ce did not affectthe texture of adductor muscle, but10μmol/L Ce（NO₃）₃significantly reduced thewater-holding capacity of the muscle. Exposure to La（NO₃）₃and Ce（NO₃）₃significantly reduced the contents of Ca, Cu, and Zn. After exposure to1,5, and10μmol/L La for30days, the Zn contents decreased by48%,60%,39%and the Cu contents decreased by63%,32%,39%. After exposure to the three concentrations ofCe（NO₃）₃, the contents of Zn and Cu decreased by8%,44%,18%and0%,11%, and28.6%respectively.It was concluded that both La and Ce significantly inhibited the accumulation ofCd in Crassostrea gigas and did not induce oxidative damage or cause markedchanges in texture and nutrient contents. Because rare earth elements have gainedwide practical applcations freshwater aquaculture, La and Ce have potentialapplications in reducing Cd accumulation in marine bivalves. However, La and Cewere accumulated in very high levels in oyster soft tissues and cannot be eliminatedwithin a short time, which differed significantly from the metabolism of rare earthelements in livestocks; besides, long-term exposure to rare earth element led tooxidatve damage and reduced Zn contents, which possibly cause adverse effect on thephysiological activities of bivalves. Further researches are necessary for promotingthe application of La and Ce in marine bivalve rearing and Cd bioaccumulationihnibition.