重金属在海洋食物链中的传递

The transfer of metals in marine food chains: A review

近年来 ,金属在不同海洋食物链中摄食富集的定量研究得到越来越多的关注。自然环境中生物体内金属的浓度并不一定和生物在食物链中所处的营养级有相关关系 ,金属在生物体内的富集还受到生物的同化、排出等过程以及其它生理生化因子的影响。在经典的海洋浮游生物食物链中 (浮游植物→桡足类→鱼类 ) ,桡足类往往可以很有效地排出体内的金属 ,同时鱼类的金属同化率又很低 ,所以该食物链中金属的浓度随食物链水平增加而减少。目前研究发现只有甲基汞和铯 Cs会被食物链所放大。在以腹足动物为顶级捕食者的底栖食物链中 ,因为生物结合金属的效率很高 ,高同化率和低排出率导致金属浓度在生物体内得到放大。重金属在生物体内的可利用性可以通过测定同化率、排出率等参数、并结合考虑生物对该金属的消化行为 ,运用一个简易的动态模型来估算。已有的研究中人们多考虑金属的化学性质对食物链传递的影响。着重介绍了近年来国外对金属在不同海洋食物链 (底栖和浮游 )中的传递的研究成果 ,强调在金属的生物可利用性评估中 ,要充分考虑到动物的生理、生化过程的影响 。

There are now increasing interests in the food chain transfer of metal contaminants in different marine food chains. The importance of metal dietary uptake in aquatic animals has been highlighted in many recent studies using both empirical experimental and modeling approaches. The application of bioenergetic-based kinetic model has played an important role in the delineation of the exposure of metal contaminants in the animals. This review summarizes the recent progress in this field, particularly on the measurements of a few important metal physiological parameters (e.g., assimilation efficiency from ingested food sources-AE, efflux rate constant during physiological turnover period) in several representative marine animals, including copepods, bivalves, gastropods and fish. Differences of these metal physiological parameters in different groups of marine animals are discussed. The potential trophic transfer factor (or biomagnification potential) of metals along marine food chains can be predicted based on a simple kinetic equation by incorporating metal AE, metal efflux rate and ingestion activity of the animals. With such approach, it is possible to dissect the complicate metal-food chain interaction in marine ecosystems. The transfer of a few metals (e.g., Cd, Hg, Zn) may be related to the trophic levels in the food chain, whereas for other metals, such trends are less obvious. At each trophic level, the potential biomagnification of metals is influenced either by metal AE only, or by metal efflux only, or by both assimilation and efflux. In the classical marine planktonic food chain (phytoplankton to copepods to fish), copepods can very efficiently remove the metals by their efflux systems, leading to a low metal concentration in the animals. The inter-species difference in metal AE is relatively small and metal AE is controlled by both metal distribution in the phytoplankton cytoplasm and metal gut passage across the copepod's gut. Typically particle reactive metals are less assimilated by the copepods as compared with those less particle reactive metals. In contrast, the metal assimilation efficiency in fish is generally low, resulting in a low trophic transfer factor of metals in the fish. The metal AEs show less variation among different functional groups of fish. Consequently, the potential biodiminution of metals in the planktonic food chain is caused by the efficient efflux in zooplankton and the low assimilation in fish. Metals showing exception to this general trend are the methylmercury and Se. Radiocesium also has the potential of being biomagnified in the marine fish as a result of its very high AE. In the marine benthic food chains, metals are efficiently assimilated and retained in the bivalves, resulting in high trophic transfer. There is a large inter-species difference in metal AEs among different species of bivalves, reflecting the complexity of metal handling strategies in marine bivalves. In the benthic top predators such as the gastropods, metals are further efficiently assimilated and retained in the animals, thus there is a further increase in trophic transfer factor. Marine gastropods develop rather sophisticated mechanisms (e.g., granule particles, metallothioneins) in binding with the metals. Thus, marine benthic food chains may possess the potential of metal biomagnfication, even for those metals that are biodiminished in the planktonic food chains (e.g., Cd). Although the physico-chemical properties of metals in controlling food chain transfer are generally considered in many past studies, we suggest that the physiological and biochemical processes need to be emphasized in any future study. Finally, we identify several important research areas in metal-biota interaction in marine food chains.