The cellular processes involved in metal metabolism in molluscs are reviewed, with emphasis on the contribution of microscopy (AMG, ARG, EPMA, and SIMS) to both basic research of metal cell biology and applied environmental research. In molluscs, metal uptake may occur by facilitated diffusion, active transport, or endocytosis, and can be enhanced by MT synthesis or formation of mineralized granules. In aquatic molluscs, gills constitute a key interface for dissolved metal uptake, where metals are bound to MT, incorporated into lysosomes, and released basally towards the blood plasma and circulating hemocytes. However, particulate metal uptake is mainly achieved via the digestive tract by endocytosis; further metals are transferred first to lysosomes and then to residual bodies, especially in the digestive cells of the digestive gland. Additionally, metals can be accumulated selectively in specific cell types. As ligands pools differ from cell to cell, different metals may be retained in different cell types. Class "a" metals are localized in cells with granules composed of carbonate, oxalate, phosphate, and sulfate (oxygen donors), whereas "b" metals are associated with those cell types rich in sulfur and nitrogen ligands (sulfur donors). In molluscs, oxygen donors occur in connective tissue calcium cells and basophilic cells, whereas sulfur donors are present in digestive cells, podocytes, nephrocytes, and rhogocytes. Hemocytes, which constitute the most relevant system for metal transport between tissues, move around the body and may penetrate tissues and remove metals from the inner medium to be accumulated in lysosomes as nondigested products. Rhogocytes also participate in metal mobilization, accumulation, and release. The assessment of metal levels in target cells of sentinel molluscs by microscopic techniques provides an early-warning measure, with promising applications as an exposure biomarker for environmental monitoring programs.

Cellular and subcellular distribution of metals in molluscs.

Seaweeds are important primary producers, and as such contributesignificantly to nearshore ecosystems. Studies on the effects ofanthropogenic stresses on these organisms have largely been concernedwith the vegetative adult stages of the life cycle. Here we review thelimited information on the sensitivity of early stages in the life cycleof seaweeds to global change (UV increase; global warming;increased storm frequencies) and pollution (eutrophication, trace metalsand oil). Impacts on fertility, substrate attachment, development,photosynthesis, growth and mortality are highlighted. In their naturalhabitats, early stages are shade-adapted, as they live shel tered underadult canopies and in pores of the substrata. Although some acclimationunder increased moderate irradiance is seen, higher solar irradiance,and especially ultraviolet-B, inhibits early development. Global warmingmay decrease the fertility and shorten the fertile period of somespecies. With the increasing likelihood of storms associated with globalwarming, gamete release may be inhibited while scouring by suspendedsediments may detach newly settled stages. Succession and localdistribution patterns are likely to be affected. Eutrophication canresult in accelerated development of the early stages of some algalspecies but sewage discharges have a negative impact on sperm motility,fertilisation and can cause increased mortality in germlings. Impacts ofother, indirect effects of eutrophication, including increased sedimentcover of substrata, scouring caused by wind-induced resuspension ofsediments, and grazing, are also expected to be negative. Toxic tracemetals affect gamete viability, inhibit fertilisation and development,and reduce growth rates. Gametes are particularly susceptible to oilpollution and interactions between hydrocarbons and the adhesive mucussurrounding the embryonic stages seem to inhibit settlement.Recommendations for future studies are provided that are aimed atgaining greater insight into the effects of anthropogenic stress on theweakest links in the development cycle of seaweeds.

Impacts of anthropogenic stresses on the early development stages of seaweeds

It has been well established that a number of trace metals are essential for various biological functions and are critical in many of the enzymatic and metabolic reactions occurring within an organism. The essentiality of nickel is now generally accepted, based on the numerous symptoms caused by nickel deficiency (mainly in terrestrial vertebrates) and its essential role in various enzymes in bacteria and plants. The information on optimal and deficient concentrations of nickel, however, is limited and the essentiality of nickel to aquatic animals is not established. The purpose of this review is to synthesize the available information on nickel essentiality and homeostasis in aquatic organisms. There is less information on these topics compared to that for other essential metals. Nickel essentiality to aquatic organisms can only be confirmed for plants and (cyano)bacteria due to the documented role of nickel in the urease and hydrogenase metabolism. Deficiency levels ranged from 10 −12 Mt o 2× 10 −6 M Ni in different species. No studies were identified that had the explicit objective of evaluating homeostatic mechanisms for nickel in aquatic life. However, inferences could be made through the evaluation of nickel bioconcentration and tissue distribution data and a comparison to other metals that have been more thoroughly studied. Data suggest active regulation and therefore nickel essentiality, since there are no known examples of active regulation of non-essential metals in invertebrates.

Nickel essentiality and homeostasis in aquatic organisms

Trace metal concentrations in marine macroalgae from different biotopes in the Aegean Sea.

Heavy metals concentrations in some commercially key species from Sicilian coasts (Mediterranean Sea): Potential human health risk estimation.

The purpose of this chapter is to provide a synthesis of the very extensive literature pertaining to the factors that influence the uptake, accumulation and handling of trace metals by marine biota. For more extensive reviews refer to the key references: Eisler (1981), Fowler (1990), Samiullah (1990), Vernet (1991), Bryan and Langston (1992) and Tessier and Turner (1995). The literature is replete with examples of the quantification of metal concentrations in an array of marine organisms collected from numerous locations. However, it is evident that information concerning the factors that determine the metal burdens of these organisms is patchy, with certain groups more thoroughly investigated than others; for example, contrast the molluscs with macroalgae.

Michael H. Depledge

Papers

Determinants of trace metal concentrations in marine organisms