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No abstract

It has been demonstrated that the deposit-feeding oligochaete Limnodrilus hoffmeisteri inhabiting Foundry Cove (FC), a severely cadmium (Cd)-contaminated cove located on the Hudson River, New York, USA, has evolved resistance to Cd. In this study we investigate how this resistance influences Cd trophic transfer from this oligochaete to the grass shri.mp Palaernonetes p u g~o . Cadmium-resistant worms collected from FC and nonresistant worms collected from an adjacent unpolluted site were investigated for differences in Cd tolerance, accumulation, subcellular distribution and bioavailability to shrimp. FC worms were more tolerant of Cd, surviving twice as long as worms from the unpolluted site during a toxicity bioassay The 7 d concentration factor of Cd-resistant worms was 4 times greater than that of nonresistant worms (2020 vs 577). There were also differences between worm populations with respect to subc~llular Cd distributions. Cd-resistant worms produced metallothionein-like proteins (MT) as well as metal-nch granules (MRG) for Cd storage and detoxification; nonresistant worms only produced MT These differences in subcellular Cd distributions led to large differences in Cd bioavailability to shrimp; shrimp fed Cd-resistant worms absorbed 21 % of the ingested Cd, while those fed nonresistant worms absorbed roughly 4 times that amount (-757:,). These absorption efficiencies were in good agreement with the proportions of Cd bound to the worm's most biologically available subcellular fractions (i e , the cytosol and organelles). Although Cd-resistant worms predominantly stored the toxic metal In biologically unavailable MRG, their increased accumulation of Cd would still result in substantial trophic transfer to shrimp because of the storage of Cd in the biologically available fractions. This work demonstrates that the evolution of Cd resistance can have profound implications for Cd bioavailability and cycling within aquatic ecosystems.

The deposit-feeding oligochaete Limnodrilus hollmelsteri possesses metallothionein-like proteins and metal-rich granules for storing and detoxifying cadmium (Cd). In this study we investigated the bioavailability of Cd sequestered within this oligochaete by conducting feeding experiments with '""Cd-labeled oligochaetes and the omnivorous grass shnmp Palaemonetes puglo. We also make predictions on Cd trophic transfer based on oligochaete subcellular Cd distnbutions and absorption efficiencies of Cd by shrimp. Cytosol (~ncluding metallothionein-like proteins and other proteins) and a debris fraction (including metal-rich granules and tissue fragments) isolated from homogenized '""Cdlabeled oligochaetes were embedded in gelatln and fed to shrimp. The '""Cd absorption efficiencies of shrimp fed these subcellular fractions were 84.8 and 48.6'!.), respectively, and were significantly different (p < 0.001), indicating that ""d bound in these fractions was not equally available to a predator. Mass balance equations demonstrate thdt shrimp fed whole worms absorb 61.5% of the ingested In9Cd, an absorption efficiency similar to that obtained experimentally (57.1 %). Furthermore, the majority of the absorbed In"Cd comes from the fraction containing metallothionein-like proteins (1 e. cytosol). ' O v d absorbed from the debris fraction probably comes from the digestion of tissue fragments, rather than metal-rich granules. The ecological significance of these f~n d~n g s is that prey detox~fication mechanisms may mediate the bioreduction or bioaccumulation of toxic metals along food chains by altering metal bioavailability. Another important finding is that trophic transfer of metal can be predicted based on the subcellular metal distribution of prey.

Microbial cells in natural environments arc often encased in different types of exopolymcr secretions (EPS), ranging from tight capsules surrounding individual cells to the looser slime matrices of biofilms. The diffcrcnt physical and chemical propertics of exopolymers could have secondary effects on trophic interactions between microbial cells and consumer animals. Laboratory studies showed that capsule EPS is significantly less digestible to consumers than slime EPS, even when extracted from the same bacterial strain. Bacterial cells with EPS capsules are less efficiently digested than noncapsuled cells, suggesting that capsules protect against digestion. Follow-up experiments determined that polysaccharide-rich fractions of slime EPS are absorbed with very high eficiencies while protein portions, which are more abundant in capsular polymers, are absorbed relatively poorly. Another series of cxpcriments showed that dissolved organic matter (DOM), when adsorbed directly to the mineralogical portions of sediment particles, is available to deposit feeders. However, the further presence of an exopolymer coating on sediments more than doubled the bioavailability of adsorbed DOM to the consumer. Observations using cold-stage scanning electron microscopy indicated that exopolymer microenvironments are a common feature of natural marine sediments. Microbial exopolymers range from easily digestible carbon sources to relatively refractory ones that effectively protect some microbial cells from consumer digestion. Exopolymer microcnvironments may also make recently adsorbed DOM highly accessible to particle-ingesting animals.