Influence of Pre-Exposure to Dietary Metal

On the Assimilation and Subcellular Distribution of Cadmium by Grass Shrimp

David R. Seebaugh (1), William G. Wallace (2)

(1) Department of Biology, Graduate School and University Center, City University of New  York, 365 Fifth Avenue, New York, NY 10016, 718-982-3876, dseebaugh@gc.cuny.edu

(2) Department of Biology, College of Staten Island, City University of New York, 2800 Victory  Boulevard, 6S-143, Staten Island, NY 10314, 718-982-3876, wallace@mail.csi.cuny.edu

Recent studies have shown that metal cycling through aquatic food chains may be more closely related to the subcellular partitioning of metal within prey than to whole tissue metal burdens. For example, a direct relationship between metal (Cd and Zn) stored within specific subcellular fractions (heat-stable proteins [HSP], heat-denatured proteins [HDP] and organelles) in soft-bodied estuarine invertebrates and metal assimilation by grass shrimp suggests that metal stored within these fractions may be considered collectively as a subcellular compartment containing trophically available metal (TAM). Beyond the subcellular partitioning of metal within prey, biological factors including metal-induced changes in predator digestive physiology (e.g., enzyme activities, gut residence time and gut pH) may impact digestion and influence the assimilation of dietary metal. The objective of this study was to investigate the influence of pre-exposure to dietary Cd and Hg on the subsequent assimilation and subcellular distribution of Cd by grass shrimp (Palaemonetes pugio) using radiotracer pulse-chase feeding experiments and subcellular fractionation techniques. Oligochaetes (Tubifex tubifex) were exposed for 96 to Cd (control, 0.22, 0.44 or 0.88 μM) or Hg (control, 0.014, 0.028, 0.055 μM) through solution with renewal of exposure solutions at 48 h. A separate sample of worms was exposed for 96 h to the radioisotope ¹⁰⁹CdCl₂ (2.22 x 10² kBq l^-1; 0.032 μM Cd) through solution. Subsamples of ¹⁰⁹Cd-labeled worms were subjected to subcellular fractionation to estimate the percentage of metal potentially available to predators (TAM-¹⁰⁹Cd%). Grass shrimp were collected from Great Kills Harbor, Staten Island, NY, USA, acclimated to laboratory conditions and pre-exposed to dietary metal by feeding on Cd- or Hg-exposed worms for 15 d (~5 worms shrimp^-1 day^-1). Following pre-exposure, grass shrimp were fed ¹⁰⁹Cd-labeled worms for ~30 min and analyzed periodically for ¹⁰⁹Cd activity for 1 week. A linear regression was fit to the physiological loss component of each ¹⁰⁹Cd retention curve (t > 24 h) and the corresponding y-intercept was used to estimate ¹⁰⁹Cd assimilation efficiency (AE-¹⁰⁹Cd%). Pre-exposed shrimp were also subjected to subcellular fractionation to estimate the percentage of ¹⁰⁹Cd distributed to each of five fractions (HSP, HDP, organelles, cellular debris and insoluble) and to the TAM compartment. TAM-¹⁰⁹Cd% in radiolabeled oligochaetes was estimated at ~83%. AE-¹⁰⁹Cd% by grass shrimp pre-exposed to control Cd worms was ~51%, but decreased to ~31% in shrimp pre-exposed to 0.22 μM Cd worms. AE-¹⁰⁹Cd% by shrimp fed 0.44 and 0.88 μM Cd worms were more similar to controls at ~43% and ~55%, respectively. The percentage of ¹⁰⁹Cd associated with the organelles, cellular debris and insoluble fractions was constant over the range of Cd pre-exposures, however, a dramatic shift from HDP to HSP was observed in shrimp pre-exposed to 0.22 μM Cd worms. TAM-¹⁰⁹Cd% in grass shrimp did not differ over the range of Cd pre-exposures (~70%). AE-¹⁰⁹Cd% by grass shrimp did not vary over the range of dietary Hg pre-exposures (~45%). Collection of subcellular fractionation data for Hg pre-exposed shrimp is in progress.