VII. Animal Feeding and Metal Uptake and Bioconcentration

 

The benthic oligochaete, Limnodrilus hoffmeisteri, inhabiting cadmium-, nickel-, and cobalt-polluted Foundry Cove has evolved resistance to these metals (Klerks & Levinton 1989a,b). In survival experiments in which oligochaetes from Foundry Cove and South Cove (control site) are exposed to sediment with highly elevated metal levels, Foundry Cove worms survived the 28 day exposure while control worms did not (see Chapter VI). Second generation offspring of Foundry Cove worms reared in clean sediment also possess metal resistance.

Cluster of Limnodrilus hoffmeisteri

An increased resistance to a metal can be achieved by a reduced accumulation of the pollutant. Reduced uptake rates have been reported for a number of different organisms, e.g., bacteria, algae, annelids, and fish (references cited in Klerks & Bartholomew 1991). But several studies comparing metal accumulation in populations differing in resistance did not find reduced uptake rates in resistant populations; some authors even found increased uptake rates. If resistant individuals have increased metal uptake rates, then they must possess some physiological mechanism for metal detoxification. The ultimate research goal is to determine the mechanism(s) by which resistance has evolved in Limnodrilus hoffmeisteri inhabiting Foundry Cove.

The first objective towards this ultimate goal is to determine whether resistant worms accumulate less cadmium than their sensitive conspecifics from South Cove. Secondly, if metal resistant and sensitive individuals have similar uptake rates, is resistance achieved by an increased cadmium detoxification mechanism? This latter question will be addressed in Chapter VIII. The following experiments determine metal uptake rates in resistant and control populations using the radioisotope 109Cd in water and Foundry Cove sediments.

The Oligochaete, Limnodrilus hoffmeisteri

Limnodrilus hoffmeisteri (Annelida, Oligochaeta, Tubificidae) is a simultaneous hermaphrodite, which reproduces sexually by cross-fertilization. This oligochaete is a deposit-feeder and the most common macrobenthic species at both coves (Klerks & Levinton 1989b). Worms and sediment were collected by Ekman grab from both Foundry and South coves. Worms were then sorted from the >500 µm fraction. Laboratory cultures were set up in polystyrene dishes with 1 cm layer of sediment and 9 cm of continuously aerated Hudson River water. Sediments collected from the two coves were sieved to < 500 µm, boiled, washed with filtered Hudson River water, then frozen and thawed shortly before use. Ground fish food flakes were added to dishes once per week and cultures were kept at 24oC under a 13:11 light:dark cycle.

It is likely that resistance in Limnodrilus in Foundry Cove evolved mainly in response to Cd pollution at this site, rather than Ni or Co pollution. This is based on several observations: these worms accumulate much more Cd than Ni, but they do not accumulate Co at all, and Cd is generally much more toxic than Ni (Khangarot & Ray 1987). This investigation into the mechanism(s) underlying the resistance in Limnodrilus in Foundry Cove will thus focus on the fate of Cd in these worms.

Cadmium Uptake from Sediments

Cadmium accumulation in L. hoffmeisteri from Foundry Cove and South Cove was determined by exposing worms to sediment with different metal levels. These exposure experiments were set up as a bioassay for comparison of sediment toxicity among populations (Klerks & Bartholomew 1991). Three replicates of 10 worms each from stock cultures were exposed to 6 different sediment metal levels (ranging from 15 to 34,000 µg Cd per g dry sediment) for 28 days. Worms which survived the exposure and laboratory stock worms were collected for metal analyses (6-10 worms per replicate). These worms were kept in filtered river water for 2 days to exclude gut contents from analyses. The worms were then rinsed in distilled water, pooled by replicate, and frozen. Worms from each replicate were then thawed, transferred to a beaker and dried. Ultrex grade nitric acid (2 ml) was then added to each beaker, refluxed for 2-4 h at 120oC, and evaporated. This procedure was repeated twice, after which each sample was brought to 5ml volume with addition of nitric acid. Cadmium concentrations were determined with a Perkin Elmer 4000 graphite furnace atomic absorption spectrophotometer. Blanks, tissue samples, and standards were run concurrently; the use of the National Bureau of Standards Oyster Tissue resulted in values within the range specified for this reference material.

Cadmium Uptake from Solution

To determine Cd accumulation from water, resistant and sensitive worms were exposed in plastic petri dishes to 8.9 µM (= 1mg/l) Cd in reconstituted fresh water (pH 7.8-8.0) for 6 days (Klerks & Bartholomew 1991). The addition of 109Cd resulted in a radioactivity of 22.2 kBq/ml in the exposure water. Three replicates of 10 worms each from Foundry Cove and South Cove stock cultures were exposed and survivors collected and frozen at -80oC. Thawed samples were later homogenized in 50 mM Tris-HCl buffer (pH 7.4) using a 50:1 ratio of buffer to tissue. Cadmium concentrations of each homogenate were determined by gamma counting, using a Beckman 4000 gamma counter with a 3-inch sodium iodide crystal.

Uptake of Cd by South Cove worms versus Foundry Cove worms. Note that Foundry Cove worms took up and accumulated more Cd. Thus the mechanism of resistance of Foundry Cove worms does not involve avoidance of uptake but, instead, some form of sequestration.

Results

Cadmium concentrations of Limnodrilus hoffmeisteri from the sediment toxicity bioassays did not differ among groups exposed to sediment with Cd levels raging from 5,400 to 34,000 µg Cd per g dry sediment (p > 0.05, ANOVA) for worms from Foundry as well as South Cove (Figure 1 & 2). These data were thus pooled to compare Cd accumulation of Foundry Cove worms to that of their conspecifics from the control area (South Cove). The data do not provide evidence for a reduced Cd accumulation in Foundry Cove worms; worms from Foundry Cove accumulated significantly more Cd in both sediment and water bioassays (almost twice as much as South Cove worms in the bioassay in water).

The data show that Cd-resistance in Limnodrilus from Foundry Cove is not due to a reduced accumulation of the metal. Worms from Foundry Cove actually accumulate more Cd than their metal-sensitive conspecifics from South Cove. Since Foundry Cove worms accumulate more Cd than their sensitive conspecifics, other mechanisms must be responsible for the Cd resistance. These mechanisms will be taken up in Chapter VIII.

Other Animals Are Affected by Cadmium

Muskrats were conspicuously rare in Foundry Cove. Cadmium was measured in kidneys and here are the results:

It was thought that muskrats might have trouble with physiological function and lesions on the liver were counted in Foundry Cove muskrats, as compared to other marshes:

Clearly, Foundry Cove muskrats were having problems.