Daphnia need to be gut-cleared too: the effect of exposure to and ingestion of metal-contaminated sediment on the gut-clearance patterns of D. magna.
The presence of sediment particles in the gut indicated that Daphnia magna used in whole-sediment bioassays ingest sediment. If gut contents are not removed prior to whole-body tissue-burden analysis, then the bioavailability of any sediment-associated contaminants (e.g. metals) can be overestimated. Gut clearing patterns were determined for D. magna after exposure to both clean and metal-contaminated (Cu and Zn) field-collected sediments. D. magna exposed to reference sediment had fuller guts than those exposed to metal-contaminated sediment (95% versus 60% full). Neither reference- nor metal-exposed D. magna could clear their gut completely of sediment particles when held in clean water for 24 h. When Daphnia were transferred to clean water after exposure to metal-contaminated sediment, there was no significant decrease in gut-fullness (P>0.05) even after 48 h of purging. By comparison, animals transferred to water containing 5 x 10(5) cells of algae (Pseudokircheriella subcapita) after exposure to contaminated sediment showed a significant drop in gut fullness from 56% immediately after exposure to 17% after 4 h of gut-clearance. Although gut fullness did not change significantly beyond 2 h of purging, data were much less variable after 8 h of gut-clearance than after 2 h or 4 h. The depuration of Cu was well described with a two-compartment first-order kinetic model (r2=0.78, P<0.0001) indicating that D. magna exposed to metal-contaminated sediment have one pool of Cu that is quickly depurated (0.2 h(-1)), and one that has been incorporated into the tissues (<<0.00001 h(-1)). Assuming tissue background of 48 microg/g, an exposed animal which has not been depurated or which has been purged with water alone would yield whole-body tissue Cu concentrations that are 5.6- and 4-fold higher, respectively, than that purged with algae + water (8 h). We recommend that D. magna used to estimate metal bioavailability from sediment be gut-cleared in the presence of algae for 8 h prior to determination of whole-body metal concentrations.
Gillis
P L
PL
Department of Biology, McMaster University, 1280 Main Street West, Hamilton, Ont., Canada L8S-4K1. gillisp@mcmaster.ca
Chow-Fraser
P
P
Ranville
J F
JF
Ross
P E
PE
Wood
C M
CM
D000818:Animals; D003120:Colorado; D003621:Daphnia / Q000502:physiology*; D004068:Digestive System Physiological Phenomena / Q000187:drug effects*; D004784:Environmental Monitoring / Q000379:methods*; D005618:Fresh Water; D005766:Gastrointestinal Contents / Q000187:drug effects*; D019015:Geologic Sediments / Q000032:analysis*; D007700:Kinetics; D019216:Metals, Heavy / Q000493:pharmacokinetics / Q000633:toxicity*; D008954:Models, Biological; D013997:Time Factors
D019216:Metals, Heavy