Individual responses of trace-element assimilation and physiological turnover by the marine copepod Calanus sinicus to changes in food quantity

Radiotracer techniques were used to quantify the assimilation and subsequent efflux of silver, cadmium, iron, mercury, thallium, and zinc by mesozooplankton fed ciliates, heterotrophic dinoflagellates, or heterotrophic flagellates, and the results were compared with published values measured for phytoplankton prey. The subcellular distribution of the metals within the prey cells was also determined and related to their bioavailability. Marine copepods assimilated 59-82% of Ag, 83-92% of Cd, 32-66% of Fe, 14-49% of Hg, and 71-78% of Zn ingested with protozoan prey. Higher Ag, Cd, Fe, and Hg assimilation efficiencies were observed for at least one of the species of protozoa than reported in the literature for phytoplankton. Significant differences in assimilation were not observed for Zn or for Tl fed to freshwater Ceriodaphnia in either protozoa or phytoplankton. The higher assimilation efficiencies of protozoan metals, when observed, were matched by higher fractions of metals in the cytoplasm of protozoa. A biokinetic model used to calculate steady-state metal concentrations in copepods indicates that copepods may contain 119% more Ag and 44% more Zn when feeding on ciliates instead of diatoms, possibly resulting in sublethal toxic effects at Ag and Zn concentrations reported for the Hudson River estuary. Further, higher assimilation and efflux rates of protozoan Fe may enhance remineralization of this limiting nutrient by mesozooplankton in high nutrient, low chlorophyll regions.Aquatic animals can accumulate metals both directly from their aqueous environment and from the prey they ingest, but metals accumulated through these two routes can have different physiological effects and geochemical fates. Food is often the dominant uptake pathway for metals in crustacean zooplankton (Munger and Hare 1997;Wang and Fisher 1998), and zooplankton can be more sensitive to metals accumulated through this pathway (Hook and Fisher 2001a,b). In addition to having greater physiological effects, metals that are ingested and assimilated by zooplankton can build up in food chains or be biologically recycled; these metals generally display longer residence times in surface waters than unassimilated metals, which get packaged into fecal pellets and sink out of surface waters (Fowler and Knauer 1986;Fisher et al. 1991). Metals bound to the exoskeleton may also be exported with molts (Fowler 1977).Most studies that have examined the assimilation of metals by zooplankton grazers have focused on phytoplankton prey (e.g., Sick and Baptist 1979;Fisher et al. 1991;Wang et al. 1996), but heterotrophic protists can also serve as an important food source for mesozooplankton. As the dominant grazers of heterotrophic and autotrophic picoplankton in many aquatic systems (Sherr and Sherr 1994), protozoa are an important trophic link between the microbial loop and the metazoan food web (Stoecker and Capuzzo 1990). Mi-

Section: Discussionmentioning

confidence: 96%

Section: Methodsmentioning

confidence: 99%

Trophic transfer of trace metals from protozoa to mesozooplankton

Twining

Fisher

2004

“…Discussion-Our study suggests that metal AEs in the copepod Acartia spinicauda fed on different types of detritus are comparable to those fed on phytoplankton. Xu et al (2001), working with the same copepod species (Acartia spinicauda), found that the metal AEs were 43 to 67% for Cd and 35 to 38% for Se at different metal concentrations in the algal cells (Prorocentrum minimum). The AEs of Zn (4 to 9%), however, were much lower than our present measurements from biogenic particles.…”

Section: Results-mentioning

confidence: 99%

The assimilation of detritus‐bound metals by the marine copepod Acartia spinicauda

Wang

2002

Abstract-The bioavailability of Cd, Se, and Zn associated with detritus of different biological origins to the marine copepod Acartia spinicauda was examined under laboratory conditions using the radiotracer approach. Three types of detritus, from the macroalga Ulva lactuca, cell debris of the diatoms Thalassiosira weissflogii and Thalassiosira rotula derived after natural decomposition or extraction, and copepod fecal pellets, were examined. The assimilation of metals from the ingested natural phytoplankton assemblage was also experimentally quantified. The assimilation efficiencies (AEs) of metals in copepods fed with different types of detritus were 47-83% for Cd, 30-59% for Se, and 41-75% for Zn. These values were comparable to the AEs measured previously for copepods fed on living algal cells, indicating that metals associated with biogenic particles were directly available to marine copepods. Labeling time had no effect on the metal AEs in copepods fed on detritus derived from U. lactuca. The metal AEs were higher in copepods fed on the freshly prepared diatom debris than on naturally decomposing debris. Zn associated with fecal pellets was assimilated at comparable efficiencies to those associated with macroalgal and microalgal debris, but Cd and Se were assimilated at a lower efficiency when associated with fecal pellets. The AEs of Cd (84-94%) in copepods fed on the natural phytoplankton assemblage were the highest among all the experiments. There was no obvious effect of the metal distribution in the cytosol of the natural phytoplankton assemblage on the metal AE. The calculated elimination rates of the metals were independent of detritus type and were similar to previous studies. Our study indicated that detritus is a potential food source for marine copepods and the metals associated with the detritus can be efficiently used with a relatively high AE. Detritus may thus play an important role in the overall biogeochemical cycling of metals in the ocean.

“…The shorter residence times of diatoms in the gut of T. longicornis might have caused physiological constraints on digestion or assimilation. Recent studies on the fate of phytoplankton minerals, trace metals, or lipids have indicated that the assimilation efficiency of some trace elements and sterols may decrease with a shorter gut residence time, while other compounds, such as PUFAs, are not affected (Harvey et al 1987;Cowie and Hedges 1996;Xu and Wang 2001). Therefore, it is conceivable that the saturating concentrations at which diatoms are regularly fed to copepods in studies of egg production and egg viability induce high feeding rate and feces production, and short gutpassage time, which thus create imbalances in lipids or unknown compounds important for the reproductive success of copepods.…”

Section: Discussionmentioning

confidence: 99%

Copepod reproduction is unaffected by diatom aldehydes or lipid composition

Dutz

Koski

ttir

2008

117

We investigated whether reduced reproductive success of copepods fed with diatoms was related to nutritional imbalances with regard to essential lipids or to the production of inhibitory aldehydes. In 10-d laboratory experiments, feeding, egg production, egg hatching success, and fecal pellet production of Temora longicornis were measured for six different diatom species as well as for a nondiatom control diet (Rhodomonas sp.). The experiments were accompanied by determinations of fatty acids, sterols, and polyunsaturated aldehydes (PUA) in the food. Although diatoms were generally ingested at high rates, they yielded a variable egg production response in copepods, ranging from high egg production in four species (two strains of Thalassiosira rotula, Chaetoceros affinis, and Thalassiosira weissflogii) to low egg production in two species (Leptocylindricus danicus and Skeletonema costatum). Egg hatching rates decreased after 4 d in all diatom treatments, irrespective of the egg production rate and without any relationship to diatom aldehyde production. Similarly, no evidence was found that diatoms are per se nutritionally inferior to nondiatom food. The lack of a distinct mechanism for the observed inhibitory activity of diatoms suggests that the cause(s) might be more complex. We suggest, as one possible explanation, that hatching-specific nutritional deficiencies might be induced by incomplete digestion following from the low gut passage time of diatoms, as indicated by a strong correlation between egg viability and fecal pellet production.