Selenium contamination in aquatic ecosystems provides management challenges because bioaccumulation in animals is largely a function of dietary exposure, whereas regulatory entities have traditionally focused on direct water to organism interactions. Selenium is known to be readily absorbed by primary producers and can potentially biomagnify in food webs and elicit adverse effects in higher trophic levels. However, selenium bioaccumulation in the invertebrate prey of many predatory animals is poorly understood. Here, we used 75Se (as selenite) as a radiotracer to characterize Se bioaccumulation into natural periphyton biofilms and subsequent dietary and maternal transfer in the mayfly, Centroptilum triangulifer, in a life-cycle assay. On average periphyton biofilms bioconcentrated selenium 1113 (+/-430)-fold following 7-9 days of exposure to a range of environmentally relevant dissolved concentrations (2.4-13.9 microg L(-1)). Mayflies grown to adulthood on these diets further biomagnified Se with trophic transfer factors averaging 22 (+/-0.4)-fold in postpartum maternal tissues. Adults then transferred 46.5 (+/-8.8)% of their body burdens to eggs with an observed reduction in fecundity for mayflies fed on diets greater than approximately 11 microg g(-1). These results suggest that at environmentally feasible dietary Se concentrations insects are potentially affected by Se exposure, and that the current presumption that insects are simply conduits of Se to higher trophic levels is inaccurate.
DNA barcoding of aquatic macroinvertebrates holds much promise as a tool for taxonomic research and for providing the reliable identifications needed for water quality assessment programs. A prerequisite for identification using barcodes is a reliable reference library. We gathered 4165 sequences from the barcode region of the mitochondrial cytochrome c oxidase subunit I gene representing 264 nominal and 90 provisional species of mayflies (Insecta: Ephemeroptera) from Canada, Mexico, and the United States. No species shared barcode sequences and all can be identified with barcodes with the possible exception of some Caenis. Minimum interspecific distances ranged from 0.3–24.7% (mean: 12.5%), while the average intraspecific divergence was 1.97%. The latter value was inflated by the presence of very high divergences in some taxa. In fact, nearly 20% of the species included two or three haplotype clusters showing greater than 5.0% sequence divergence and some values are as high as 26.7%. Many of the species with high divergences are polyphyletic and likely represent species complexes. Indeed, many of these polyphyletic species have numerous synonyms and individuals in some barcode clusters show morphological attributes characteristic of the synonymized species. In light of our findings, it is imperative that type or topotype specimens be sequenced to correctly associate barcode clusters with morphological species concepts and to determine the status of currently synonymized species.
Selenium effects in nature are mediated by the relatively large bioconcentration of aqueous Se by primary producers and smaller, yet critical, dietary transfers to primary consumers. These basal processes are then propagated through food webs to higher trophic levels. Here we quantified the movement of dissolved Se (as selenite) to periphyton, and used the resultant periphyton as a food source for conducting full life-cycle dietary Se exposures to the mayfly Centroptilum triangulifer. Periphyton bioconcentrated Se ~2,200-fold from solution in a log-linear fashion over dissolved Se concentrations ranging from 1.1 to 23.1 μg L(-1). We examined the influence of two feeding ration levels (1x and 2x) on trophic transfer, tissue Se concentrations, maternal transfer, and functional endpoints of mayfly performance. Mayflies fed a lesser ration (1x) displayed greater trophic transfer factors (mean TTF, 2.8 ± 0.4) than mayflies fed 2x rations (mean TTF, 1.1 ± 0.3). In 1x exposures, mayflies exhibited significant (p < 0.05) reductions in survivorship and total body mass at dietary [Se] ≥ 11.9 μg g(-1), reduced total fecundity at ≥ 4.2 μg g(-1), and delayed development at ≥ 27.2 μg g(-1). Mayflies fed a greater ration (2x) displayed reduced tissue Se concentrations (apparently via growth dilution) relative to 1x mayflies, with no significant effects on performance. These results suggest that the influence of Se on mayfly performance in nature may be tied to food resource availability and quality. Furthermore, nutritional status is an important consideration when applying laboratory derived estimates of toxicity to risk assessments for wild populations.
Little is known about the bioaccumulation dynamics, biotransformation processes, or subsequent toxicity to consumers of dissolved selenite (SeO3) versus selenate (SeO4) uptake into aquatic primary producer communities. To address these data gaps, we examined SeO3 and SeO4 bioconcentration into complex freshwater periphyton communities under static and static-renewal conditions. Further, we explored periphyton biotransformation of Se species using X-ray absorption near edge structure (XANES) spectroscopy analysis and changes in the periphyton associated microbial consortium using denaturing gradient gel electrophoresis (DGGE). Last, we fed differentially treated periphyton to the mayfly Centroptilum triangulifer in full life cycle exposures to assess toxicity. Selenite exposed periphyton readily bioconcentrated Se while, in contrast, initial periphyton uptake of SeO4 was negligible, but over time periphyton [Se] increased steadily in conjunction with the formation of dissolved SeO3. XANES analyses revealed that both SeO3 and SeO4 treated periphyton biotransformed Se similarly with speciation dominated by organo-selenide (∼61%). Mayfly survival, secondary production, and time to emergence were similar in both SeO3 and SeO4 treated periphyton exposures with significant adverse effects at 12.8 μg g(-1) ((d.w.) secondary production) and 36 μg g(-1) ((d.w.) survival and development time). Overall, dissolved selenium speciation, residence time, and organisms at the base of aquatic food webs appear to be the principal determinants of Se bioaccumulation and toxicity.
Diet is often the predominant route of trace metal exposure in aquatic insects. In freshwater ecosystems, periphyton serves as a primary source of food to many aquatic insects and is a major sink for trace metals. We investigated the bioconcentration of the essential metal Zn by periphyton using (65)Zn as a radiotracer. At relatively low dissolved concentrations (2-20 μg L(-1)), non steady state Zn bioconcentration by periphyton averaged 6,099 ± 2,430-fold, with much of the variability determined by loading regime (number of renewals and duration of exposures). Labeled periphyton was used as a food source for dietary accumulation studies with the mayfly Centroptilum triangulifer. After 29 days, larvae concentrated Zn 19-, 16- and 17-fold relative to dietary Zn concentrations of 8.1, 43.2 and 82.3 μg g(-1) (dry weight), respectively. Adults from that same cohort only concentrated Zn 8-, 3- and 3- fold relative to those same dietary concentrations, revealing that mayflies lose significant Zn prior to reaching adulthood. Anecdotal evidence suggests that this loss occurs prior to emergence to the subimago, as negligible Zn was found in the subimago to imago exuvium. Across a range of adult tissue concentrations, maternal transfer consistently averaged 26.7 %. Uptake (k(u), 0.26 L g(-1 )d(-1)) and efflux rate constants (k(e), 0.001-0.007 d(-1)) were measured and assimilation efficiencies from dietary Zn concentrations of 4.9 and 59.7 μg Zn g(-1) were estimated to be 88 ± 4 % and 64 ± 15 %, respectively. Both life cycle and biodynamic modeling approaches point towards diet being the primary route of Zn bioaccumulation in this mayfly.
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