Lethal effects of five metals on the freshwater rotifers Asplanchna brigthwellii and Brachionus calyciflorus

Efectos letales de cinco metales en los rotíferos dulceacuícolas Asplanchna brigthwellii y Brachionus calyciflorus

Gustavo Emilio Santos-Medrano and Roberto Rico-Martínez

Centro de Ciencias Básicas, Departamento de Química, Universidad Autónoma de Aguascalientes, Avenida Universidad 940, Ciudad Universitaria Aguascalientes, Ags., 20131. México e-mail: gesantosmed@yahoo.com.mx

Recibido: 9 de septiembre del 2011.
Aceptado: 07 de enero del 2013.

ABSTRACT

Acute toxicity tests of five metals (aluminum, cadmium, iron, lead, zinc) were performed to determine LC50 values in two species of freshwater rotifers: Asplanchna brigthwellii and its prey Brachionus calyciflorus. We conducted the tests using neonates less than 24 hr-old, each test consisted of five replicates, negative control and five metal concentrations (Al, Cd, Fe, Pb, Zn). We found that the prey rotifer B. calyciflorus was more sensitive to Al, Cd, Pb and Fe than the predator rotifer A. brightwellii. For both rotifers Cd was the most toxic of the five metals. It was established that the strain of B. calyciflorus studied is sensitive when compared with other B. calyciflorus strains and other species and genera of the family Brachionidae. In the other hand, LC50 values of A. brigthwellii are compared with rotifer and copepod predators.

Key words: Acute toxicity, aquatic toxicology, LC50, metal toxicity, trophic interactions.

RESUMEN

Se realizaron pruebas de toxicidad aguda con cinco metales (aluminio, cadmio, hierro, plomo, zinc) para determinar los valores CL50 en dos especies de rotíferos dulceacuícolas: Asplanchna brigthwellii y su presa Brachionus calyciflorus. Para las pruebas se usaron neonatos menores de 24 horas, cada prueba consistió de cinco replicas, un control negativo y cinco concentraciones (Al, Cd, Fe, Pb, Zn). Se determinó que el rotífero B. calyciflorus fue más sensible al Al, Cd, Pb y Fe que su rotífero depredador A. brigthwellii. Para ambas especies el Cd fue el metal más tóxico de los cinco evaluados. Se comprobó que la cepa estudiada de B. calyciflorus es sensible al compararla con otras cepas de B. calyciflorus y otros géneros y especies de la familia Brachionidae. También se compararon los valores de CL50 del rotífero A. brigthwellii con otros rotíferos y copépodos depredadores.

INTRODUCTION

The assessment of metals effects in water is important due to their high toxicity and persistence, and rapid uptake by organisms. Metals are difficult to eliminate in the environment, since organisms incorporate them into their tissues and transferred to predators (Förstner & Prosi, 1979). Toxicity is proportional to the amount of metal absorbed by aquatic organisms. A metal dissolved in ionic form can be absorbed more easily than in its elemental form, while the reduced form increases the likely metal toxicity due to oxidation and retention in different organs. Usually metals are not removed from aquatic ecosystems by natural processes because they are not biodegradable (Förstner & Wittmann, 1981). Metals tend to form associations with minerals (carbonates, sulfates) as well as with organic substances, by phenomena of ion exchange, adsorption, chelation, formation of chemical combinations, etc., and therefore accumulate in the environment, mainly in the sediments of rivers and lakes (Förstner & Wittmann, 1981; Dekov et al., 1998). Metals are indicators of the ecological quality of water because of their high toxicity and bioaccumulative behavior (Purves, 1985). Rotifers are common in freshwater zooplankton communities. Species of the genus Brachionus are sensitive to different toxicants (metals and organic compounds), consequently they can be used as standard toxicity test organisms. On the other hand, Asplanchna is an important predator on smaller zooplankton; rotifers, ciliates and cladocerans (Wallace et al., 2006). Both genera are important organisms in freshwater food webs.

Toxicity tests using rotifers are becoming an interesting alternative to traditional methods using cladocerans. These tests measure various parameters such as mortality, reproduction, behavior, physiology, biochemistry or molecular biology, and microcosms (Snell & Janssen, 1995). As a starting point of a broader research it has been proposed to assess the toxicity in a battery of freshwater organisms of different trophic levels and to determine the 24-h LC50 (median lethal concentration) values of aluminum, cadmium, iron, lead and zinc in the freshwater predator rotifer Asplanchna brigthwellii (Gosse 1850) and its rotifer prey Brachionus calyciflorus, Pallas 1766 (Gilbert, 1967). Our hypothesis states than since B. calyciflorus is located in a lower trophic level it should be more sensitive to toxicant effects of metals than A. brigthwellii Gosse 1850 the predator of B. calyciflorus.

MATERIALS AND METHODS

Asplanchna brightwellii females carrying embryos were placed in a 24-well polystyrene plate (one female per well to avoid cannibalism), then incubated at 25 °C, photoperiod 16:8 (l:d) in absence of food. Neonates less than 24-hr-old were collected next day. Ten neonate females were placed in bottles of 20 ml total volume; with a test volume of 5 ml. On the other hand, Brachionus calyciflorus neonates less than 24-hr-old hatching from cysts were placed in a 24-well polystyrene plate (ten neonates per well) with a test volume of 1 ml. For both species, each test consisted of five replicates, negative control EPA medium pH 7.5 (USEPA, 1985), and five metal concentrations (atomic absorption standards of aluminum, cadmium, iron, lead, zinc from Sigma-Aldrich Co). All metals were in 2% nitric acid solution). In order to derive LC50 for all metals, we performed a range-finding test. For the definitive test, five toxicant concentrations are chosen covering the 0 and 100% mortality concentration range (Table 1) determined in the range-finding test. The animals were Incubated for 24 hours at 25 °C, photoperiod 16:8 (l:d) in absence of food. At the end of incubation, dead animals were counted using a stereomicroscope. A one-way analysis or variance (ANOVA) and Duncan's test were calculated to compare mortality percentages for each toxicant concentration to that of the control. From these data the NOEC (No observed effect concentration) and LOEC (Lowest observed effect concentration) values were calculated. The LC50 values were calculated using regression between probit units and the logarithm of each toxicant concentration using the software Statistica 6.0 (StatSoft Inc., 2001).

RESULTS

We found that B. calyciflorus was more sensitive to all metals, except Zn, than A. brightwellii when the LC50 values were compared (compare Table 2 and Table 3). Regarding NOEC values B. calyciflorus was more sensitive to all metals than A. brightwellii. When LOEC values were compared B. calyciflorus was more sensitive to Al, Cd and Zn than A. brightwellii. There were the same LOEC values for Fe, and A. brightwellii was more sensitive than B. calyciflorus for Pb (Compare Table 2 and Table 3).

DISCUSSION

We found enough data on the literature to compare the sensitivity (24-h LC50) of B. calyciflorus with other species of the family Brachionidae (Table 4). For Al B. calyciflorus (0.105 mgl^-1, this work) was more sensitive than the strain used by Snell et al. (1991b) (>3.0 mgl^-1 ), the same effect was observed with Pb (B. calyciflorus 0.77 mg l^-1, this work), (Snell et al., 1991a), B. plicatilis Müller, 1786 (4.0 mgl^-1 Snell et al., 1991b) and B. patulus Müller, 1776 (6.15 mgl^-1 García-García et al., 2007) (Table 4).

Cadmium was the metal that showed greater variation in the LC50 values in the family Brachionidae with a range of 0.09 mgl^-1 (B. patulus, Sarma et al., 2006) -39 mgl^-1 (B. plicatilis, Snell et al., 1991a). In addition LC50 values of Zn (B. calicyflorus 0.324 mgl^-1, this work) was more sensitive than others strains of B. calyciflorus, 1.3 mgl^-1 (Snell et al. 1991b), 1.32 mgl^-1 (Couillard et al., 1989), and 1.67 mgl^-1 (Nelson & Rolin, 1998) (Table 4).

Anuraeopsis fissa Gosse, 1851 was the most sensitive to Zn (0.31 mgl^-1) its CL50 value being similar to that of B. calyciflorus (0.324 mgl^-1) used in this work (Table 4).

We found enough data on the literature to compare the sensitivity (24-h LC50 and EC50) of A. brightwellii with other copepod or rotifer predator species preying on rotifers (Table 5). A. brightwellii (0.174 mgl^-1, this work) was more sensitive to Al than Acanthocyclops vernalis Fischer, 1853 (Havens, 1991; Al LC50 value = 0.54 mgl^-1) and Mesocyclops edax Forbes 1891 (Havens, 1991) with a very close value (0.58 mgl^-1) (Table 5). The toxic effects of Cd were LC50 value for A. brightwellii (0.146 mgl^-1) and Megacyclops viridis Jurine, 1820 (LC50 0.0005 mgl^-1, Braginskij & Shcherban, 1979) have a difference of 292-fold lower than A. brightwellii ( this work, see table 5) otherwise, it showed a value very close to Eurytemora affinis Poppe, 1880(LC50 0.13 mgl^-1) . The EC50 value (0.05 mgl^-1) of the copepod T. prasinus mexicanus Kiefer, 1938 (Lalande & Pinel-Alloul, 1986) was 4.44-fold lower than the LC50 (0.222 mgl^-1) value for A. brigthwellii for Zn (Table 5).

The high toxicity of Al is influenced by the aqueous chemistry, and is extremely complex. The changes in the molecular form or concentration of Al are largely dependent on its pH. Other factors include complexes with ligands (Wauer et al., 2004). While Zn is an essential trace element for living organisms, it was more toxic than Pb to A. brightwellii.

When we compared rotifer predators, A. brigthwellii was more sensitive than A. vernalis (copepod) to Al. Our hypothesis was partially fulfilled; B. calyciflorus was more sensitive to four metals (Al, Cd, Fe, Pb) than A. brigthwellii.

ACKNOWLEDGEMENTS

We thank to Dr. F. J. Flores-Tena for his comments and recommendations. Thanks to the National Council for Science and Technology for a grant (CONACYT Ref. 209263).

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