Doklady Biological Sciences, Vol. 379, 2001, pp. 378–381. Translated from Doklady Akademii Nauk, Vol. 379, No. 3, 2001, pp. 426–429. Original Russian Text Copyright © 2001 by Ostroumov, Kolesnikov.

GENERAL BIOLOGY

Pellets of Some Mollusks in the Biogeochemical Flows of C, N, P, Si, and Al S. A. Ostroumov and M. P. Kolesnikov Presented by Academician M.E. Vinogradov February 1, 2001 Received February 1, 2001

Aquatic invertebrates excrete pellets [1,2], which consist of products of incomplete digestion and pseudofeces. Different taxons are characterized by the following values of food assimilability (in percents): Rotatoria, 48–80; Bryozoa, 41.6; Gastropoda, 42–82; Bivalvia, 40–47; Cladocera, 50.5–85.5; Copepoda, 30– 88; Mysidacea, 84.2–95; Isopoda, 68; Amphipoda, 5.5– 98; Decapoda, 38.7–96.1; larvae of Odonata, 20–97.2; Ephemeroptera, 41–72; Plecoptera, 9–73; Trichoptera, 5–51; and Diptera, 1–31.4 [2]. When settling to the bottom of water bodies by gravity, pellets contribute to vertical fluxes of chemical elements through the ecosystem level of the biospheric biogeochemical cycles or into “biogenic migration of atoms in the biosphere” [3]. The purpose of this study was to estimate the capacity of mollusk pellets for contributing into the vertical transfer of chemical elements through an aquatic ecosystem, using Limnaea stagnalis (L.) and bivalves (Unionidae) as examples. The mollusks L. stagnalis were collected in June in a pond in the floodplain of the upper Moskva River and were kept afterwards as described in [4]. Bivalve mollusks (Unionidae) collected from the partly silted sand bottom of the Moskva River upstream of the town of Zelinograd represented a sample from a natural benthic community dominated by Unio tumidus and U. pictorum (63.21 and 27.36%, respectively, of the overall number of specimens in the sample). The proportions of Crassiana crassa and Anodonta cygnea were lower (7.55 and 1.89%, respectively). The bivalves sampled from the natural ecosystem (the total biomass was 3302 g wet weight, including the shells; the average weight of one mollusk was 21.9 g), where they filtered natural seston, were incubated for 24 h in a wide flask containing settled tap water to obtain the pellet precipitate, which was afterwards resuspended in a 300-ml glass cylinder. The sand fraction, which precipitated

Moscow State University, Vorob’evy gory, Moscow, 119899 Russia Bakh Institute of Biochemistry, Russian Academy of Sciences, Leninskii pr. 33, Moscow, 117071 Russia

within 15 s (fraction 1), was separated from the remaining suspension of the pellet material per se. The latter was transferred into another glass cylinder. From this suspension, pellets precipitated within 3 h (fraction 2); this precipitate was separated by means of decanting the supernatant. The dry weight of fraction 2 was 1434 mg. To determine the carbon content in the vegetative material and in pellets, they were oxidized by 10% K2Cr2O7 in the presence of a mixture of concentrated H2SO4 and H3PO4 [5]; the CO2 formed was trapped by 0.5 M NaOH, and the remaining alkaline was titered with hydrochloric acid. The photometric method was also used; the optic density of the solution was measured at a wavelength of 590 nm after oxidation of the material with bichromate. The amount of organic nitrogen was measured using a Kjeltec Auto 1030 Analyzer (Tecator, Sweden) by the Kjeldahl method after mineralization with a mixture of H2SO4 and H2O2. Phosphorus was assayed using phosphoromolybdenum blue (PMB) in a working solution containing ions of orthophosphate and orthosilicic acid in 15.0 ml of 1 N H2SO4 [6]. The sum of organogenic and soluble (mineral) silicon was determined using the modified silicomolybdenum blue method without preliminary ashing of the vegetative material [6]. Aluminum was determined in an aliquot of HNO3 hydrolyzate (1.5–2.5 ml) neutralized with ammonium and dissolved in water to a final volume of 25.0 ml; afterwards, 2.0 ml of 1% ascorbic acid were added and pH 2.5 was obtained. Then, 0.04% aquatic solution of eriochromcyanogen R (pH 2.5) and 5.0 ml of CH3COONH4 (50% solution, pH 7.0) were added. After addition of water to obtain a final volume of 50.0 ml, the optical density was measured at 535 nm. Chemical analysis is described in detail elsewhere [4, 6]. The element composition of L. stagnalis pellets after the mollusks were fed on the leaves of Nuphar lutea Smith and Taraxacum officinale Wigg is shown in Table 1. The compositions of L. stagnalis pellets were similar in both cases. The contents of N and P were slightly higher in the pellets of L. stagnalis than in the phytomass of N. lutea, whereas the content of Si was noticeably higher than that in the phytomass of both plant species that served as a food for the mollusks. Pellets of the bivalves were similar in composition to those

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Table 1. Element composition of pellets (% of dry weight) formed by the mollusk L. stagnalis feeding on the leaves of N. lutea and T. officinale N. lutea Element C N P Si Al

T. officinale

leaves

pellets

leaves

– 2.11–2.19 0.34–0.39 0.81–0.85 0.043–0.047

69.4 2.83 0.5 1.73 0.054–0.059

– 2.57 0.44 1.15 0.076

4.17

5.97

5.43

The sum of ash elements (ash content)

pellets 67.5 2.89 0.48 1.87 0.094 –

Note: The element composition of leaves (in percents of the leaf dry weight) is shown for comparison.

Table 2. Formation of pellets by the mollusk L. stagnalis feeding on the leaves of various plant species Parameter

N. lutea

Wet weight of the biomass consumed, g

15.89

Dry weight of the pellets formed, mg

T. officinale 9.66

2434

Dry weight of the pellets formed per 1 g of wet weight of the leaf mass

153.2

143.6

30

55

309

48

Number of mollusks Incubation time, h

1387

Note: The incubation conditions when N. lutea was used as food: 309 h, 22–24°C, eight flasks; the incubation conditions when T. officinale was used as food: 48 h, 22–23°C, nine flasks.

Table 3. Transfer of matter and chemical elements (mg) with pellets of bivalves (a sample from a natural community of Unionidae) per unit biomass of mollusks and per unit area of the ecosystem from which the mollusks were collected mg/1 kg of mollusk biomass (wet weight including shells)

Pellets and chemical elements (% of dry weight)

mg/1 m2 of the ecosystem

per day

120 days

per day

120 days

Pellets (total dry weight, mg)

434.28

52114

717.00

86040

C

279.24

3509

461.03

55324

N

11.86

1423

19.57

2349

P

1.69

203

2.80

336

Si

4.95

594

8.17

981

Al

0.31

37

0.51

61

30.88

3705

50.98

6117

The sum of ash elements (ash content), 7.11

Note: Pellets (fraction 2) were obtained from mollusks (total biomass, 3302 g of wet weight with shells) collected from the area of 2 m 2 in August in the Moskva River. The element composition of pellets (% of dry weight) was as follows: C, 64.3; N, 2.73; P, 0.39; Si, 1.14; Al, 0.071.

of L. stagnalis. Note that pellets of bivalves were characterized by a somewhat higher ash content and lower silicon content. Based on data on L. stagnalis nutrition in a microcosm, it was calculated that 143.6 to 153.2 mg of pellets DOKLADY BIOLOGICAL SCIENCES

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2001

(dry weight) were formed per 1 g of consumed plant biomass, and carbon accounts for as much as 67.5 to 69.7% of this amount (from 96.9 to 106.8 mg) (Table 2). It is also interesting that pellet formation occurred at a similar rate when the mollusks were fed on the phytomass of different plant species.

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OSTROUMOV, KOLESNIKOV

Table 4. Chemical elements (mg) with pellets of the mollusk L. stagnalis per unit biomass of mollusks and per unit area of the ecosystem from which the mollusks were collected

Pellets and chemical elements, % of dry weight

mg/1 kg of mollusk biomass (wet weight with shells)

mg/1 m2 of the ecosystem

per day

120 days

per day

120 days

Pellets (total dry weight, mg)

3372.3

404676

178.42

21410

C

2351.8

282221

124.43

14931

N

95.44

11452

5.05

606

P

6.86

2023

0.89

107

Si

58.34

7001

3.09

370

Al

2.02

243

0.11

13

201.33

24160

10.65

1278

The sum of ash elements (ash content), 5.97

Note: Pellets from mollusks (total biomass, 158.72 g of wet weight with shells) were collected from an area of 3 m 2 in August in a pond in the floodplain of the upper Moskva River. The element composition of pellets (% of dry weight) was as follows: C, 69.74; N, 2.83; P, 0.5; Si, 1.73; Al, 0.06.

The data shown in Tables 3 and 4 can be used to calculate both specific rates of element transfer (per 1 g of mollusk biomass) and the magnitude of the transfer (per unit area of the ecosystem). In L. stagnalis, the specific rate of matter transfer (dry weight of pellets per 1 g of mollusk biomass) (Table 4) was significantly higher than in bivalve mollusks (Table 3). However, the magnitudes of transfer (as calculated per unit area of the ecosystem) differed less, because the biomass density of bivalves (per 1 m2) was significantly higher than that of L. stagnalis. However, this is only a conservative estimate, because, in natural ecosystems, the mollusk biomass density considerably varies, and, hence, the matter transfer also varies. Therefore, we regard these estimates with caution and do not extrapolate them to other ecosystems or other areas of the same ecosystem that differ in the mollusk population density. In previous experiments with microcosms, we studied the effect of the surfactant tetradecyltrimethylammonium bromide (TDTMA) on the trophic activity of L. stagnalis, which was measured from the consumption of leaf phytomass for three days of incubation [4]. At the surfactant concentration of 2 mg/l, the feeding rate decreased with time, as well as the pellet excretion by the mollusks. The inhibition of trophic activity ranged from 27.9 to 70.9%, depending on time. Under the exposure to TDTMA, formation and total accumulation of pellets on the bottom per unit biomass of mollusks was only 58.3% of the control value [4]. In similar experiments, the surfactant sodium dodecylsulfate (SDS) also inhibited the trophic activity of L. stagnalis feeding on T. officinale biomass during 17 h. The degree of inhibition was 52.2 and 44.9% at SDS concentrations of 1 and 2 mg/l, respectively. Similarly, the synthetic detergent Tyde-Lemon inhibited the trophic activity of L. stagnalis feeding on the same plant spe-

cies. During 23 h of incubation, the degree of inhibition was 36% at the detergent concentration of 75 mg/l. Phytomass consumption was estimated per unit biomass of the mollusks (per 1 g of wet weight of L. stagnalis). Thus, trophic activity of molluscs and the related transfer of matter and energy along the trophic chain (phytomass–phytophage–excreted pellets) are inhibited by surfactants. These results, as well as other data [7–12], indicate one more potential anthropogenic hazard to biogeochemical flows in the biosphere. ACKNOWLEDGMENTS We are grateful to V.D. Fedorov, A.F. Alimov, V.V. Malakhov, E.A. Kuznetsov, M. V. Chertoprud, and other researchers at the Moscow State University and Russian Academy of Sciences for their help and useful comments on this study. The work of S.A. Ostroumov was supported by the Open Society Support Foundation (RSS grant no. 1306/1999). RFERENCES 1. Alimov, A.F., Funktsional’naya ekologiya presnovodnykh dvustvorchatykh mollyuskov (Functional Ecology of Fresh-Water Bivalve Mollusks), Leningrad: Nauka, 1981. 2. Monakov, A.V., Pitanie presnovodnykh bespozvonochnykh (Feeding of Freshwater Invertebrates), Moscow, 1998. 3. Vernadskii, V.I., Khimicheskoe stroenie biosfery Zemli i ee okruzheniya (Chemical Structure of the Biosphere of the Earth and Its Surroundings), Moscow: Nauka, 1965. 4. Ostroumov, S.A. and Kolesnikov, M.P., Dokl. Akad. Nauk, 2000, vol. 373, no. 2, pp. 278–280. DOKLADY BIOLOGICAL SCIENCES

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PELLETS OF SOME MOLLUSKS 5. Dalal, R.C., Analyst (Cambridge, UK), 1979, vol. 104, pp. 151–154. 6. Kolesnikov, M.P. and Abaturov, B.D., Usp. Sovrem. Biol., 1997, vol. 117, no. 5, pp. 534–548. 7. Ostroumov, S.A., Biologicheskie effekty poverkhnostnoaktivnykh veshchestv v svyazi s antropogennymi vozdeistviyami na biosferu (Biological Effects of Surfactants in Connection with the Anthropogenic Impact of the Biosphere), Moscow: MAKS, 2000.

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8. Ostroumov, S.A., Donkin, P., and Staff, F., Dokl. Akad. Nauk, 1998, vol. 362, no. 4, pp. 574–576. 9. Ostroumov, S.A., Dokl. Akad. Nauk, 2000, vol. 372, no. 2, pp. 279–282. 10. Ostroumov, S.A., Dokl. Akad. Nauk, 2000, vol. 374, no. 3, pp. 427–429. 11. Ostroumov, S.A., Dokl. Akad. Nauk, 2000, vol. 371, no. 6, pp. 844–846. 12. Ostroumov, S.A., Dokl. Akad. Nauk, 2000, vol. 375, no. 6, pp. 847–849.