Uptake of trace metals by aquatic invertebrates

Transcription

1 Uptake of trace metals by aquatic invertebrates

2 Principles Organisms take up trace metals. Aquatic invertebrates take up metals from solution and diet. Metals are then accumulated or excreted. Some accumulated metal can remain in metabolically available form; the rest must be stored in detoxified form. Toxic effects occur when the rate of uptake exceeds the rates of excretion and detoxification combined (too much metabolically available metal can bind in the wrong places). Factors that affect the rate of uptake will therefore affect toxicity (toxicity only starting at very high rates of uptake). Measurements of accumulation are only measurements of uptake if there is no excretion, or (in the case of radiotracer experiments) no time for excretion.

3 Factors affecting uptake from solution Dissolved concentration. Trace metal uptake rates are typically proportional to external dissolved concentration. The uptake rate constant K u is expressed as µg g -1 d -1 per µg L -1 or L g -1 d -1. Temperature. Trace metal uptake rates increase with temperature. Salinity. Uptake rates of Cd and Zn typically increase when salinity is decreased. Other trace metals. The presence of other trace metals may increase or decrease the uptake rate of a trace metal, and the effect may be concentration-dependent. Water-soluble chelating agents. EDTA typically decreases the rate of uptake of trace metals like Zn and Cd. Lipid-soluble ligands or organometals. Metals in lipidsoluble form usually have faster uptake rates than inorganic equivalents. PHYSICOCHEMICAL CONTROL OF TRACE METAL UPTAKE

4 Salinity Uptake rates of Cd and Zn typically increase when salinity is decreased. eg. a) Wright (1977) Shore crab Carcinus maenas 14 d Cd accumulation (= uptake) 10 µm Cd % SW Body Cd µmol g -1 wet wt b) Hutcheson (1974) Blue crab Callinectes sapidus TOXICITY Salinity h LC 50 11,600 4, µg Cd L -1 c) Nugegoda & Rainbow (1989a) Palaemon elegans 56.2 µg Zn L -1 (radiotracer) % SW Zn uptake rate ± 0.08 µg Zn g -1 d ± ± 0.39

5 Palaemon elegans Rate of Zn uptake from 56.2 µg/l Zn decreases with salinity increase Nugegoda & Rainbow (1989a)

6 Palaemon elegans Rate of Zn uptake from 120 µg/l Zn a) 75% seawater 2.27 ± 0.65 µg/g/d b) 25% seawater 5.57 ± 2.30 µg/g/d c) 25% seawater, 5.11 ± 2.08 µg/g/d isosmotic with 75% seawater Increased uptake at low salinity is a salinity effect not an osmolality effect Nugegoda & Rainbow (1989b)

7 Water-soluble chelating agents EDTA typically decreases the rate of uptake of trace metals like Zn and Cd, eg. a) Sunda et al. (1978) Palaemonetes pugio TOXICITY at salinity 5 NTA µmol L h LC ,000 3,000,000 µg Cd L -1 b) Nugegoda & Rainbow (1988) Palaemon elegans 100 µg L -1 Zn (1.53 µmol L -1 ) Sequential reduction of uptake rate from 3 µg Zn g -1 d -1 to almost zero as EDTA concentration changes from 0 to 3 µmol L -1.

8 EDTA decreases rate of zinc uptake from solution

9 Lipid-soluble ligands or organometals Metals in lipid-soluble form usually have faster uptake rates than inorganic equivalents. eg. a) Corner & Sparrow (1958) Elminius modestus larvae TOXICITY relative to inorganic mercuric chloride Cl - Hg - Cl 1 CH 3 - Hg - Cl 4.7 (methyl) C 2 H 5 - Hg - Cl 6.8 C 4 H 9 - Hg - Cl 16 C 5 H 11 - Hg -Cl 20 Increased lipophilic character increases toxicity b) Ahsanullah & Florence (1984) Allorchestes compressa Lipid soluble ligands increase toxicity: eg. k ethyl xanthogenate and oxine Water-soluble ligands decrease toxicity: eg. NTA, tannic acid, HSA

10 BIOAVAILABILITY OF DISSOLVED TRACE METALS Solutions of the same total dissolved trace metal concentration may cause different uptake rates in an organism, ie they have different bioavailabilities to that organism. Different bioavailabilities have been caused by physico-chemical characteristics of the solution, ie independently of the organism. eg salinity, other metals, chelating agents The Free Metal Ion Model is used to explain many such data.

11 DISSOLVED BIOAVAILABILITY and the FREE METAL ION MODEL The Free Metal Ion is the form typically taken up from solution across the cell membrane. Many trace metals in seawater are complexed with inorganic complexing agents, particularly chloride. eg. Cd 2.5% Cd ++, 51% CdCl 20, 39% CdCl +, 6% CdCl 3 - Zn 40% Zn ++ with ZnCl +, ZnCl 20, etc. In seawater (or freshwaters) naturally occurring DOM (Dissolved Organic Matter) eg humic acids may also chelate trace metals (eg Cu in coastal waters). Low salinity reduces chloride complexation of Cd and Zn. Higher percentage of dissolved metal is in the form of the free metal ion. ie M ++ availability has increased. EDTA chelation reduces the concentration of M ++. Other trace metals may change complexing equilibria in solution (or directly compete for uptake sites).

12 Complexation of Metal ions in the Sea Metal Probable dissolved species Ag Silver AgCl 2-, AgCl 3-4, AgCl 2-3 Cd Cadmium CdCl 2o, CdCl +, CdCl - 3 Cu Copper CuCO 30, Cu(OH) +, Cu 2+ Fe Iron Fe(OH) 0 3 Hg Mercury HgCl 2-4, HgCl 3 Br 2-, HgCl - 3 Mn Manganese Mn 2+, MnCl + Ni Nickel Ni 2+, NiCO 30, NiCl + Pb Lead PbCO 30, Pb(CO 3 ) 2-2, PbCl + V Vanadium HVO 2-4, H 2 VO 4-, NaHVO - 4 Zn Zinc Zn 2+, Zn(OH) +, ZnCO 30, ZnCl +

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15 Trace Metal Uptake from Solution Potential routes across the membrane 1) Carrier protein (transporter): passive facilitated diffusion. The free metal ion is bound to the carrier protein and transferred to cell interior. The metal ion is immediately bound to an intracellular ligand (eg protein) and may be passed onto other ligands of higher affinity. Intracellular binding causes a concentration gradient of free metal ion from out to in (even if the free metal ion is only a small proportion of total outside dissolved trace metal). 2) Entry via a major ion channel. The Cd free metal ion has a diameter (109 pm) close to the (free) Ca ion (114 pm) and will enter via Ca channels. 3) Diffusion across the lipid bilayer a) Non-polar neutral complexes (eg AgCl 0 ) will diffuse across the membrane b) Organometal compounds will diffuse across (eg tributyl tin, methyl mercury). 4) Endocytosis. Metal-rich particles in bivalve gill. MORE THAN ONE ROUTE MAY BE IN USE - RELATIVE IMPORTANCE???

16 external internal carrier protein mediated M ++ M ++ major ion channel M ++ MX 0 eg AgCl MR n 0 eg Hg(CH 3 ) 2 endocytosis M eg Fe(OH) 3?

17 Uptake of trace metals from solution Physicochemistry can explain much, especially the free metal ion availability but, is there any physiological intercession? Physiology v physicochemistry

18 Palaemon elegans Rate of Zn uptake from 120 µg/l Zn a) 75% seawater 2.27 ± 0.65 µg/g/d b) 25% seawater 5.57 ± 2.30 µg/g/d c) 25% seawater, 5.11 ± 2.08 µg/g/d isosmotic with 75% seawater Increased uptake at low salinity is a salinity effect, not an osmolality effect ie PHYSICOCHEMICAL CONTROL OF UPTAKE Nugegoda & Rainbow (1989b)

19 Other species?

20 Orchestia gammarellus amphipod crustacean

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22 Orchestia gammarellus Physicochemical control (free metal ion availability) of Zn and Cd uptake at high salinity. Physiological change to low salinity reduces expected Zn and Cd uptake rates. Apparent water permeability?

23 Physicochemistry, Physiology and Crabs Carcinus maenas (the shore crab) Zn and Cd uptake rates decrease with salinity decrease Unexpected from free ion activity model Physiological response to low salinity (decreasing Apparent Water Permeability with decreasing salinity) offsets physicochemical change Eriocheir sinensis (Chinese mitten crab) Adapted to fresh water; low AWP unchanging with salinity change Zn and Cd uptake rates low (existing low AWP) Physicochemical control of uptake via free metal ion availability

24 Physicochemistry v Physiology In most cases uptake from solution by aquatic invertebrates is controlled externally by physicochemistry, depending on the relative significance of uptake routes. Physiological changes by some invertebrates (eg. to salinity change) may have a secondary effect on trace metal uptake rates.

25 Bioavailability in diet Digestion releases metal ions for uptake in the gut across apical cell membranes of gut epithelial cells by previous routes. Uptake from the diet (Assimilation Efficiency) is therefore affected by digestive enzymes, gut ph ingestion rate gut passage time and efficiency of digestion nature of chemical binding of metals in the food

26 Bioavailability in diet Food organisms Phytoplankton: accumulated metals may be differently distributed on the outside or inside of the cell (adsorption v absorption), soluble v insoluble binding, and nature of soluble binding. Invertebrate prey: Is the metal adsorbed or absorbed (zooplankton)? What is the form of (detoxified) binding of accumulated trace metal? Inert insoluble granules? Digestible storage proteins?

27 Assimilation of trace metals by herbivores from phytoplankton Reinfelder & Fisher (1991) 1:1 relationship between metal assimilated by copepods from diatoms and the metal partitioned in the cytoplasm of the diatoms. Science 251,

28 Barnacle Balanus trigonus feeding on 4 phytoplankton species: Cd, Cr and Zn Wang & Rainbow (2000) Mar Ecol Prog Ser 204,

29 Balanus trigonus Trace Metal Assimilation Efficiencies (%) Phytoplankton Cd Zn Thalassiosira 62.0 ± ± 7.0 weissflogii Skeletonema 71.4 ± ± 7.5 costatum Prorocentrum 40.8 ± ± 2.9 minimum Tetraselmis 49.0 ± ± 5.4 levis

30 Balanus trigonus

31 Balanus trigonus Assimilation Efficiency of Cd and Zn, but not Cr, correlated with percentage of phytoplankton metal in cytoplasm Wang & Rainbow (2000)

32 Barnacle Elminius modestus feeding on 5 species of phytoplankton Rainbow & Wang (2001)

33 Elminius modestus Assimilation Efficiency of Cd and Se, but not Cr or Zn, correlated with percentage of phytoplankton metal in cytoplasm Inconsistency for Zn between B. trigonus and E. modestus

34 Ng, Amiard-Triquet, Rainbow, Amiard, Wang (2005) Mar Ecol Prog Ser 299, Barnacle: Balanus amphitrite Mussel: Perna viridis Clam: Ruditapes philippinarum feeding on 7 phytoplankton species 3 metals: Ag, Cd, Zn Accumulated metal: Exchangeable v. Soluble v. Insoluble

35 Were there any correlations between Assimilation Efficiency (AE) and fractionation of accumulated metal in the phytoplankton into a) Exchangeable b) Soluble c) Exchangeable plus soluble d) Insoluble fractions?

36 Some but, Only in 8 cases out of 48 Inconsistent or contradictory Therefore, no general principle has emerged control of assimilation results from more subtle differences in the nature of the chemical binding in the phytoplankton

37 Invertebrates as the prey of predators 1) Wallace & Lopez (1996): Cd in oligochaete worm Limnodrilus hoffmeisteri preyed on by decapod crustacean Palaemonetes pugio Only metal bound to soluble fraction is trophically available. 2) Nott & Nicolaidou (1994): Gastropod mollusc prey and hermit crab Clibanarius erythropus Insoluble metal can be trophically available but less so than soluble metal, and trophic availability varies with granule type. 3) Wallace & Luoma (2003): Bivalve mollusc prey and Palaemon macrodactylus Trophically Available Metal (TAM) is bound to proteins (soluble) and organelles (insoluble).

38 Wallace & Luoma (2003)

39 Nereis diversicolor the prey The Diet of Worms Rainbow et al. (2006)

40 Palaemonetes varians the predator

41 PREY from 2 sites Blackwater, Essex, UK (control) Restronguet Creek, Cornwall, UK (metal-rich) radiolabelled with one of cadmium, silver or zinc via solution, or sediment Therefore: Range of distribution of accumulated labelled metals in 5 fractions (after Wallace & Luoma, 2003)

42 Wallace & Luoma (2003)

43 Is Assimilation Efficiency (AE) correlated with the proportion of metal present as a) TAM (defined by Wallace & Luoma, 2003) b) Total Protein (soluble) Metal (HSP + MTLP)?

44 Ag AE (%) Ag TAM (%) Ag Only in 1 case out of a possible 6 for individual metals Ag AE and Ag TAM

45 AE (%) Ag Cd Zn TAM (%) If Ag + Cd + Zn data are combined, then AE is correlated with TAM

46 AE (%) Ag Cd Zn Total Protein (%) and AE is correlated with Total Protein (soluble)

47 100 AE (%) Ag Cd Zn MRG (%) AE is negatively correlated with metal bound in MRG

48 Wallace & Luoma (2003)

49 Conclusions There is a correlation between the proportion of metals bound in the combination TAM in the prey and subsequent assimilation, but this relationship only explained about 21% of the variance observed. There is also a correlation between the proportion of metals bound in the combination Total Protein in the prey and subsequent assimilation, but this relationship also only explained about 21% of the variance observed. There is also a negative correlation between the proportion of metals bound in Metal-Rich Granules in the prey and subsequent assimilation.

50 Conclusions The combination TAM alone does not completely account for all bioavailable metal in invertebrate prey. It is likely that different fractions of accumulated metal in prey show a range of Assimilation Efficiencies (with soluble proteinbound metals having high AEs). The different fractions combine to produce a particular integrated AE for the trophic transfer of metal from prey to predator.

51 Bioavailability in diet Ingested sediment Not all metal is bioavailable (able to be assimilated). Nature and strength of binding is important, eg binding to organic component association with Fe in sediment Luoma & Bryan (1978) Scrobicularia plana surface deposit feeder (fresh sediment) Pb concentration in body tissues explained by Pb/Fe ration in sediments; high Pb in sediments is not available if Fe is high.

52 Luoma & Bryan (1978)

53 Bioavailability in diet Chemical nature of metal binding in the diet is important in determining the bioavailability of dietary metals to animals. Metals bound in a range of chemical forms can be bioavailable but to different extents.