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Trace Metal Biogeochemistry (Marine Bioinorganic Chemistry) 12.755. MIT-WHOI Joint Program Graduate Course Lecture 1 Mak Saito, Marine Chemistry and Geochemistry Department Course website: www.whoi.edu/sites/12.755 Outline:

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Trace metal biogeochemistry marine bioinorganic chemistry 12 755 l.jpg

Trace Metal Biogeochemistry(Marine Bioinorganic Chemistry) 12.755

MIT-WHOI Joint Program Graduate Course

Lecture 1

Mak Saito, Marine Chemistry and Geochemistry Department

Course website: www.whoi.edu/sites/12.755

Outline:

Introductions, comments on course schedule, structure, approach, assignments, and pedagogy

Introduction to Trace Metal Biogeochemistry: an evolving field

Classifications of TM profiles

Metal Speciation lecture


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Class Topics

  • Introduction to trace metal biogeochemistry, broad categories

  • Metal Speciation

  • Free ion model

  • Algal uptake kinetics

  • The Droop model and colimitations

  • Mercury Biogeochemistry (Lamborg as guest lecturer)

  • Iron biogeochemistry (limitation, light colimitation, redox, speciation, uptake mechanism, colloids, and policy)

  • Trace elements and the ancient ocean

  • Metalloenzymes

  • Analytical approaches (in silico and proteomic/mass spec)

  • Specific elemental biogeochemistries (Mn, Al, Pb, Co, Zn, Cd, Cu)


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Events

  • Phone conference with Bill Sunda, expert trace metal phytoplankton interactions

  • JGI bioinformatics module with hands on computer experiments and phone conference with JGI genomic scientist

  • Anonymous review of papers

  • Readings on ideas in science for discussion throughout semester

  • Discussion of iron fertilization

  • Discussion of Mercury policy (Lamborg)


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Iron as a limiting nutrient in HNLC regions (Review of Iron Fertilization Experiments Boyd et al., 2007, Science)

Purposeful (white crosses) and natural (red crosses) Fe enrichment studies have shown Fe limitation of phytoplankton growth.


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Science Plan

Download PDF from

http://www.geotraces.org/


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GEOTRACES Goal: making WOCE-like sections for Trace Elements and Isotopes

Meridional Pacific, Hiscock, Measures and Landing, GBC 2008


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RIP: CEBIC

1999-2007


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Trace metal biogeochemistrya.k.a Marine Bioinorganic Chemistry:a field developing its own identity

Driven originally by analytical chemistry

  • Initial measurements of many metals far too high due to contamination

    Biological or “Bioinorganic” component has grown in:

  • Bioactive metals: Fe, Co, Cd, Zn, Cu, Ni, Mn, Mo etc.

  • Iron limitation discovered

  • The Role Complexation on Bioavailability

  • Metalloenzymes

  • Other limitations and colimitations

  • Future roles for genomics, metagenomics, proteomics


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Four Categories of Trace Metal Profiles in 2D

  • Conservative distributions

    • Residence time greater than 100000 years

    • Much greater than the residence time of the oceans

    • Molybdenum, tungsten, antimony, rubidium: are involved in particle cycling, but the quantities are insignificant relative to their large seawater inventory

    • Concentrations of some are quite high: Mo = 105nM

    • Don’t increase with thermohaline circulation

    • Searching for the kink in Molybdenum due to nitrogen fixation


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Four Categories of Trace Metal Profiles in 2D

  • Nutrient-type distributions:

    • Significantly involved with internal cycles of biologically derived particulate material

    • Distributions are dominated by phytoplankton uptake in surface waters followed by export of some of this material below the surface layer and subsequent remineralization and release to intermediate and deep waters

    • Have a low level of scavenging in intermediate and deep waters

    • (N, P, Si) Zinc, Cadmium, Barium, Silver, Nickel

    • Increase in concentration with thermohaline circulation

    • Can be used as paleoproxies for P (Cd) or Si (Zn) in foram tests and diatom opal.


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Four Categories of Trace Metal Profiles in 2D

3. Scavenged-type distributions

  • Strong interactions with particles

  • Short residence times (~100-1000y)

  • Increased concentration near sources

  • Decreased concentrations away from sources

  • Decreased concentrations along flow path due to continual scavenging

  • Aluminum, lead, manganese


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Tangent:

  • Tomatoes and Tomatoes

  • Aluminium (British and Aussies) and Aluminum (Elsewhere)

  • International Union of Pure and Applied Chemistry uses Aluminium

  • Probably most importantly for oceanography: Chris Measures is British


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Four Categories of Trace Metal Profiles in 2D

4. Hybrid-Type Metals

  • Strongly influenced by both micronutrient use and remineralization and scavenging processes.

  • Does not accumulate with thermohaline circulation

  • Can depend on geographic location: high dust input can obscure surface drawdown signal

  • “Hybrid-Type” is a relatively new descriptor

  • Bruland and Lohan (assigned reading this week): Iron, copper

  • Although not included, Cobalt is undoubtedly a hybrid-type metal

  • Mn could be one as well, but only at high latitudes, where nutrient-like drawdown occurs


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These four geochemical categories of metals in seawater are a direct result of their chemical properties:

  • Solubility

  • Inorganic speciation

  • Organic Speciation

  • Redox chemistry

  • Biological properties is debatable as a fifth, since there appear to be non-biological elements with nutrient-like profiles


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Background Aquatic Chemistry of Trace Elements: a direct result of their chemical properties:A marine water column context

Solubility Products: Example for Fe(OH)3(s)

Ksp= [Fe][OH]3= 1042.7

Stability constants for metal complexes (where L is ligand, M is Metal):

K = [ML]/[M][L]

Ligands can include inorganic chemical species:

In oxic systems: OH-, CO32-,SO42-, Cl-, PO43-,

In anoxic systems add: HS-,, S2-

Ligands can also include organic chemical species:

EDTA, DTPA, NTA, Citrate, Tris, siderophores, cobalophores,

DFB, TETA, and the famous unknown ligand(s) “L”


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Background Aquatic Chemistry of Trace Elements: a direct result of their chemical properties:A marine water column context

Detailed balancing: Principle of Microscopic Reversibility

kf

Mn+ + L- ML

kb

d[ML]/dt = kf [M+] [L-]

-d[M+]/dt = -d[L-]/dt = kb [ML]

At steady state:

kf [M+] [L-] = kb [ML]

kf / kb = [ML]/([M+][L-]) = K


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Background Aquatic Chemistry of Trace Elements: a direct result of their chemical properties:A marine water column context

However, there can be Non-Ideal effects (Morel and Hering 76-82):

  • The effects of other solutes on the free energy of ion(s) of interest

  • Solubility product and stability constants need to be corrected, or better, determined to/at the appropriate ionic strength.

  • The activity of the metal is: {Mn+} = [Mn+]gMn+

  • The activity coefficient, gMn+, can be estimated by the Debye-Huckel correction or the Davies expression (modified Debye-Huckel)

  • I = ionic strength

  • Z=charge, A = 1.17 M-1/2, B=0.3M-1/2

  • Thermodynamic databases (Martell and Smith) will provide the ionic strength experimental conditions for each constant (e.g. 0.1M)


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Quasi constant value between I=0.3-0.7 a direct result of their chemical properties:

From Morel and Hering, 1993, p77


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Definitions a direct result of their chemical properties:

  • Ligand – an atom, ion, or molecule that donates/shares electrons with one or more central atoms or ions. Metal-ligand bonds (inner sphere) are covalent.

  • Chelate – (from Greek chelos = crab, with two binding claws) two or more donor atoms from a single ligand to the central metal atom


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Vraspir and Butler 2009 that a metal can have. Most metals have a # of 6, forming


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Characteristics of Metal Ion Binding to Ligands that a metal can have. Most metals have a # of 6, forming

  • Soft vs Hard

    • Soft: Ions are large and easily polarizable

    • Hard: Small and less easily polarizable

  • Soft metals tend to “like” soft ligands

  • Hard metals tend to “like” hard ligands

  • Examples:

  • Hard: Fe3+,Co3+ and OH-

  • Soft: Cd2+, Cu+, Hg2+ and sulfide groups


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Valence that a metal can have. Most metals have a # of 6, forming

  • Metal chemistry strongly influenced by the removal of electrons from a neutral atom

  • Main group: outer electron shells consist of s and p orbitals (Li, Na, K)

    • React violently with water (e.g. pure sodium to NaOH, +1 ions)

  • Transition metals have incomplete d electron shell

  • Most transition metals have variable valence, a major component of their chemistry

    • Fe: +2, +3

    • Mn: +2, +3, +4, +6, +7


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Ionic radii of Cd that a metal can have. Most metals have a # of 6, forming 2+ > Co2+ > Fe3+


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Characteristics of Metal Ion Binding to Ligands that a metal can have. Most metals have a # of 6, forming

  • Soft vs Hard

    • Soft: Ions are large and easily polarizable

    • Hard: Small and less easily polarizable

  • Soft metals tend to “like” soft ligands

  • Hard metals tend to “like” hard ligands

  • Examples:

  • Hard: Fe3+,Co3+ and OH-

  • Soft: Cd2+, Cu+, Hg2+ and sulfide groups


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Average Major Seawater Ions (mM) that a metal can have. Most metals have a # of 6, forming

(Morel and Hering, p291)

HCO3- 2.38

SO42- 28.2

Cl- 0.545

Ca2+ 0.0102

Mg2+ 0.0532

Na+ 0.0468

K+ 0.0102


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Average Major Seawater Ions ( that a metal can have. Most metals have a # of 6, forming mM)

(Morel and Hering, p291)

HCO3- 2.38

SO42- 28.2

Cl-

Ca2+

Mg2+ 0.0532

Na+

K+ 0.0102


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The Irving-Willliams Series that a metal can have. Most metals have a # of 6, forming

  • Observations that complex stability for each ligand have a tendancy to rank:

    Mn2+ < Fe2+ < Co2+ < Ni2+ < Cu2+ > Zn2+

  • Caused by increases in ionic radius and ligand field stabilization effects

  • Many implications both for ligands “L’s” in seawater and for protein binding of metals inside cells, area for much future research

    • For example it is hard to find any cobalt(II) ligand that is stronger than a nickel(II) ligand


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Complexation Environment that a metal can have. Most metals have a # of 6, forming

  • “Free ions” is really a misnomer

  • Cu2+ is actually Cu(H2O)62+, if not bound by other inorganic species

  • Water is a ligand, ligand-exchange rxn constants indicative of rate of reactivity, or the kinetics

  • Dissociation of water molecules dependent on size and inversely to the size of the metal cation


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Water loss exchange rates that a metal can have. Most metals have a # of 6, forming


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Abundance (or lack there of) is our friend that a metal can have. Most metals have a # of 6, forming

Seawater constituents:

  • Major ions (the salt) – millimolar and higher

    • Na+

    • Cl-

    • Mg2+

    • Ca2+

    • HCO3-

  • Organic ligands/chelators - nanomolar

    • “L”

  • Trace metals/elements – picomolar to nanomolar

    • Mn+

  • With major ions, everything depends on everything (and must be considered simultaneously

  • With trace elements, we can consider one element at a time, independently of other constituents


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Preview: Software for Metal Speciation that a metal can have. Most metals have a # of 6, forming

  • Mineql – Westall et al. a program made for calculating aqueous speciation and solubility at low temperature geochemical conditions

  • Critical.exe – Smith and Martell volumes built into a DOS based-database.

  • But need to know how to do it by hand well in order to use software effectively. I usually use both hand calculations and computer assisted calculations to cross-check assumptions.


Readings available on website www whoi edu sites 12 755 l.jpg
Readings – available on website that a metal can have. Most metals have a # of 6, forming www.whoi.edu/sites/12.755

  • Bruland and Lohan -Treatise on Geochemistry Chapter

  • Morel and Hering, Principles of Aquatic Chemistry Chapter 6

  • Background: Lippard and Berg Bioinorganic Chemistry chapter 2

  • Goldberg Biography


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