Trace metal stable isotope biogeochemistry

In nature, variations in isotopic ratios of trace metals on the order of a few tenths to several ‰ are observed within and across diverse environments spanning the geosphere, hydrosphere and biosphere.

Stable isotope ratios of trace metals can be used to answer a variety of questions spanning diverse fields, including oceanography, geochemistry, biology, medicine, anthropology and astronomy.

In addition to their modern applications, trace metal isotopic compositions can provide insight into ancient biogeochemical processes operated on Earth.

Studies of the acidophilic Fe(II)-oxidizing bacterium, Acidthiobacillus ferrooxidans, have been used to determine the fractionation as a result of iron-oxidizing bacteria.

[26] Iron isotopes are distributed heterogeneously throughout the body, primarily to red blood cells, the liver, muscle, skin, enzymes, nails, and hair.

[8] Isotopic variations observed on planetary bodies can help to constrain and better understand their formation and processes occurring in the early Solar System.

Their ability to act as isotopic tracers allows for their use to determine information regarding the formation of geologic units and as a potential proxy for life on Earth and other planets.

[35] Iron isotopic studies can reveal the details of the formation of BIFs, which allows for the reconstruction of redox and climatic conditions at the time of deposition.

[36] Ultimately, the improved understanding of BIFs using iron isotope fractionations would allow for the reconstruction of past environments and the constraint of processes occurring on the ancient Earth.

Thus, the continued study and improved understanding of biologic and abiologic fractionation effects would be beneficial in providing better details regarding BIF formation.

[38] Isotopic analyses similar to the one above are utilized throughout all of the world's oceans to better understand regional variability in the processes which control iron cycling.

One of the primary challenges in the study of planetary accretion is the fact that many tracers of the processes occurring in the early Solar System have been eliminated as a result of subsequent geologic events.

Thus, the continual improvement of knowledge regarding the biological fractionations of iron observed on Earth can have applications when studying extraterrestrial samples in the future.

[42] While this field of research is still developing, this could provide evidence regarding whether a sample was generated as a result of biologic or abiologic processes depending on the isotopic fractionation.

[48] Copper's powerful redox capability makes it critically important for biology, but comes at a cost: Cu1+ is a highly toxic metal to cells because it readily abstracts single electrons from organic compounds and cellular material, leading to production of free radicals.

In the context of catalytic activity, copper proteins function as electron or oxygen carriers, oxidases, mono- and dioxygenases and nitrite reductases.

Biological processes that fractionate Cu isotopes are not well-understood, but play an important role in driving the δ65Cu values of materials observed in the marine and terrestrial environments.

However, based on a compilation of measurements already made, it appears that Cu isotope ratios vary somewhat widely within and between environmental materials (e.g., plants, minerals, seawater, etc.

[67] The Cu isotope compositions of copper-containing minerals vary over a wide range, likely due to alteration of the primary high-temperature deposits.

δ65Cu values of Cu minerals (including chrysocolle, azurite, malachite, cuprite and native copper) in low-temperature deposits have been observed to vary widely over a range of -3.0 to +5.6‰.

[64][66][73] δ65Cu values of the surface layers of FeMn-nodules are fairly homogenous throughout the oceans (average = 0.31‰),[69] suggesting low biological demand for Cu in the marine environment compared to that of Fe or Zn.

[51] Due to its relatively short turnover time of ~6 weeks in the human body,[74] Cu serves as an important indicator of cancer and other diseases that rapidly evolve.

Examples of zinc-based enzymes include superoxide dismutase (SOD), metallothionein, carbonic anhydrase, Zn finger proteins, alcohol dehydrogenase and carboxypeptidase.

Kafantaris and Borrok[83] grew model organisms B. subtilis, P. mendocina and E. coli, as well as a natural bacterial consortium collected from soil, on high and low concentrations of Zn.

However, based on a compilation of some reported measurements, it appears that Zn isotope ratios do not vary widely among environmental materials (e.g., plants, minerals, seawater, etc.

Zn isotope ratios vary between individual blood components, bones and the different organs in humans, though in general, δ66Zn values fall within a narrow range.

[86] Additionally, in a small sample set of volunteers, whole blood δ66Zn values were ~+0.15‰ higher for vegetarians than for omnivores,[87] suggesting diet plays an important role in driving Zn isotope compositions in the human body.

In addition to its critical role in many metalloenzymes (see Zinc Biology section), Zn is an important component of the carbonate shells of foraminifera[93] and siliceous frustules in diatoms.

[73] In some photic zones in the ocean, Zn is a limiting nutrient for phytoplankton,[95] and thus its concentration in surface waters serves as one control on marine primary productivity.

[69] Additionally, the surface layers of FeMn-nodules are 66Zn enriched at high-latitudes (average δ66Zn = +1‰), while δ66Zn values of low-latitude samples are smaller and more variable (spanning +0.5 to +1‰).