Abstract: Zinc is a trace metal responsible for the normal operation of living organisms but can be toxic at elevated concentrations. The biogeochemical processing of Zn in microorganisms may be reflected in changes in the ratios of stable Zn isotopes. If so, the isotopic signatures of Zn might be used as a tool to learn more about cellular metal processing and toxicity. Despite pioneering work on how Zn isotopes are fractionated during interaction with diatoms and plants, Zn isotopes in bacterial systems remain largely unexplored.
In this study, we conducted a series of metabolic uptake experiments with bacteria using different concentrations of Zn-citrate. Experiments were conducted using the Gram-negative bacteria, Pseudomonas mendocina and Escherichia Coli, as well as with a natural consortium of desert soil bacteria. Each experimental system was sampled as a function of time/cell growth and the elemental and isotopic compositions of Zn were determined for both the bacterial cells and the solution.
When exposed to 2 ppm of Zn, the growth curves for both E. coli and the natural consortium were similar in that the stationary phase occurred 8 days after inoculation. The stationary phase for P. mendocina occurred at 6 days after inoculation. However, when P. mendocina cells were exposed to 20 ppm of Zn, there was a time lag and the populations reached stationary phase 9 days after inoculation. Cell counts also suggested that fewer bacteria grew under the elevated Zn concentrations. In the 2 ppm experiments, E. coli, P. mendocina and the natural consortium incorporated a maximum of 17, 16 and 29 ppm of Zn, respectively. In contrast, when exposed to 20 ppm of Zn in the growth solution, P. mendocina incorporated 509 ppm of Zn. Zn isotopes, reported as Δ66Znbacteria-solution, for the 2 ppm Zn experiments varied as a function of the growth phase. A separation factor of +0.22‰ was measured for the log phase of E. coli, but this decreased to slightly negative values during the death phase. In contrast, P. mendocina showed the most negative Δ66Znbacteria-solution during the log phase of its growth (-0.4‰). When P. mendocina cells were exposed to 20 ppm of Zn, the Δ66Znbacteria-solution was positive, ranging from +1.02 to +1.5‰. The largest values were again found in the log phase of growth. The natural bacterial consortium did not substantially fractionate Zn isotopes (Δ66Znbacteria-solution = -0.04 to +0.05‰) when exposed to 2 ppm of Zn.
Despite some broad similarities, our experiments demonstrate that the bacterial species, growth phase, and the dosage of Zn all impact the Zn isotopic composition of bacteria. This may suggest that Zn isotopes could be used as a chemical tool for understanding Zn homeostasis within cells. The observation of a dose-dependent fractionation further suggests that Zn isotopes might serve as a tool for understanding toxicological impacts on these microorganisms. However, because of these same complexities, the use of Zn isotopes as a possible biological marker in natural systems deserves further scrutiny.
Abstract: The average abundance of U in carbonate sedimentary
rocks is 2.2 ppm while the concentration of U in relevant
Tethyan rocks from SE Europe and the Middle East is
reported to be in the range 1–10 ppm (e.g. [1]). Uranium is
removed from the oceanic water due to chemical processes at
the interface of organic-rich sediments whereas the reduction
of U(VI) to U(IV) occurs relatively late in the diagenetic
sequence [2]. However, additional diagenetic processes as
well as further tectonic and weathering processes may modify
the primary distribution and speciation of U in marine
sediments. Here we demonstrate the first Synchroton-based
investigation, performed in the SUL-X beamline of ANKA
(Germany), of typical marine carbonates originating in the
Tethys paleo-ocean. The samples represent stratified organicrich
calcitic limestone and tectonized/wheathered dolomitic
limestone of Triassic age from Mt. Kithaeron, central Greece,
exhibiting unusually elevated U concentrations (up to ca. 53
ppm). The SR µ-XRF study revealed that U is not particularly
enclosed into Fe and/or Mn phases (containing V, Cr, Zn and
As) but it is accumulated (together with Y and potential REEs)
in certain areas of the surrounding Ca-rich –carbonate- matrix
which includes, according to supplementary microscopic data,
minor phosphate and aluminosilicate compounds. The
corresponding µ-XANES spectra indicated the presence of
U(IV) in Ca-rich regions around distinct Mn phases. This is
the second mention for U(IV) in carbonate geologic materials,
after a similar study of a 35 Ma-old spar calcite (5-35 ppm U)
from a Mississippi Valley–type zinc ore deposit [3].
[1] Ehrenberg, Svånå & Swart (2008) AAPG Bull. 92, 691-
707. [2] Klinkhammer & Palmer (1991) Geochim.
Cosmochim. Acta 55, 1799-1806. [3] Sturchio et al. (1998)
Science 281, 971-973
Abstract: According to an early survey, performed by Greek authorities in the years between 1979 and 1986, the most significant radiometric anomalies in central Greece (Sterea Hellas) are related to lignites, bauxites and limestones. In the case of lignites and limestones the elevated natural radioactivity was attributed to the relatively high U concentrations (up to ca. 95 ppm in clay-bearing lignite from Atalanti, Phthiotis) while in the case of bauxites the sources of radiation were not clearly determined. In the greater Attica region, including the urban area of Athens and the surrounding metropolitan area (Attica basin) with a population of about 4 million, the only remarkable radioactive geological material was reported to be the Triassic “phosphorus-bearing bituminous limestone” of Mt. Kithaeron (up to ca. 47 ppm U). This formation had also been described in a previous unpublished report as “U-V-bearing phosphorite”. Nevertheless, a recent investigation [1] revealed that radioactive rock samples, which can currently be found in the Mt. Kithaeron area, concern in fact uraniferous carbonate rocks (ca. 14 to ca. 55 ppm U) with no evidence of increased phosphorus concentrations. In the present work we present natural radioactivity measurements (by laboratory γ-ray spectrometry/HPGe detector) in the above rocks, which can be described as stratiform bitumenous calcitic limestones of relatively lower radioactivity (164 Bq/Kg for 238U, total dose rate: 76 nGy/h) and tectonized dolomitic limestones of higher radioactivity (724 Bq/Kg for 238U, total dose rate: 344 nGy/h). Further investigations in the greater Attica region indicated insignificant radioactivity in limestones and marbles of the Attica basin (Mt. Lycabettus and Tourkovounia, Mt. Hymettus, Mt. Penteli, Mt. Aegaleo, Liosia area) as well as in the low-grade metasedimentary rocks known as the “Athens schists”. However, the volcanic rocks (dacites) of Ag. Theodori area (Sousaki volcano) and the crystalline basement rocks (orthogneisses) of Mt. Penteli exhibit significant natural radioactivity (105 and 147 nGy/h respectively) mainly due to the contribution of 232Th series (54 and 90 Bq/Kg respectively) and 40K (1080 and 1737 Bq/Kg respectively).
1. F.-C. Kafandaris, A. Godelitsas, D. Kostopoulos, S. Xanthos, E. Chatzitheodoridis and E. Baltatzis (2007), Geochim. Cosmochim. Acta, Vol. 71 (15S), A457.
Abstract: The average abundance of U in carbonate sedimentary rocks is 2.2 ppm. Sandstones and shales contain 0.5 ppm and 3.5 ppm respectively while the typical U concentration in seawater is 3.2 ppb (Krauskopf and Bird 1994). Uranium in the oceans follows anoxic pathways and it is mainly removed from the water due to chemical processes taking place at the interface of organic-rich sediments. Uranium is therefore correlated to organic carbon whereas the diagenetic cycle of the element may include reduction of U(VI) to U(IV) related to sulfate bio-reduction (Mo et al. 1973, Klinkhammer and Palmer 1991). The concentration of U in marine carbonates from SE Europe and the eastern Mediterranean is reported to be in the region ca. 1 – 7 ppm. Here we present the occurrence of carbonate rocks, limestones and dolomitic limestones with variable organic content, from Mt. Kithaeron (central Greece) containing unusually elevated U concentrations up to ca. 56 ppm. We also present a relationship between the U content and radioactivity (see Figure below). They are typical Alpine (Neotethyan) sediments, of likely Triassic age (according to existing paleontological data) belonging to the SubPelagonian zone of the internal Hellenides. The rocks were investigated using a combination of microscopic, spectroscopic and wet-chemical techniques. Our study revealed that U is probably associated to the non-carbonate part of the rocks (organic matter, Fe- and Mn-oxides, phosphates, etc.).
Krauskopf K.B. and Bird. D. (1994), Introduction to Geochemistry-3rd Edition, McGraw-Hill, 640pp.
Mo T., Suttle A.D. and Sackett W.M. (1973), Geochim. Cosmochim. Acta, 37, 35-51.
Klinkhammer G.P. and Palmer M.R. (1991), Geochim. Cosmochim. Acta, 55, 1799-1806.
