Little is known about the dynamics of dissolved phosphate in oligotrophic areas of the world's oceans, where concentrations are typically in the nanomolar range. Here, we have budgeted phosphate uptake by the dominant microbial groups in order to assess the effect of the microbial control of this depleted nutrient in the North Atlantic gyre. Low concentrations (2.2 +/- 1.2 nM) and rapid microbial uptake (2.1 +/- 2.4 nM day(-1)) of bioavailable phosphate were repeatedly determined in surface waters of the North Atlantic oligotrophic gyre during spring and autumn research cruises, using a radiotracer dilution bioassay technique. Upper estimates of the concentration of bioavailable phosphate were 7-55% of the dissolved mineral phosphate suggesting that a considerable part of the chemically measured nanomolar phosphate was in a form unavailable for direct microbial uptake. A 1:1 relationship (r(2) = 0.96, P < 0.0001) was observed between the bioavailable total phosphate uptake and the phosphate uptake of all the flow sorted bacterioplankton cells, demonstrating that bacterioplankton were the main consumers of phosphate. Within the bacterioplankton a group of heterotrophic bacteria and Prochlorococcus phototrophic cyanobacteria, were the two major competing groups for bioavailable phosphate. These heterotrophic bacteria had low nucleic acid content and 60% of them comprised of SAR11 clade cells based on the results of fluorescence in situ hybridization. Each of the two competing bacterial groups was responsible for an average of 45% of the phosphate uptake, while Synechococcus cyanobacteria (7%) and picoplanktonic algae (0.3%) played minor roles in direct phosphate uptake. We have demonstrated that phosphate uptake in the oligotrophic gyre is rapid and dominated by two bacterial groups rather than by algae.

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(35)S-Methionine and (3)H-leucine bioassay tracer experiments were conducted on two meridional transatlantic cruises to assess whether dominant planktonic microorganisms use visible sunlight to enhance uptake of these organic molecules at ambient concentrations. The two numerically dominant groups of oceanic bacterioplankton were Prochlorococcus cyanobacteria and bacteria with low nucleic acid (LNA) content, comprising 60% SAR11-related cells. The results of flow cytometric sorting of labelled bacterioplankton cells showed that when incubated in the light, Prochlorococcus and LNA bacteria increased their uptake of amino acids on average by 50% and 23%, respectively, compared with those incubated in the dark. Amino acid uptake of Synechococcus cyanobacteria was also enhanced by visible light, but bacteria with high nucleic acid content showed no light stimulation. Additionally, differential uptake of the two amino acids by the Prochlorococcus and LNA cells was observed. The populations of these two types of cells on average completely accounted for the determined 22% light enhancement of amino acid uptake by the total bacterioplankton community, suggesting a plausible way of harnessing light energy for selectively transporting scarce nutrients that could explain the numerical dominance of these groups in situ.

Radionuclides of caesium are environmentally important since they are formed as significant high yield fission products ( 135 Cs and 137 Cs) and activation products ( 134 Cs and 136 Cs) during nuclear fission. They originate from a range of nuclear activities such as weapons testing, nuclear reprocessing and nuclear fuel cycle discharges and nuclear accidents. Whilst 137 Cs, 134 Cs and 136 Cs are routinely measurable at high sensitivity by gamma spectrometry, routine detection of long-lived 135 Cs by radiometric methods is challenging. This measurement is, however, important given its significance in long-term nuclear waste storage and disposal. M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPTseparation using an ICP-MS equipped with a reaction cell. The removal of the peak tailing interference is dependent on the instrument used for final measurement. This review summarizes and compares the analytical procedures developed for determination of 135 Cs/ 137 Cs, with particular focus on ICP-MS detection and the methods applied to interference separation. Sources and properties of caesium isotopesHigh yield fission products 135 Cs and 137 Cs (Table 1) are present in environmental samples as a result of releases from nuclear power plants and reprocessing sites, nuclear accidents, and fallout from atmospheric weapons testing [1]. Caesium-137 is established as an important radionuclide in radiation protection, environmental monitoring, sediment dating, land erosion and waste disposal [2]. By comparison, 135 Cs is a long-lived radioisotope with a comparatively low radiation risk; however it is a significant contributor to the long term radiological risk associated with deep geological disposal [3]. Furthermore, the 135Cs/ 137 Cs ratio varies with reactor, weapon and fuel type, and therefore can be used as a forensic tool to identify the source of radioactive contamination [1,[4][5][6]. Other isotopes of caesium include the activation products ( 134 Cs and 136 Cs, Table 1). These shorter-lived activation products (2.06 years and 13.2 days, respectively) provide information on fuel irradiation history and have importance in health physics terms but are not considered further here.( Table 1) Caesium-137 decays by beta emission to short-lived metastable isomer Cs is therefore achievable using beta counting or gamma spectrometry. Gamma spectrometry is generally favoured because it exploits the high gamma intensity of the 662 keV energy that is not susceptible to significant absorption. The ability to directly count most samples without the need for any chemical separation is also beneficial, and a high-purity germanium (HPGe)well detector can readily achieve a detection limit of ~2.5 mBq g -1 (for a 20 g sample counted for half a day), which is suitable for many environmental samples [3]. By comparison, 137 Cs measurement by beta counting in environmental samples requires chemical separation from other beta-emitting nuclides prior to measurement.Caesium-135 decays with a maximum beta particle energy of 2...

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Recent advances in sector field inductively coupled plasma mass spectrometry (ICP-SFMS) have led to significant sensitivity enhancements that expand the range of radionuclides measurable by ICP-MS. The increasing capability and performance of modern ICP-MS now allows analysis of medium-lived radionuclides previously undertaken using radiometric methods. A new generation ICP-SFMS was configured to achieve sensitivities up to 80,000 counts per second for a 1 ng/L (133)Cs solution, providing a detection limit of 1 pg/L. To extend this approach to environmental samples it has been necessary to develop an effective chemical separation scheme using ultrapure reagents. A procedure incorporating digestion, chemical separation and quantification by ICP-SFMS is presented for detection of the significant fission product radionuclides of cesium ((135)Cs and (137)Cs) at concentrations found in environmental and low level nuclear waste samples. This in turn enables measurement of the (135)Cs/(137)Cs ratio, which varies with the source of nuclear contamination, and can therefore provide a powerful dating and forensic tool compared to radiometric detection of (137)Cs alone. A detection limit in sediment samples of 0.05 ng/kg has been achieved for (135)Cs and (137)Cs, corresponding to 2.0 × 10(-3) and 160 mBq/kg, respectively. The critical issue is ensuring removal of barium to eliminate isobaric interferences arising from (135)Ba and (137)Ba. The ability to reliably measure (135)Cs/(137)Cs using a high specification laboratory ICP-SFMS now enables characterization of waste materials destined for nuclear waste repositories as well as extending options in environmental geochemical and nuclear forensics studies.

Tritium ((3)H) is produced in nuclear reactors via several neutron-induced reactions [(2)H(n, gamma)(3)H, (6)Li(n, alpha)(3)H, (10)B(n, 2alpha)(3)H, (14)N(n, (3)H)(12)C, and ternary fission (fission yield <0.01%)]. Typically, (3)H is present as tritiated water (HTO) and can become adsorbed into structural concrete from the surface inward where it will be held in a weakly bound form. However, a systematic analysis of a sequence of subsamples taken from a reactor bioshield using combustion and liquid scintillation analysis has identified two forms of (3)H, one weakly bound and one strongly bound. The strongly bound tritium, which originates from neutron capture on trace lithium ((6)Li) within mineral phases, requires temperatures in excess of 350 degrees C to achieve quantitative recovery. The weakly bound form of tritium can be liberated at significantly lower temperatures (100 degrees C) as HTO and is associated with dehydration of hydrous mineral components. Without an appreciation that two forms of tritium can exist in reactor bioshields, the (3)H content of samples may be severely underestimated using conventional analytical approaches. These findings exemplify the need to develop robust radioactive waste characterization procedures in support of nuclear decommissioning programs.

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