Most stratified sulfidic holomictic lakes become oxygenated after annual turnover. In contrast, Lake Rogoznica, on the eastern Adriatic coast, has been observed to undergo a period of water column anoxia after water layer mixing and establishment of holomictic conditions. Although Lake Rogoznica's chemistry and hydrography have been studied extensively, it is unclear how the microbial communities typically inhabiting the oxic epilimnion and a sulfidic hypolimnion respond to such a drastic shift in redox conditions. We investigated the impact of anoxic holomixis on microbial diversity and microbially mediated sulfur cycling in Lake Rogoznica with an array of culture-independent microbiological methods. Our data suggest a tight coupling between the lake's chemistry and occurring microorganisms. During stratification, anoxygenic phototrophic sulfur bacteria were dominant at the chemocline and in the hypolimnion. After an anoxic mixing event, the anoxygenic phototrophic sulfur bacteria entirely disappeared, and the homogeneous, anoxic water column was dominated by a bloom of gammaproteobacterial sulfur oxidizers related to the GSO/SUP05 clade. This study is the first report of a community shift from phototrophic to chemotrophic sulfide oxidizers as a response to anoxic holomictic conditions in a seasonally stratified seawater lake.
Mercury electrodes preconcentrate metal chalcogenide nanoparticles effectively, enabling their detection at submicromolar concentrations (as ∑chalcogenide) by adsorptive cathodic stripping voltammetry. Understanding the unique behavior of nanoparticle analytes during preconcentration is critical for lowering detection limits and for quantification. A multistep mechanism is proposed on the basis of accumulation experiments with polydisperse copper sulfide (CuxS) nanoparticles. Particles first diffuse and adsorb at the Hg0 surface. When both the electrode and particles have negative surface potentials, this process resembles charge-impeded coagulation, obeying the Schulze−Hardy rule at various electrolyte strengths. Consequently, accumulation rates are surprisingly sensitive to electrolyte concentration. Choosing accumulation potentials where the electrode and particles have opposite surface potentials greatly improves collection efficiency, especially for the smallest particles. After adsorption, particles undergo transformations. One product is a more stable (harder to reduce) form of CuxS, interpreted to consist of adclusters or adlayers. A very significant (∼0.3 V) negative shift in reduction potential results from this transformation. Loss of analyte to at least one nonelectroactive product is also observed. Loss is greatest for the smallest particles and is sensitive to choice of accumulation potential. To improve accumulation efficiency, accumulation potentials more positive that the potential of zero charge of Hg electrodes are advantageous but care must be taken to remove dissolved chalcogenides under these conditions in order to avoid artifacts.
<p>In oxic waters, Re<sup>VII </sup>is the stable oxidation state which undergo hydrolysis to the relatively unreactive perrhenate ion, Re<sup>VII</sup>O<sub>4</sub><sup>- [1]</sup>. The oceanic dissolved Re exhibits quite conservative behaviour with the concentration of about 40 pM <sup>[2]</sup>. Despite the frequent utilization of Re for the atmosphere and the ocean past redox state reconstructions, the geochemical behaviour of Re in the modern surface environments such as rivers, estuaries as well as in seawater is not well studied. Understudy is partially arising from the fact that Re has low seawater and riverine concentration of 4&#160;pM and 16.5 pM, respectively<sup></sup><sup>[1, 3]</sup>. In the Amazon and the Hudson estuaries, in crease of Re concentration at low and middle salinity regions is observed<sup> [4]</sup>. On the other hand, Re exhibits complete conservative behaviour in Indian river estuaries, i.e. Narmada, Tapi and the Mandovi estuaries in the Arabian Sea and the Hooghly estuary in the Bay of Bengal <sup>[5]</sup>. Deviation from conservative behaviour in Re can be explained as the interplay of variety of factors including the nature and composition of the particles, Eh-pH conditions, biological productivity and fate of the organic matter. <sup>[5]</sup>.</p><p>Here we present the Re concentration profiles in the freshwater part of the karstic Krka river (Croatia) and its 23 km long estuarine segment, covering a full salinity range (0.1 to 38.6). Analysis of Re was performed by its preconcentration and separation using an anion exchange resin (Dowex 1X8) followed with the ICP-MS quantification using isotope dilution (ID) method. The Krka River spring is characterised by the low Re concentration (~6 pM). A noticeable anthropogenic influence at the point of the wastewater discharge of the Knin town was observed (27 pM). This input probably caused a progressive downstream increase of Re concentration to 12 pM at the freshwater end-member in the winter period (with a high Krka River discharge) and 17 pM in the summer period (low Krka River discharge). In the estuarine segment, a near-conservative behaviour of Re was found, with the "oceanic" concentration of 38 pM in the seawater end-member.</p><p>&#160;</p><p>References:</p><p>[1] Hasse AA et al., (2019) Coordination chemistry reviews 394: 135-161.</p><p>[2] Anbar AD et al., (1992) Geochimica et Cosmochimica Acta<strong> </strong>56:4099-4103.</p><p>[3] Miller CA et al., (2011) Geochimica et Cosmochimica Acta 75:7146-7179.</p><p>[4] Colodner D et al., (1993) Earth and Planetary Science Letters 117:205-221.</p><p>[5] Rahaman W and Singh SK (2010) Marine Chemistry 118: 1-10.</p>