Uptake of molecular hydrogen (H2) by soil is a biological reaction responsible for approximately 80% of the global loss of atmospheric H2. Indirect evidence obtained over the last decades suggests that free soil hydrogenases with an unusually high affinity for H2 are carrying out the reaction. This assumption has recently been challenged by the isolation of Streptomyces sp. PCB7, displaying the high-affinity H2 uptake activity previously attributed to free soil enzymes. While this finding suggests that actinobacteria could be responsible for atmospheric H2 soil uptake, the ecological importance of H2-oxidizing streptomycetes remains to be investigated. Here, we show that high-affinity H2 uptake activity is widespread among the streptomycetes. Among 14 streptomycetes strains isolated from temperate forest and agricultural soils, six exhibited a high-affinity H2 uptake activity. The gene encoding the large subunit of a putative high-affinity [NiFe]-hydrogenase (hydB-like gene sequence) was detected exclusively in the isolates exhibiting high-affinity H2 uptake. Catalysed reporter deposition-fluorescence in situ hybridization (CARD-FISH) experiments targeting hydB-like gene transcripts and H2 uptake assays performed with strain PCB7 suggested that streptomycetes spores catalysed the H2 uptake activity. Expression of the activity in term of biomass revealed that 10(6)-10(7) H2-oxidizing bacteria per gram of soil should be sufficient to explain in situ H2 uptake by soil. We propose that specialized H2-oxidizing actinobacteria are responsible for the most important sink term in the atmospheric H2 budget.

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Environmental context. Mercury, in its methylated form, is a neurotoxin that biomagnifies in marine and terrestrial foodwebs leading to elevated levels in fish and fish-eating mammals worldwide, including at numerous Arctic locations. Elevated mercury concentrations in Arctic country foods present a significant exposure risk to Arctic people. We present a detailed review of the fate of mercury in Arctic terrestrial and marine ecosystems, taking into account the extreme seasonality of Arctic ecosystems and the unique processes associated with sea ice and Arctic hydrology.Abstract. This review is the result of a series of multidisciplinary meetings organised by the Arctic Monitoring and Assessment Programme as part of their 2011 Assessment 'Mercury in the Arctic'. This paper presents the state-of-the-art knowledge on the environmental fate of mercury following its entry into the Arctic by oceanic, atmospheric and terrestrial pathways. Our focus is on the movement, transformation and bioaccumulation of Hg in aquatic (marine and fresh water) and terrestrial ecosystems. The processes most relevant to biological Hg uptake and the potential risk associated with Hg exposure in wildlife are emphasised. We present discussions of the chemical transformations of newly deposited or transported Hg in marine, fresh water and terrestrial environments and of the movement of Hg from air, soil and water environmental compartments into food webs. Methylation, a key process controlling the fate of Hg in most ecosystems, and the role of trophic processes in controlling Hg in higher order animals are also included. Case studies on Eastern Beaufort Sea beluga (Delphinapterus leucas) and landlocked Arctic char (Salvelinus alpinus) are presented as examples of the relationship between ecosystem trophic processes and biologic Hg levels. We examine whether atmospheric mercury depletion events (AMDEs) contribute to increased Hg levels in Arctic biota and provide information on the links between organic carbon and Hg speciation, dynamics and bioavailability. Long-term sequestration of Hg into non-biological archives is also addressed. The review concludes by identifying major knowledge gaps in our understanding, including:(1) the rates of Hg entry into marine and terrestrial ecosystems and the rates of inorganic and MeHg uptake by Arctic microbial and algal communities; (2) the bioavailable fraction of AMDE-related Hg and its rate of accumulation by biota and (3) the fresh water and marine MeHg cycle in the Arctic, especially the marine MeHg cycle.

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Streptomyces soil isolates exhibiting the unique ability to oxidize atmospheric H 2 possess genes specifying a putative high-affinity [NiFe]-hydrogenase. This study was undertaken to explore the taxonomic diversity and the ecological importance of this novel functional group. We propose to designate the genes encoding the small and large subunits of the putative high-affinity hydrogenase hhyS and hhyL, respectively. Genome data mining revealed that the hhyL gene is unevenly distributed in the phyla Actinobacteria, Proteobacteria, Chloroflexi, and Acidobacteria. The hhyL gene sequences comprised a phylogenetically distinct group, namely, the group 5 [NiFe]-hydrogenase genes. The presumptive high-affinity H 2 -oxidizing bacteria constituting group 5 were shown to possess a hydrogenase gene cluster, including the genes encoding auxiliary and structural components of the enzyme and four additional open reading frames (ORFs) of unknown function. A soil survey confirmed that both high-affinity H 2 oxidation activity and the hhyL gene are ubiquitous. A quantitative PCR assay revealed that soil contained 10 6 to 10 8 hhyL gene copies g (dry weight) ؊1 . Assuming one hhyL gene copy per genome, the abundance of presumptive high-affinity H 2 -oxidizing bacteria was higher than the maximal population size for which maintenance energy requirements would be fully supplied through the H 2 oxidation activity measured in soil. Our data indicate that the abundance of the hhyL gene should not be taken as a reliable proxy for the uptake of atmospheric H 2 by soil, because high-affinity H 2 oxidation is a facultatively mixotrophic metabolism, and microorganisms harboring a nonfunctional group 5 [NiFe]-hydrogenase may occur.