Annette Klæstrup Møller:
Bacterial reduction of mercury in the high arctic

Date: 15-08-2010    Supervisor: Søren J. Sørensen

It is well-established that mercury (Hg) from lower latitudes is transferred to and pollutes the Arctic environment. One mechanism of Hg transfer is through the atmosphere where Hg is deposited in the Arctic in the spring time during Atmospheric Mercury Depletion Events (AMDE): large amounts of Hg is believed to be depleted from the atmosphere and deposited onto snow and sea-ice through photochemical reactions. The faith of mercury after deposition is poorly understood and while bacteria are known to play an important role in the bio-geochemical Hg cycle in various temperate environments, their role in the dynamics of Hg deposited in the Arctic is unknown. In this PhD study the bacterial communities in snow, freshwater and sea-ice in Northeastern Greenland were examined with focus on Hg resistant bacteria. The PhD study consists of three parts: 1) examination of the bacterial communities in snow and freshwater both by applying culture dependent and independent techniques (pyrosequencing) 2) Identification of Hg resistant bacteria from snow, freshwater and sea-ice and 3) Identification and investigation of Hg resistance genetic elements in arctic Hg resistant isolates.

Cultivation of bacterial isolates from three snow depths and freshwater only showed a scattered representation of the phyla and genera in comparison to strains identified by culture independent methods. The microbial composition of all arctic sample sites was significantly different, with the two uppermost snow layers being most similar to each other. The freshwater environment was less diverse as compared to all snow environments most likely reflecting the freshwater environment as a less extreme and more stable environment than in snow. For both snow and freshwater, abundant bacterial phyla included higher numbers of genera than the rare phyla, suggesting that the ecological success of a bacterial phylum depends on the diversity rather than the dominance of a few genera. The most dominant phyla included Proteobacteria, Actinobacteria, Bacteroidetes, Cyanobacteria and Firmicutes in the snow and Proteobacteria, Bacteroidetes, Actinobacteria and Planctomycetes in freshwater. The bacteria identified in this study both included phylotypes commonly found in cold environments as well as rare phylotypes.

During the time of sampling atmospheric ozone measurements and total Hg measurements in the snow indicated that Atmospheric Mercury Depletion Events were taking place, therefore the bacterial resistance to mercury was assesed. In snow, Hg resistant bacteria accounted for up to 31% of the culturable bacteria, but were below 2% in freshwater and sea-ice. The resistant bacteria belonged to the α-, β- and γ-Proteobacteria, Firmicutes, Actinobacteria, and Bacteriodetes. It was found that 25% of the isolates resistant to Hg also reduced HgII to Hg0, although there was no correlation between level of resistance, ability to reduce HgII, and taxonomic group. An estimation of the potential bacterial reduction of HgII in snow suggested that this may be important in the deeper snow layers. This highlights the importance of microbial mercury transformation in the biogeochemical mercury cycling in the High Arctic. While bacterial Hg reduction by the mercuric reductase, MerA, is widespread in temperate environments, its distribution and abundance in the Arctic is largely unexplored. MerA loci were found in six taxonomic classes (α-, β- and γ-Proteobacteria, Actinobacteria, Flavobacteria and Bacilli) among the high Arctic mercury resistant isolates. Eight different merA sequences were identified; five of which (from α-, β- and γ-Proteobacteria) showed high similarity (99-100%) to proteins in the Genbank database while the three others were less similar (82-92%) to any protein sequences. Of the 71 mercury resistant isolates, only 26 carried a detectable merA and, thus, several other merA sequences or Hg reduction mechanisms may be found in the Arctic. Of the Hg resistant isolates, 24% carried plasmids and two out of the five sequenced plasmids contained a mer-operon. The presence of plasmids carrying mer-operons, an uneven distribution of merA, the level of sensitivity and the ability to volatilize Hg within the different taxonomic groups could indicate lateral transfer of the merA genes. Whole genome sequencing revealed a simple mer-operon in a Flavobacterium isolate. Unique for this putative mer-operon was the regulatory element; instead of the common merR this operon was initiated by arsR, which is common for mer-operons in Archaeal species. Clustering of this putative MerA sequence along with other putative MerA sequences from other Bacteroidetes showed a closer phylogenetic distance to Archaeal MerA sequences than any other bacterial MerA sequences. The results suggest that bacterial communities in the Arctic, especially in snow covers, may play in important role in the Hg transformation in the Arctic environment. Furthermore, the results indicate that a diverse and yet undiscovered pool of merA exists in arctic bacterial assemblages, and these genes may be distributed in the community through lateral gene transfer.