Antecedentes artísticos

The microbial community of the elemental sulfur (S0) on the surface of the Borup Fiord

Pass Glacier, Canadian High Arctic, is dominated by members which possess the genetic

capability to undertake a range of sulfur redox reactions, and in particular to oxidize reduced

sulfate (the most oxidized form of sulfur). The geochemical data, bioenergetic calculations, small

subunit ribosomal RNA (SSU rRNA) data and the quantitative functional gene data from the

metagenome all support the conclusion that this community is driven by sulfur lithotrophy as the

main energy source for primary productivity. The deep deposit on which the metagenome

analysis was performed was dominated by Sulfurovum and Sulfuricurvum, Epsilonproteobacteria

known to be capable of sulfur lithoautotrophy. Their numerical dominance in the deposits,

combined with the fact that the most abundant sulfur redox and carbon fixation genes in the

metagenome appear, by sequence comparison, to be of Epsilonproteobacterial origin, indicate

that these Epsilonproteobacteria are most likely the major primary producers and sulfur cyclers.

This is the first time that these Genera have been reported as being dominant in a sub-aerial

environment, as they have previously been found in anoxic and sulfidic environments such as

hydrothermal vents and sulfidic caves. By far the most abundant energy source available in the

glacial deposits is the S0 and as cultured representatives of these Genera have previously been

shown in culturing studies to be capable of oxidizing S0 to sulfate it is likely that the Borup

Epsilonproteobacteria are using this reaction to obtain energy for growth. The genes responsible

for oxidizing S0 to sulfate are not fully known, but the metagenome shows high levels of

abundance of the sox gene cluster, known to be able to oxidize S0 to sulfate in vitro. There is

disproportionately-high relative abundance of DsrE genes in the metagenome compared to the

other Dsr genes, and as the DsrE gene product is known to be involved in mobilization of

internal S0 deposits in bacteria which possess the entire Dsr cluster, this raises the interesting

question of whether the DsrE gene in these Epsilonproteobacteria might be involved in

There is a curious absence of both SSU rRNA and functional genes from phototrophs in

the metagenome, indicating that photosynthesis is not being significantly utilized by this

community despite the presence of abundant light (24-hour daylight during the arctic summer

when the samples were collected). There was also an absence of genes for both the aerobic and

anaerobic oxidation of ammonium (nitrification and anammox, respectively) despite the fact that

both metabolisms should be energetically favorable.

The cultured representatives of Sulfurovum and Sulfuricurvum have been shown to prefer

microaerophilic rather than fully oxygenated conditions and it was noteworthy that the surface of

S0 deposits on the glacier are dominated by Flavobacterium. It is possible that the aerobic

metabolism of this Flavobacterium is drawing down the oxygen level sufficiently to create a

microaerophilic climate in the lower regions of the deposit. This dominant Flavobacterium was

isolated, and culturing studies indicated that it was capable of oxidizing thiosulfate to sulfate, but

was not able to gain energy for growth from this reaction. It is therefore most likely gaining

energy from the oxidation of carbon compounds. One possibility that would explain its

dominance near the spring is that the spring water contains methane, and some Flavobacterium

sp. have shown the ability to utilize C1 compounds to obtain energy for growth. However, a key

gene known to be involved in methane oxidation, methane monooxygenase, was not strongly

represented in the metagenome.

A Gillisia sp. isolate, also a member of the Flavobacteriaceae family, was also isolated

from cultures originally inoculated with sulfur from the Borup glacial deposits. This isolate

showed the ability to create unusual S0-biomineralized structures although it is not clear whether

it is actively metabolizing sulfur species or simply catalyzing a novel precipitation process.

process is significant in the Borup system as the Gillisia is numerically a minor constituent and

the biomineralized structures have not so far been observed in the environmental S0 deposits.

The dominant members of the community in the S0 deposits are very different from that

of the stream. This suggests that the change in environment going from an anoxic, sulfide-rich,

subsurface to the surface S0 deposits which have ample access (at least at their surface) to

atmospheric oxygen allows different microbes to be successful in each environment. The

dominant members of the stream microbial community are Burkholderiaceae members

Burkholderia and Ralstonia, but their role in the system was not investigated as part of this

study. Both Burkholderia and Ralstonia representatives have shown the ability to oxidize

thiosulfate, but while Burkholderia could conserve energy for growth from this reaction,

Ralstonia has not so far shown this ability (Anandham et al., 2008, Cramm 2009). The role that Burkholderia and Ralstonia are playing within the Borup Glacial spring and sulfur deposits

therefore remains unknown.