A Contemporary Microbially Maintained Subglacial Ferrous "Ocean"

Abstract

An active microbial assemblage cycles sulfur in a sulfate-rich, ancient marine brine beneath Taylor Glacier, an outlet glacier of the East Antarctic Ice Sheet, with Fe(III) serving as the terminal electron acceptor. Isotopic measurements of sulfate, water, carbonate, and ferrous iron and functional gene analyses of adenosine 5'-phosphosulfate reductase imply that a microbial consortium facilitates a catalytic sulfur cycle. These metabolic pathways result from a limited organic carbon supply because of the absence of contemporary photosynthesis, yielding a subglacial ferrous brine that is anoxic but not sulfidic. Coupled biogeochemical processes below the glacier enable subglacial microbes to grow in extended isolation, demonstrating how analogous organic-starved systems, such as Neoproterozoic oceans, accumulated Fe(II) despite the presence of an active sulfur cycle.

Subglacial Lake Whillans microbial biogeochemistry: a synthesis of current knowledge

“…It is possible that these genes are related to uncharacterized sulfur-oxidizing prokaryotes. Similar sequences are found in other environmental samples including lake sediments [81,82] and in Blood Falls [8] but the organisms containing them have not been isolated or characterized. Below 8 cm, key genes involved in sulfate reduction were observed [52].…”

“…For example, cell abundances of 2.1 × 10 4 cells ml −1 were detected in Grímsvötn lake water in Iceland, and 3.8 × 10 7 cells g −1 of lake sediment [67], whereas abundances that are comparable with SLW were observed in a different subglacial volcanic lake in Iceland, West Skaftá (4.7-5.7 × 10 5 cells ml −1 ; [68]). Blood Falls, a subglacial outflow in the McMurdo Dry Valleys of Antarctica, contained an average 6 × 10 4 cells ml −1 during an outflow event in 2004 [8].…”

“…It is now accepted that there are a variety of aqueous features beneath ice sheets including 'wetlands' or saturated sediments, streams, rivers and lakes [3,4]. As of this writing, 379 perennial subglacial lakes have been identified below the Antarctic ice sheets [5] and these lakes likely represent a variety of chemistries ranging from fresh [6,7] to highly saline [8,9]. Subglacial hydrological environments require meltwater, which forms from pressure-induced melting of basal ice or heating from geothermal flux at the bed [10].…”

“…Nonetheless, these differences in community structure may result in part from the hydrology of each environment and residence time of water in the system. Rapid turnover of the water in the lake system (years to decades as in the case of SLW) probably provides more frequent replenishment of 'high-energy' electron acceptors such as oxygen and nitrate from basal ice melt while slow turnover (millennia or longer as in the case of Blood Falls; [8]) results in the depletion of these compounds.…”

“…The E h level for the water column of SLW has been reported as 382 mV [7] or equivalent to pε = 7.06 at in situ temperatures. In Blood Falls, the outflow of subglacial water from beneath the Taylor Glacier in the McMurdo Dry Valleys, E h was determined to be 90 mV ( [8]; or pε = 1.69). These results are consistent with the low but measureable levels of oxygen and nitrate reported for SLW, and the lack of oxygen and oxidized forms of nitrogen in Blood Falls outflow [7,8], respectively.…”