MICROBIAL UTILISATION OF BEDROCK COMPONENTS DURING CHEMICAL WEATHERING IN SUBGLACIAL ENVIRONMENTS

Sub-glacial environments have been considered to be too extreme for life due to a combination of low temperatures and lack of energy sources. However, a range of recent research has demonstrated the presence of significant bacterial populations in this habitat, with activity occurring down to at least 0.3°C. But what energy sources are available for these organisms? Indirect geochemical evidence indicates that sub-glacial bacteria may be oxidizing bedrock kerogen and metal sulphides. For example, ¹⁸O-SO₄^2- and ¹³C-HCO₃- of runoff from Finsterwalderbreen, a polythermal-based glacier with a predominantly shale, sandstone and limestone bedrock on Svalbard, shows clear evidence of sulphate reduction and methanogenesis. This is consistent with excess HCO₃- relative to SO₄^2- in the runoff at this site. In addition, low temperature (7°C) laboratory microbial weathering studies demonstrate significant SO₄^2- production and changes in NO₃- and Mg²⁺, compared to sterile controls, indicating an important role for bacteria in low temperature mineral weathering reactions. Comminution of the bedrock produces reactive, large surface area particles which may continuously provide bacteria with organic and inorganic substrates and nutrients. Sequential microbial utilization of these compounds may drive the system anaerobic enabling a range of different reactions to occur e.g. S^2- and CH₄ production. For example, sulphate-reducing bacteria are present in both the Greenland and Norwegian Ice Sheets. In Greenland, these bacteria exhibit a discrete range of temperature optima (4, 8, 17-22°C) and thus it is likely that different psychrophilic/psychrotolerant populations will dominate at different stages of the melt season and in different parts of the glacier. These results demonstrate that bacterial activity is important for sub-glacial weathering reactions and these supply energy to maintain bacterial populations in a seemingly hostile environment.