Organics From Microbes

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Fig 1. Lake Chad and the Seuss Glacier in Taylor Valley in the McMurdo Dry Valleys, South Victorialand, Antarctica.
Rebecca Witherow, www.mcmlter.org

The McMurdo Dry Valleys and other desert oases on the coast of East Antarctica are essentially "plant-free" environments. As a result the dissolved organic material (DOM) present in the water of the dry valley lakes and streams is derived only from the breakdown of biomass originally produced by microbes, e.g. algae and bacteria.

These microbes grow in the water column of the lakes, such as Lake Fryxell in Taylor Valley (Fig.1), and form mats that cover the streambeds. Studies by MCM scientists have shown that this microbial DOM has chemical characteristics that are distinct from those of DOM derived from leaf litter on the forest floor or from wetland plants which can give water a yellow-color. Based on these chemical differences, a simple spectroscopic measurement (the fluorescence index, FI) was developed (McKnight et al. 2001). This index has been used by many scientists to understand processes that control carbon and nutrient cycling in diverse aquatic and soil ecosystems and can be used to design in situ sensors. For example, studies using the index have shown that in lakes DOM can become more "microbial" in character during summer blooms of algae (Miller et al 2009).

The carbon to nitrogen ratio in DOM and the fluorescence index (FI) for all samples from the 13 LTER and other field sites from across the United States. DOM samples with a higher FI have a greater contribution from algae and bacteria, i.e. are more similar to Antarctic lake DOM and are richer nitrogen compared to carbon (lower C:N ratio). The asterisk indicates significant at the 95% confidence level.
Jaffe R, McKnight D, Maie N, Cory, R., McDowell, W. H., Campbell, J. L. 2008. Spatial and temporal variations in DOM composition in ecosystems: The importance of long-term monitoring of optical properties. Journal of Geophysical Research-Biogeosciences113G4G04032 .
For further reading: 
McKnight, D.M., E.W. Boyer, P.K. Westerhoff, P.T. Doran, T. Kulbe, and D.T. Andersen (2001). Spectrofluorometric characterization of dissolved organic matter for indication of precursor organic material and aromaticity. Limnology and Oceanography, v. 46, p. 38-48.
Miller, M. P., D. M. McKnight, S. C. Chapra and M.W. Williams. 2009. A model of degradation and production of three pools of dissolved organic matter in an alpine lake. Limnol.Oceanogr.54:2213-2227.
Monteith et al.. 2007. Dissolved organic carbon trends resulting from changes in atmospheric deposition chemistry. Nature 450: 537-540.
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