Annette Summers Engel
Jones Professor of Aqueous Geochemistry
University of Tennessee at Knoxville
Department of Earth and Planetary Sciences
This research focuses on understanding geochemical and enzymatic controls that affect microbial transformations of dissolved organic matter (DOM) as water moves from the surface into the subsurface, including changes functional microbial diversity (i.e. at the level of genes and enzymes). 
This research direction started from the Edwards Aquifer study to determine how much organic matter from the surface enters the deep karst aquifer. There are notable difference between surface-derived DOM and DOM produced within a system by microbial communities, such as chemolithoautotrophs. Surface water is dominated by dissolved humic substances that are generally recalcitrant, such as fulvic acids. Even photosynthetic microbial activity produces humic-like substances. But, microbial activity in the subsurface produces polysaccharides, alkyl alcoholds, ketones, amides, and other hydrophilic and neutral compounds that are more labile. DOM in the subsurface is also protected from photoxidation processes, making caves ideal habitats to investigate DOM production and cycling.
 
We use spectroscopy, wet chemistry, and molecular methods to conduct the research. We also conduct experiments with cave samples to determine how different types of DOM, sourced from different surface plants, soils, and sediments, may affect microbial activity and stimulate growth. In one experiment, surface and cave waters (with distinct microbial communities) that were incubated with different DOM, sourced from either deciduous tree leachate, or mixed grass leachate, or a protein-rich substance, had differing responses. Faster DOM degradation rates were observed for surface waters compared to cave waters, with tree leachate having the slowest rates. This research is ongoing, and we are now using metagenomics to understand the genetic potential of the microbial communities in the waters (surface and cave) and potential enzymatic diversity for DOM transformations. 
 
Current funding: Jones Endowment, University of Tennessee; Cave Conservancy Foundation (to PhD students Terri Brown and Kathleen Brannen)
 
Collaborators: Terri Brown, Kathleen Brannen (Engel group PhD students)
 
Several UTK undergraduates have also conducted research on this project: Scott Beeler, Lauren Parker, and E. Ross Larsen

Student Work

Brannen-Donnelly, K. (2015) Microbial diversity and function within an epigenic cave system and implications for other limestone cave systems. Ph.D. dissertation, University of Tennessee, Knoxville, TN. 

Related Publications and Conference Abstracts from Group

Brannen-Donnelly, K., Engel, A.S. (2015) Bacterial diversity differences along an epigenic cave stream reveal evidence of community dynamics, succession, and stability. Frontiers in Microbiology. doi:10.33 89/fmicb.2015.00729
 
Brannen-Donnelly, K., Engel, A.S. (2015) Evidence for stable functional niches in an epigenic cave system prone to disturbance. National Speleological Society convention, Waynesville, Missouri, July 13-17, 2015. (talk)  
 
Brannen, K.M., Engel, A.S. (2014) How prevalent are transported microorganisms to the microbial community of an epigenic cave stream? International Symposium on Subsurface Microbiology, Pacific Grove, CA, Oct. 5-10, 2014. (poster)
 
Brown, T., Brannen, K.M., Engel, A.S., Pfiffner, S. (2014) Diurnal changes in carbon cycling dynamics in an oligotrophic karst aquifer under base flow conditions. International Symposium on Subsurface Microbiology, Pacific Grove, CA, Oct. 5-10, 2014. (oral presentation)
 
Brown, T., Engel, A.S. (2014) Nitrogen enrichment of autochthonous dissolved organic matter in a surface-influenced karst aquifer. International Symposium on Subsurface Microbiology, Pacific Grove, CA, Oct. 5-10, 2014. (oral presentation)
 
Parker, L., Brown, T., Engel, A.S. (2014) Extended alkaline incubation times to quantify protein content in karstic aquifers and other oligotrophic groundwater systems. 1st Annual Southeastern Biogeochemistry Symposium, Atlanta, GA, April 5-6, 2014. (poster)
 
Brannen, K.M., Engel, A.S. (2014) Seasonality influences allochthonous organic matter and microbial community diversity in a cave stream. 1st Annual Southeastern Biogeochemistry Symposium, Atlanta, GA, April 5-6, 2014. (poster presentation)
 
Engel, A.S., Brannen, K.M., Brown, T., Beeler, S. (2013) Seasonal changes to cave stream microbial communities influence dissolved organic matter production and transformations. Program guide to the 70th annual National Speleological Society convention, Shippensburg, PA, 2013, p. 41. (oral presentation)
 
Beeler, S.A., Brannen, K.M., Engel, A.S. (2013) Seasonal variations in degradation of dissolved organic matter by groundwater microbial communities in a karst systems. Geological Society of America meeting, Denver, CO, October 27-30, 2013. (poster)
 
Brannen, K.M., Engel, A.S., Larson, E.R. (2013) Microbial controls on in situ production of dissolved organic matter, in Fong, D.W., Culver, D.C., Veni, G., Engel, S.A. (eds.) Carbon and Boundaries in Karst, Abstracts of the conference held January 7 – 13, 2013, Carlsbad, New Mexico.  Karst Waters Institute Special Publication 17, Karst Waters Institute, Leesburg, Virginia. p. 2.  (poster presentation).
 
Brown, T., Engel, A.S., Pfiffner, S. (2013) Component isolation and lipid profiling to characterize dissolved organic matter transformations along a groundwater flow path, in Fong, D.W., Culver, D.C., Veni, G., Engel, S.A. (eds.) Carbon and Boundaries in Karst, Abstracts of the conference held January 7 – 13, 2013, Carlsbad, New Mexico.  Karst Waters Institute Special Publication 17, Karst Waters Institute, Leesburg, Virginia. p. 4.  (poster presentation).
  
Brown, T., Engel, A.S., Brannen, K.M., Birdwell, J.E. (2011) Estimating aquifer contributions to carbon cycle budgets based on dissolved organic matter characterization in karts aquifers, in Engel, A.S., Engel, S.A., Moore, PJ, and DuChene, H. (eds.), Carbonate Geochemistry: Reactions and Processes in Aquifers and Reservoirs, Karst Waters Institute Special Publication 16: Leesburg, Virginia, p. 8-9.
  
Brown, T., Engel, A.S., Birdwell, J.E. (2011) Isolation and analyses of dissolved organic matter to identify microbial processes in karst aquifers. National Speleological Society Convention, Glenwood Springs, Colorado, July 16-24. (oral presentation)  
 
Brown, T., Engel, A.S. (2011) Comparison of field-employed extraction and isolation methods to characterize dissolved organic matter functional fractions from karst waters. 3rd International Multidisciplinary Conference on Hydrology and Ecology: Ecosystems, Groundwater and Surface Water – Pressures and Options. Vienna, Austria. May 2-5. (poster presentation)
 
(102) Brown, T., Engel, A.S. (2011) Alternative approaches to DOM characterization. HydroDays, US Geological Survey Karst Interest Group Meeting, University of Arkansas, Fayetteville, Apr. 26-29, 2011. (poster presentation)
  
Brannen, K.M., Birdwell, J.E., Engel, A.S. (2011) Qualitative assessment of dissolved organic matter in cave and karst waters using the Synch-75 Index. International Symposium for Environmental Biogeochemistry, Istanbul, Turkey, Sept. 25-30, 2011. (oral presentation)
 
Brannen, K.M., Engel, A.S., Birdwell, J.E. (2011) The SYNCH-75 index concept for the qualitative assessment of dissolved organic matter in cave and karst waters. National Speleological Society Convention, Glenwood Springs, Colorado, July 16-24. (oral presentation)  
  
Brannen, K.M., Maas, B., Engel, A.S. (2010) Impact of microbial processes on dissolved organic matter transformations in karst aquifers: A case study from the Cascade Cave System, Kentucky. Geological Society of America meeting, Denver, CO. (poster presentation)
  
Brannen, K.M., Engel, A.S., Birdwell, J.E., (2010) Creating humic matter indices for interpreting ecosystem energetics. 20th International Conference on Subterranean Biology, Postjona, Slovenia. Aug 27-Sept. 3, 2010. (oral presentation; K.M. Brannen won 1st place for oral student presentations)
 
Brannen, K.M., Engel, A.S., Birdwell, J.E., Creating humic matter indices for the interpretation of ecosystem energetics, in International Training Workshop on Organic Matter Characterization Using Spectroscopic Techniques May 19-21, 2010 - Granada, Spain, p. 1-2 (poster abstracts) http://spectroscopyworkshop.weebly.com/poster-info.html. (poster presentation)
  
Birdwell, J.E., Engel, A.S. (2010) Characterization of dissolved organic matter in cave and spring waters using UV-Vis absorbance and fluorescence spectroscopies. Organic Geochemistry. 41: 270-280.
  
Birdwell, J.E., Engel, A.S. (2009) Variability in terrestrial and microbial contributions to dissolved organic matter fluorescence in the Edwards Aquifer, Central Texas. Journal of Cave and Karst Studies. 71: 144-156. (includes published cover photograph for the issue)
 
Brannen, K.M., Engel, A.S., and Birdwell, J.E. (2009) Tracing the origins of chromophoric dissolved matter in karst aquifers, in White, W.B. (ed.), Proceedings of the 15th International Congress of Speleology, vol. 1, symposia part 1, p. 354-359.
 
Birdwell, J.E., Schulz, C., Engel, A.S. (2008) Chromophoric dissolved organic carbon from cave and karst waters, in Kato, K., and Kimura, H., 7th International Symposium for Subsurface Microbiology, Shizuoka, Japan, p. 62. (oral presentation)
 
Birdwell, J.E., Engel, A.S. (2008) Changes in the fluorescence of chromophoric dissolved organic matter in a central Texas aquifer between extreme climatic events, in Kato, K., and Kimura, H., 7th International Symposium for Subsurface Microbiology, Shizuoka, Japan, p. 37 (poster presentation).
  

Field Photographs

Geochemistry. Geomicrobiology. Geology. Ecology.

Interdisciplinary Research at the University of Tennessee