Annette Summers Engel
Donald and Florence Jones Professor of Aqueous Geochemistry
University of Tennessee at Knoxville
Department of Earth and Planetary Sciences
Karst landscapes comprise 20-25% of the ice-free land surface, and nearly 20% of society worldwide relies on karst aquifers for economic, urban, and environmental fresh water. Because most karst aquifers have strong hydrological connections with the surface, which bring rapid and focused recharge and potentially polluted water under turbulent flow regimes, karst systems are vulnerable to disturbance and environmental change. Over the past decade, our research has focus on he Edwards Aquifer in Central Texas, and the fresh water to saline water interface (FWSWI), known locally as the “bad-water” line. The juxtaposition of freshwater with saline, hydrogen sulfide-rich water creates a unique, energy-rich habitat for microbes. Our work was the first to investigate the FWSWI microbes and to demonstrate their geological and ecological significance. Microbes change water-rock equilibria, which can lead to carbonate dissolution due to colonizing surfaces and lowering local pH. Based on our recent experiments, thick biofilms can create a hydrophobic barrier to solute transport that can decrease carbonate dissolution rates over time.
These biofilms also serve as important food sources for animals in the aquifer, mostly proximal to the FWSWI. With increasing depth, surface to subsurface interactions and terrigenous inputs into the aquifer diminishes, water-rock interactions increase because of longer water and solute residence times, and microbial metabolism shifts from heterotrophy (i.e. energy from organic carbon consumption) to chemolithoautotrophy (i.e. fixation of inorganic carbon to organic carbon via chemical energy). Changes in microbial metabolism (i.e. heterotrophy versus chemolithoautotrophy) are evident in the carbon and nitrogen isotopic signature of invertebrates in the aquifer because they record the signature of their food source(s) and of the food web structure. 
We recently began a project focused on microbial mediation in karstification during  organic carbon cycling in caves not associated with sulfide, and tfrom caves that are closer to UT in Kentucky. Our primary field site is Cascade Cave, in Carter Caves State Resort Park, Kentucky. The system is relatively simple, having <2 km underground flowpath (in at least four caves) with one main recharge point, minimal other inputs, and one main discharge point. Our continued research addresses several questions: What are the sources of microbes into the subsurface? Can recharge and colonization sources (i.e. inputs) be tracked over time? How does the rate of carbonate dissolution differ between surface (epigenic) caves and the sulfidic section of the Edwards Aquifer?
Edwards Aquifer, Texas
Cascade Cave, Kentucky
Current Support: Jones Endowment, University of Tennessee
Previous Support: Louisiana Board of Regents
Dr. Benjamin Schwartz, Texas State University
Dr. Benjamin Hutchins, Texas Parks and Wildlife, Austin, Texas

Publications from Group

Hutchins, B.T., Engel, A.S., Nowlin, W.H., Schwartz, B.F. (2016) Chemolithoautotrophy supports subterranean food webs and affects regional diversity and long-term stability of stygobiont communities. Ecology. 97: 1530–1542. DOI: doi/10.1890/15-1129.1/.
Hutchins, B.T., Engel, A.S., Nowlin, W.H., Schwartz, B.F. (2016) Biodiversity, stability, and trophic complexity in the Edwards Aquifer, United States: The influence of chemolithoautotrophy on stygobiont community structure. International Conference for Subterranean Biology, Fayetteville, Arkansas. June 12-17, 2016. p. 44. 

Hutchins, B., Schwartz, B., Engel, A.S. (2013) Environmental controls on organic matter production and transport across surface-subsurface and geochemical boundaries in the Edwards Aquifer, Texas, USA. Acta Carsologica 42/2-3: 245–259. 
Gray, C.J., Engel, A.S. (2013) Microbial diversity and impact on carbonate geochemistry across a changing geochemical gradient. ISME Journal. 7: 325:337. DOI:10.1038/ismej.2012.105. 
Engel, A.S., Randall, K.W. (2011) Experimental evidence for microbially-mediated carbonate dissolution from the saline water zone of the Edwards Aquifer, Central Texas. Geomicrobiology Journal. 28(3): 313-327.
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)
Engel, A.S. (2009) Sulfur-rich karst systems, in Palmer, A.N. and Palmer, M.V. (eds.), Caves and Karst of the USA. National Speleological Society: Huntsville, Alabama, p. 404.
Porter, M.L., Engel, A.S. (2008) Balancing the conservation needs of sulphidic caves and karst with tourism, economic development, and scientific study: Cave and Karst Science. 35:19-24.  

Related Conference Abstracts from Group

Engel, A.S., Hutchins, B., Nowlin, W., Schwartz, B.F. (2018) Profiling the functional diversity of microbial communities to understand the development and maintenance of the Edwards Aquifer ecosystem. Geological Society of America Abstracts with Programs. Indianapolis, IN; 50(6), doi: 10.1130/abs/2018AM-322716.

Engel, A.S., Hutchins, B., Schwartz, B., Gray, C., Birdwell, J.E. (2011) Evidence for a non-terrigenous food web base in the Edwards Aquifer, Texas. National Speleological Society Convention, Glenwood Springs, Colorado, July 16-24. (oral presentation)     
Hutchins, B., Schwartz, B. Engel, A.S. (2011) Linking hydrogeology, geochemistry, and microbiology to food web dynamics in the biodiverse phreatic karstic Edwards Aquifer, TX, USA. North American Benthological Society; Providence, Rhode Island. May 22 – 26. (poster presentation)
Gray, C., Engel, A.S. (2009) Changes in microbial diversity along a geochemical gradient in the Edwards Aquifer, Texas: Possible implications for aquifer modification and ecosystem processes. AGU Annual Meeting, San Francisco, CA: ID# B23C-0392.
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).  
Birdwell, J., Schulz, C., Engel, A.S. (2007) Autochthonous dissolved organic matter in karst waters: Evidence from fluorescence spectroscopy. Eos Trans. American Geophysical Union, 88(52), Fall Meet. Suppl., Abstract H43C-1511. (poster presentation)
Engel, A.S., Randall, K.W. (2006) Microbial colonization and weathering of carbonates in deep subsurface settings. International Mineralogical Association, Kobe, Japan, p. 248. (Invited oral presentation)
Randall, K.W., Johnson, L., Engel, A.S.  (2005) Assessing the potential impact of microbes in the Edwards and Trinity aquifers of Central Texas. GSA Abstracts with Programs, 37: 217. (poster presentation)

Field Photographs

Geochemistry. Geomicrobiology. Geology. Ecology.

Interdisciplinary Research at the University of Tennessee