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Annette Summers EngelAnnette Summers Engel

Jones Professor of Aqueous Geochemistry

Department of Earth and Planetary Sciences
713 Strong Hall
1621 Cumberland Ave
Knoxville, TN 37996-1410

Phone: (865) 974-0402
Lab: (865) 974-9777

Nearly all low-temperature geochemical and geological processes on the Earth's surface and within the shallow subsurface are influenced or controlled by microorganisms. The overall goals in my research group are to understand the nature and extent of microbial activities in geochemical reactions that involve aqueous solutions and geological materials through time, to determine the extent to which microbes affect biogeochemical cycles and ecosystem-level processes, including in symbiotic associations, and to evaluate the mechanisms by which microorganisms have come to colonize so many different environments on Earth. We are especially interested in chemolithoautotrophy because microbes capable of this metabolism are able to harness chemical energy by scavenging nutrients from rocks and water. We use a range of field and lab experimental and theoretical approaches, focusing on terrestrial habitats like caves, aquifers, hot springs, and sediments, but also coastal marine habitats like marshes and sea grass beds, as well as microbes in symbiotic association with animals, including insects, clams, and alligators.

Current projects include:

  • The geochemical and geologic consequences of subsurface (cave and groundwater) microbial communities, including karstification and the production and cycling of organic matter;
  • Sulfur gas production and cycling in the subsurface by biotic and abiotic mechanisms, and the effects of low-temperature diagenetic processes in overprinting biogeochemical signatures in the geologic record;
  • Epsilonproteobacterial molecular diversity, biogeography, and metabolic capabilities in terrestrial environments;
  • Molecular diversity, biogeography, and biogeochemical contributions of bacterial symbiotic associations in lucinid bivalves (clams) and the American alligator;
  • Hot spring geomicrobiology and biogeochemistry of arsenic and other metals;
  • Hydrocarbon degradation potential of microbes in coastal environments (e.g., beach sand, marsh grass, oyster beds, open water) that were unimpacted and impacted by the Deepwater Horizon oil spill.


Our research is currently supported by the National Science Foundation, the Gulf of Mexico Research Initiative, the Cave Conservancy Foundation, and the University of Tennessee. Previous research has been supported by the National Science Foundation, the Louisiana Board of Regents, and generous anonymous donors. Student research and travel expenses have been supported by the American Association of Petroleum Geologists, the Geological Society of America, the Coastal and Estuarine Research Federation, and Sigma Xi.

Edited Books

  • Klimchouk, A., Sasowsky, I.D., Mylroie, J., Engel, S.A., Engel, A.S. (editors) (2014) Hypogene Cave Morphologies. Selected papers and abstracts of the symposium held February 2 through 7, 2014, San Salvador Island, Bahamas. Karst Waters Institute Special Publication 18: Leesburg, Virginia, 111 p.
  • Engel, A.S., Engel, S.A., Moore, P.J., and DuChene, H. (eds.) (2011) Carbonate Geochemistry: Reactions and Processes in Aquifers and Reservoirs. Selected papers and abstracts of the symposium held August 6 through 9, 2011, Billings, Montana. Karst Waters Institute Special Publication 16, Karst Waters Institute, Leesburg, Virginia, 84 p.
  • Engel, A.S. and Engel, S.A. (eds.) (2009) Select Field Guides to Cave and Karst Lands of the United States. Karst Waters Institute Special Publication 15, Leesburg, Virginia, 184 p.


  • Turner, R.E., Overton, E.B., Ashton, B.M., Miles, M.S., McClenachan, G., Hooper-Bui, L., Engel, A.S., Swenson, E.M., Lee, J.M., Milan, C.S., Gao, H. (2014) Distribution and recovery trajectory of Macondo (Mississippi Canyon 252) oil in Louisiana salt marshes. Marine Pollution Bulletin.
  • Headd, B., Engel, A.S. (2014) Congruency among bacterial communities from terrestrial sulfidic springs. Frontiers in Microbiology – Extreme Microbiology section. DOI: 10.3389/fmicb.2014.00473.
  • Hooper-Bui, L., Rabalais, N.N., Engel, A.S., Turner, R.E., McClenachan, G., Robert, B., Overton, E.B., Justic, D., Strudivant, K., Brown, K., Conover, J. (2014) Overview of Research into the coastal effects of the Macondo Blowout from the Coastal Waters Consortium: A GoMRI Consortium. International Oil Spill Conference Proceedings. American Petroleum Institute. 1:604-617.
  • Engel, A.S., Gupta, A. (2014) Regime shift in sandy beach microbial communities following Deepwater Horizon oil spill remediation efforts. PLoS One. 9(7): e102934, DOI:10.1371/journal.pone.0102934.
  • Engel, A.S., Bovenkamp, G.L., Prange, A., Hormes, F.J. (2014) In situ speciation of sulfur vapors using X-ray absorption near edge structure spectroscopy. Chemical Geology. 380: 1-6. DOI: 10.1016/j.chemgeo.2014.04.019.
  • Yin, J., Rekepalli, B., Patel, P. Drennen, C., Engel, A.S. (2014) Instrumenting genomic sequence analysis pipeline mothur on shared memory architecture. XSEDE '14 Proceedings of the 2014 Annual Conference on Extreme Science and Engineering Discovery Environment, ACM New York, NY, USA; Article No. 19. DOI:10.1145/2616498.2616505.
  • 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.
  • Keenan, S., Engel, A.S., Elsey, R.M. (2013) The alligator gut microbiome and implications for archosaur symbiosis. Scientific Reports. 3:2877. DOI:10.1038/srep02877.
  • Engel, A.S., Paoletti, M.G., Beggio, M., Dorigo, L., Pamio, A., Gomiero, T., Furlan, C., Brilli, M., Bertonie, R., Dreon, A.L. (2013) Comparative microbial community composition from secondary carbonate (moonmilk) deposits: implications for the Cansiliella servadeii cave hygropetric food web. International Journal of Speleology.  42: 181-192. DOI: 10.5038/1827-806X.42.3.2.
  • Paoletti, M.G., Mazzon, L., Sanudo, I.Z., Simonato, M., Beggio, M., Dreon, A.L., Pamio, A., Brilli, M., Dorigo, L., Engel, A.S., Tondello, A., Baldan, B., Concheri, G., Squartini, A. (2013) A unique midgut-associated bacterial community hosted by the cave beetle Cansiliella servadeii (Coleoptera: Leptodirini) reveals parallel phylogenetic divergences from universal gut-specific ancestors. BMC Microbiology. 13:129. DOI:10.1186/1471-2180-13-129.
  • Headd, B., Engel, A.S. (2013) Evidence for niche partitioning revealed by the distribution of sulfur oxidation genes collected from areas of a terrestrial sulfidic spring with differing geochemical conditions. Applied and Environmental Microbiology. 79: 1171-1182. DOI:10.1128/AEM.02812-12.         
  • Gray, C.J., Engel, A.S. (2013) Microbial diversity and impact on carbonate geochemistry across a changing geochemical gradient. The ISME Journal. 7: 325:337. DOI:10.1038/ismej.2012.105.
  • Engel, A.S., Johnson, L.R., Porter, M.L. (2013) Arsenite oxidase gene diversity among Chloroflexi and Proteobacteria from El Tatio Geyser Field, Chile. FEMS Microbiology Ecology. 83: 745–756. DOI: 10.1111/1574-6941.12030.
  • Green-Garcia, A.M., Engel, A.S. (2012) Bacterial diversity of siliciclastic sediments in a Thalassia testudinum meadow and the implications for Lucinisca nassula chemosymbiosis. Estuarine, Coastal and Shelf Science. 112: 153-16.
  • Rossmassler, K., Engel, A.S., Twing, K., Hanson, T.E, Campbell, B.J (2012) Drivers of Epsilonproteobacteria community composition in sulfidic caves and springs. FEMS Microbiology Ecology. 79:421-432. DOI:10.1111/j.1574-6941.2011.01231.x
  • Book chapters

  • Lee, N.M., Liebl, W.E., Engel, A.S., Porter, M.L. (2013) Cave biofilm metagenomics. In: Nelson K. (ed.) Encyclopedia of Metagenomics: Springer-Verlag Berlin Heidelberg, SpringerReference DOI: 10.1007/SpringerReference_331411.
  • Engel, A.S. (2012) Microbes, in Culver, D. and W. White, (eds.) Encyclopedia of Caves. 2nd Edition, p. 490-499.
  • Engel, A.S. (2012) Chemoautotrophy, in Culver, D. and W. White, (eds.) Encyclopedia of Caves. 2nd Edition. p. 125-134.
  • Lee, N.M., Meiginger, D.B., Aubrecht, R., Kovacic, L., Saiz-Jimenez, C. Baskar, S.R., Schleifer, K.-H., Liebl, W., Porter, M.L., Engel, A.S. (2012)  Life in caves, in Bell, E. (ed.) Life at Extremes: Environments, Organisms and Strategies for Survival. CAB Publishing. p. 302-344.
  • Engel, A.S. (2011) Karst ecosystems, in Reitner, J., and Thiel, V. (eds.), Encyclopedia of Geobiology, Springer Encyclopedia of Earth Sciences Series (EESS, formerly Kluwer Edition), Berlin Encyclopedia of Earth Sciences Series, pp. 521-53. DOI: 10.1007/978-1-4020-9212-1.
  • Engel, A.S. (2010) Microbial diversity of cave ecosystems, in Barton, L., Mandl, M., and Loy, A. (eds.), Geomicrobiology: Molecular & Environmental Perspectives, Springer. p. 219-238. DOI:10.1007/978-90-481-9204-5_10.

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