Dr. Theo Dreher, Professor
Research Interests: Freshwater Cyanobacterial Blooms and Cyanophages
Courses Taught: MB 434/534 Virology; MB 490 Microbiology Capstone Experience
Not accepting grad students
Genetic Diversity of Freshwater Cyanobacterial Blooms and Drivers of their Rise and Decline, Including Cyanophages: Every summer, lakes and reservoirs across the world suffer blue-green algal (cyanobacterial) blooms that can be associated with toxins, with unpleasant tastes and odors that taint drinking water, and with changes in water chemistry (e.g., high pH, anoxia upon sudden die-offs) that disturb ecosystems. In Oregon and the Pacific Northwest (PNW), genera such as Microcystis, Anabaena, Aphanizomenon, Oscillatoria, Woronichinia and Gloeotrichia are the most common bloom-forming cyanobacteria. Several of these are potentially toxic due to the production of hepato- or neuro-toxins, such as microcystin and anatoxin, but the toxigenic status of PNW blooms is not well understood. Neither is there a good understanding of the genetic diversity among bloom-forming cyanobacterial populations, the diversity of their genes for the production of toxins or taste-and-odor compounds, the regulation of these genes, and the factors (including phages) that regulate the bloom and bust cycles commonly seen. These are questions we are investigating through the following projects, hoping to better understand the ecology of bloom-forming cyanobacteria and to inform management regarding public health decisions related to toxic blooms.
Toxic Microcystis Blooms in the Klamath River System: Two major reservoirs on the Klamath River in northern California (Copco and Iron Gate Reservoirs) support annual Microcystis blooms that produce large amounts of the hepatoxin microcystin, a cyclic heptapeptide produced by enzymes encoded by a c. 50 kbp nonribosomal peptide synthetase (NRPS) gene cluster. Both toxic and non-toxic Microcystis strains are present, but toxic strains have become more prevalent over recent years: why? Toxic Microcystis strains are present throughout the Klamath system, from Upper Klamath Lake in Oregon to the lower reaches near the Pacific Ocean. What is the relationship between the populations in these distant parts of this long watershed?
Anabaena-dominated Blooms in the Midwest and Oregon: Multi-year studies are being conducted on Cheney Reservoir (Kansas), Lake Houston (Texas) and Dexter Reservoir (Oregon) in collaboration with USGS colleagues in Lawrence, KS. These reservoirs are drinking water supplies that encounter blooms capable of producing toxins and taste-and-odor compounds that should be absent from finished drinking water. We are working towards understanding the drivers of bloom rise and fall cycles in order to model and better predict toxic or stinky bloom events to allow water utilities to anticipate enhanced treatment needs. Among the factors that may influence blooms are chemical and physical factors (nutrients, temperature, light levels, etc.), consumers (zooplankton or bacterial grazers, lytic cyanophages), levels of competing or synergistic taxa, and the genetic (ecotype) variation among cyanobacteria.
Anatoxin-a Production in Washington Lakes: Anderson Lake on Washington's Olympic Peninsula is fortunately one of very few PNW lakes to produce high levels of the neurotoxin anatoxin-a. Anabaena appears to be the main toxin producer, yet Anabaena is common in non-toxic settings in the PNW. What is special about the Anabaena strain or the conditions in Anderson Lake?
Cyanophages Infecting Freshwater Bloom-Forming Cyanobacteria: Blooms occasionally crash in the absence of an obvious triggering event, such as a sudden temperature drop. To what extent are phages responsible for such crashes and for regulating bloom populations? We are studying this question in connection with Microcystis and Anabaena blooms, beginning by identifying phages infecting these cyanobacteria. We are also interested in exploring the roles of cyanophage genes, especially host-related genes that may play important roles in regulating the infection.
Cyanobacteria and Cyanophages
Otten, T.G., Paerl, H.W., Dreher, T.W., Kimmerer, W.J., and Parker, A.E. (2017) The molecular ecology of microcystis sp. blooms in the San Francisco estuary delta. Environ. Microbiol. doi:10.1111/1462-2920.13860.
Driscoll C.B., Otten T.G., Brown N.M., and Dreher T.W. (2017) Towards long-read metagenomics: complete assembly of three novel genomes from bacteria dependent on a diazotrophic cyanobacterium in a freshwater lake co-culture. Stand Genomic Sci. Jan 19;12:9. doi: 10.1186/s40793-017-0224-8.
Li, X., Dreher, T.W., and Li, R. (2016) An overview of diversity, occurrence, genetics and toxin production of bloom-forming Dolichospermum (Anabaena) species. Harmful Algae. 2016 Apr;54:54-68. doi: 10.1016/j.hal.2015.10.015.
Otten, T.G., Graham, J.L., Harris, T.D. and Dreher, T.W. (2016) Elucidation of taste- and odor-producing bacteria and toxigenic cyanobacteria in a midwestern drinking water supply reservoir by shotgun metagenomic analysis. Appl. Environ. Microbiol. 82(17):5410-20.
Brown, N.M., Mueller, R.S., Shepardson, J.W., Landry, Z.C., Morré, J.T., Maier, C.S., Hardy, F.J., and Dreher, T.W. (2016) Structural and functional analysis of the finished genome of the recently isolated toxic Anabaena sp. WA102. BMC Genomics. 17:457.
Otten, T.G., Crosswell, J.R., Mackey, S. and Dreher, T.W. (2015) Application of molecular tools for microbial source tracking and public health risk assessment of a Microcystis bloom traversing 300 km of the Klamath River. Harmful Algae 46:71-81.
Dreher, T.W. and Bozarth, C.S. (2012) Harmful algalblooms: What can genetic techniques reveal? Lakeline, Fall 2012, pp. 12-16.
Dreher, T.W., Brown, N, Bozarth, C.S., Schwartz, A.D., Riscoe, E., Thrash, J.C., Bennett, S.E., Tzeng, S.-C., and Maier, C.S. (2011) A freshwater cyanophage whose genome indicates close relationships to photosynthetic marine cyanomyophages. Environ. Microbiol. 13:1858-74.
Bozarth, C.S., Schwartz, A.D., Shepardson, J.W., Cowell, F.S. and Dreher, T.W. (2010) Population turnover in a Microcystis bloom results in predominantly nontoxigenic variants late in the season. Appl Environ Microbiol. 76:5207-13.
Positive Strand RNA Viruses
Zan, X., Sitasuwan, P., Powerll, J., Dreher, T.W. and Wang, Q. (2012) Polyvalent display of RGD motifs on turnip yellow mosaic virus for enhanced stem cell adhesion and spreading. 2012. Acta Biomater. 8(8):2978-85.
Powell, J.D., Barbar, E., and Dreher, T.W. (2012) Turnip yellow mosaic virus forms infectious particles without the native beta-annulus structure and flexible coat protein N-terminus. Virology 422(2):165-73.
Val, R., Wyszko, E., Valentin, C., Szymanski, M., Cosset, Al, Alioua, M., Dreher, T.W., Barciszewski, J., and Dietrich, A. (2011). Organelle trafficking of chimeric ribozymes and genetic manipulation of mitochondria. Nucleic Acids Res. 39(21):9262-74.
Zeng, Q., Saha, S., Lee, L.A., Barnhill, H., Oxsher, J., Dreher, T. And Wang, Q. (2011) Chemoselective modification of turnip yellow mosaic virus by Cu(II) catalyzed azide-alkyne 1,3-dipolar cycloaddition reaction and its application in cellbinding. Bioconjug Chem. 22:58-66.
Dreher, T.W. (2010) Viral tRNAs and tRNA-like structures. Wiley Interdisicp. Rev. RNA. 1(3):402-14.
Shin, H.-I., Tzanetakis, I.E., Dreher, T.W. and Cho, T.-J. (2009) The 5´-UTR of Turnip yellow mosaic virus does not include a critical encapsidation signal. Virology 387:427-435.
Tzanetakis, I.E., Tsai, C.-H., Martin, R.R. and Dreher, T.W. (2009) A tymovirus with an atypical 3´-UTR illuminates the possibilities for 3´-UTR evolution. Virology 392:238-245.
Dreher, T. W. (2009) Role of tRNA-like structures in controlling plant virus replication. Virus Res. 139:217-229.
Matsuda, D and Dreher, T.W. (2006) Close spacing of AUG initiation codons confers dicistronic character on a eukaryotic mRNA. RNA 12:1338-49.
Cho, T.-J. and Dreher, T. W. (2006) Encapsidation of genomic but not subgenomic Turnip Yellow Mosaic Virus RNA by coat protein provided in trans. Virology 356:126-135.
Dreher, T. W. (2004) Pathogen profile. Turnip yellow mosaic virus: transfer RNA mimicry, chloroplasts and a C-rich genome. Molecular Plant Pathology 5:367-375.
Chiu, W.-W., Kinney, R.M. and Dreher, T.W. (2005) Control of translation by the 5' and 3' terminal regions of the dengue virus genome. J. Virol., 79:8303-15.