Lake Erie Microcystis: Relationship between microcystin production, dynamics of genotypes and environmental parameters in a large lake Johanna M. Rinta-Kanto a,1 , Elisabeth A. Konopko b , Jennifer M. DeBruyn c , Richard A. Bourbonniere d , Gregory L. Boyer b , Steven W. Wilhelm a, * a Department of Microbiology, The University of Tennessee, Knoxville, TN 37996, United States b SUNY College of Environmental Science and Forestry, Department of Chemistry, Syracuse, NY 13210, United States c Department of Ecology and Evolutionary Biology, The University of Tennessee, Knoxville, Tennessee 37996, United States d National Water Research Institute, Environment Canada, 867 Lakeshore Road, PO Box 5050, Burlington, Ontario, Canada L7R 4A6 1. Introduction Microcystis blooms in the Laurentian Great Lakes, especially in Lake Erie, have been observed almost annually over the past decade (Brittain et al., 2000; Vincent et al., 2004; Hotto et al., 2005; Rinta-Kanto et al., 2005). In the 1960s, cyanobacterial blooms were common in a then severely eutrophied Lake Erie, but reductions in external phosphorus loading implemented in the 1970s reduced cyanobacterial biomass (Makarewicz and Bertram, 1991); by the late 1980s water quality had improved substan- tially, and algal biomass had decreased up to 89% in off-shore waters (Makarewicz, 1993). Since the mid-1990s, cyanobacterial blooms composed primarily of Microcystis spp., have once again increased in frequency, and this has been considered a sign of returning eutrophic conditions in the lake (Conroy et al., 2005). While Microcystis spp. are not the sole microcystin-producers in the system (Rinta-Kanto and Wilhelm, 2006), Microcystis spp. are abundant and wide-spread compared to other potentially toxic cyanobacteria (Ouellette et al., 2006; Conroy et al., 2007). Microcystin concentrations >1 mgL À1 have been commonly observed during Microcystis blooms in Lake Erie (Brittain et al., 2000; Rinta-Kanto et al., 2005) and thus Microcystis has been considered as the major producer of the toxin in the lake. In natural assemblages Microcystis cells exist mostly as colonies held together by a mucilaginous matrix. These colonies can float on the water surface due to the presence of intracellular gas vesicles (Komarek, 2003). A subset of Microcystis cells carries a 55 kb microcystin synthetase (mcy) gene cluster required for the production of the toxin microcystin, whereas non-toxin producing genotypes generally lack or contain an incomplete copy of this gene cluster and thus lack the ability to produce the toxin (Kaebernick and Neilan, 2001). Microcystins are a chemically diverse group of cyanotoxins which in humans have been known to cause gastroenteritis, liver damage and, in the most severe case, Harmful Algae 8 (2009) 665–673 ARTICLE INFO Article history: Received 25 February 2008 Received in revised form 22 December 2008 Accepted 23 December 2008 Keywords: Toxic cyanobacteria qPCR Environmental controls ABSTRACT Cyanobacteria of genus Microcystis sp. have been commonly found in Lake Erie waters during recent summer seasons. In an effort to elucidate relationships between microcystin production, genotypic composition of Microcystis community and environmental parameters in a large lake ecosystem, we collected DNA samples and environmental data during a three-year (2003–2005) survey within Lake Erie and used the data to perform a series of correlation analyses. Cyanobacteria and Microcystis genotypes were quantified using quantitative real-time PCR (qPCR). Our data show that Microcystis in Lake Erie forms up to 42% of all cyanobacteria, and that Microcystis exists as a mixed population of potentially toxic and (primarily) non-toxic genotypes. In the entire lake, the total abundance of Microcystis as well as the abundance of microcystin-producing Microcystis is strongly correlated with the abundance of cya- nobacteria suggesting that Microcystis is a significant component of the cyanobacterial community in Lake Erie during summer seasons. The proportion of total Microcystis of all cyanobacteria was strongly linked to the microcystin concentrations, while the percentage of microcystin-producing genotypes within Microcystis population showed no correlation with microcystin concentrations. Correlation analysis indicated that increasing total phosphorus concentrations correlate strongly with increasing microcystin concentrations as well as with the total abundance of Microcystis and microcystin-producing Microcystis. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +1 865 974 0664; fax: +1 865 974 4007. E-mail address: wilhelm@utk.edu (S.W. Wilhelm). 1 Present address: Department of Marine Sciences, University of Georgia, Athens, Georgia 30602, United States. Contents lists available at ScienceDirect Harmful Algae journal homepage: www.elsevier.com/locate/hal 1568-9883/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.hal.2008.12.004