US Long-Term Ecological Research Network

Recent studies report large genetic diversity among cultured cyanobacteria within and between aquatic environments.  This genetic diversity is not represented well by traditional morphological microscopic identifications of cyanobacteria.  To date, the majority of information on genetic diversity of cyanobacteria in lakes is based on analyses of cultured cyanobacteria.  Yet, it is widely known that culture collections rarely reflect true microbial diversity.  To our knowledge no studies have used genetic, culture-independent methods to compare the diversity of cyanobacteria across multiple eutrophic, inland lakes where these organisms thrive.

We explore the genetic diversity of cyanobacteria in four eutrophic NTL core study lakes (Lakes Mendota, Monona, Kegonsa, and Wingra) using a culture-independent genetic method (Miller and McMahon 2011).  The goals of this study were to 1) describe the phylogeny of cyanobacteria in each lake, and 2) examine genotype richness and evenness as the determinants of cyanobacterial diversity.  Depth-integrated water samples representing the area of the water column receiving photosynthetically active radiation were taken weekly (May-October 2010) from three locations in each lake.  Total DNA was extracted from samples and DNA was pooled by lake.  Thus, pooled samples represent the community supported by each lake at three spatially separated locations throughout the open-water season.  This removes within-lake and temporal variability focusing on between-lake variability.  As a genetic marker, the alpha and beta phycobili protein genes with intergenic spacer (PC-IGS) were recovered from the pooled samples by PCR, cloning, and DNA sequencing.  A phylogenetic analysis of the PC-IGS sequences was conducted to compare the diversity of the major cyanobacteria genera supported by the four lakes.  

The PC-IGS sequences recovered were affiliated with those from Nostocales and Chroococcales, which could be subdivided into five major genera including Microcystis, Aphanizomenon, Anabaena, Chroococcus and Cylindrospermopsis.  These were then further subdivided into PC-IGS genotypes based on their occurrence in maximum likelihood phylogenetic trees (Fig. 1, left cladogram).  The resulting PC-IGS genotypes differed by >2% sequence identity.  This study provides the first genetic evidence for the presence of the tropical/sub-tropical invasive cyanobacterium, Cylindrospermopsis raciborskii in Wisconsin waters.  In all lakes, Microcystis was the most abundant closely followed by Aphanizomenon.  Of 24 unique genotypes, only three were shared across all lakes, while few were exclusive to a single lake (Fig. 1).  Overall genetic structure was similar across lakes (Unifrac P>0.06), but in a cluster analysis the Lake Wingra population was clearly separated from that of the other three lakes.  Lorenze Curves and Gini coefficients, which quantify dispersion from perfect population evenness, indicated that the Lake Wingra population was slightly uneven (G=0.5), while other lakes supported relatively even population structures (G<0.5).  This suggests that, in general, these lakes do not support a dominant genotype.  Analysis of molecular variance (AMOVA) indicated that there was significant genetic variation among all genotypes (φ=0.06, p<0.001) and 94% of the variability occurred within lakes rather than between lakes (6%).  Overall, results from this study revealed that most of the cyanobacterial diversity observed across lakes was due to temporal and within-lake spatial heterogeneity.

Miller, T.R. and K.D. McMahon. 2011. Genetic diversity of Cyanobacteria in four eutrophic lakes. FEMS Microbiology Ecology doi: 10.1111/j.1574-6941.2011.01162.x

 

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