Dataset Information

Distribution and abundance of nontoxic mutants of cyanobacteria in lakes of the Alps.

ABSTRACT: The filamentous cyanobacterium Planktothrix rubescens frequently occurs in deep and stratified lakes in the temperate region of the northern hemisphere and is a known producer of the hepatotoxic secondary metabolite microcystin. These cyclic heptapeptides are synthesized nonribosomally via large enzyme complexes encoded by the microcystin (mcy) synthetase gene cluster. The occurrence of cyanobacterial strains lacking microcystin, but containing the mcy gene cluster has been reported repeatedly; it was shown that this inactivation is due to mutations such as gene deletion events and the insertion of transposable elements. In the present study, 12 lakes in Austria, Germany, and Switzerland were sampled from July 2005 to October 2007, and the proportion of inactive mcy genotypes was quantified in relation to the total population of the red-pigmented filamentous cyanobacterium Planktothrix by means of quantitative polymerase chain reaction. In total, four different mutations were quantified, namely two insertions affecting mcyD, one insertion affecting mcyA, and a deletion within mcyH and mcyA. The mutations occurred over a wide range of population densities (40-570,000 filaments L(-1)), and their abundance was found to be positively correlated with population density. However, on average, all nontoxic mutants were found in a low proportion only (min 0%, mean 6.5% +/- 1.1 (SE), max 52% of the total population). The genotype containing the mcyHA deletion had a significantly higher proportion (min 0%, mean 3.7% +/- 1, max 52%) when compared with all the genotypes containing insertions within the mcy gene cluster (min 0%, mean 2.8% +/- 0.7, max 24%). The results demonstrate that the occurrence of inactive mcy genotypes is linearly related to the population density, and selective sweeps of nontoxic mutants did not occur during the transition from prebloom to bloom conditions.

SUBMITTER: Ostermaier V

PROVIDER: S-EPMC3044886 | biostudies-literature | 2009 Aug

REPOSITORIES: biostudies-literature

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Distribution and abundance of nontoxic mutants of cyanobacteria in lakes of the Alps.

Ostermaier Veronika V Kurmayer Rainer R

Microbial ecology 20090213 2

The filamentous cyanobacterium Planktothrix rubescens frequently occurs in deep and stratified lakes in the temperate region of the northern hemisphere and is a known producer of the hepatotoxic secondary metabolite microcystin. These cyclic heptapeptides are synthesized nonribosomally via large enzyme complexes encoded by the microcystin (mcy) synthetase gene cluster. The occurrence of cyanobacterial strains lacking microcystin, but containing the mcy gene cluster has been reported repeatedly; ...[more]

PMID: 19214623

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Project description:BackgroundBloom-forming cyanobacteria cause toxic algae outbreaks in lakes and reservoirs. We aimed to explore and quantify mutation events occurring within the large mcy gene cluster (55 kbp) encoding microcystin (MC) biosynthesis that inactivate MC net production. For this purpose we developed a workflow to detect mutations in situ occurring anywhere within the large mcy gene cluster as amplified from one single filament of the red-pigmented cyanobacterium Planktothrix rubescens. From five lakes of the Alps eight hundred Planktothrix filaments were isolated and each individual filament was analyzed for mutations affecting the mcy genes.ResultsMutations inactivating MC synthesis were either through an insertion element ISPlr1 or the partial deletion of mcy genes. Neutral mutations not affecting MC biosynthesis occurred within two intergenic spacer regions, either through the insertion of a Holliday-junction resolvase RusA or ISPlr1. Altogether, the insertions affected a few mcy genes only and their location was correlated with regions similar to repetitive extragenic palindromic DNA sequences (REPs). Taking all of the filaments together, the mutations leading to the inactivation of MC synthesis were more rare (0.5-6.9%), when compared with the neutral mutations (7.5-20.6%). On a spatial-temporal scale the ratio of MC synthesis-inactivating vs. neutral mutations was variable, e.g., the filament abundance carrying partial deletion of mcyD (5.2-19.4%) and/or mcyHA (0-7.3%) exceeded the abundance of neutral mutations.ConclusionsIt is concluded that insertion events occurring within the Planktothrix mcy gene cluster are predictable due to their correlation with REPs. The frequency of occurrence of the REPs within the mcy gene cluster of Planktothrix relates to the rather common mutation of mcy genes in Planktothrix. Spatial-temporal variable conditions may favor the emergence of partial mcy deletion mutants in Planktothrix, in particular a higher proportion of genotypes resulting in inactivation of MC synthesis might be caused by increased ISPlr1 activity.

Project description:It is often difficult to decide which cyanobacteria found in endolithic habitats of calcite spring-tufa deposits are present as ephemeral components of the biota or are persistent, structural elements. To answer this question, we repeatedly studied two microhabitats of contrasting calcareous tufa springs in the European Alps. Pigment extracts, fluorescence probe measurements of in situ samples and traditional microscopy confirmed the dominance of cyanobacteria over eukaryotic algae and their viability in both microhabitats. Spring Site 1 (Laas, Northern Italy) is characterized by a highly variable, moist to dry and sun-exposed waterfall tufa consisting of fibrous calcite. A segment of these deposits in the lateral flank of a grotto contained dark endolithic layers in dim light, 1-2 mm below the surface, where aggregated cyanobacterial cells were dominant but not directly attached to calcites, a potential sign of gentle endolithic dissolution rather than calcite precipitation induced by cyanobacteria. Site 2 (Mühlau, Austria), in contrast, is a moss-tufa microhabitat associated with a seepage spring situated in a shady gorge, where the targeted stromatolites consisted of bark-like sheets of friable, orange to light-brown when wet (drying violet) 'styrofoam'- like aggregates of minute crystallites on the day-light exposed surfaces. These calcites were observed to nucleate directly on external sheaths of viable cyanobacteria trichomes. A polyphasic approach including LM, SEM, TEM exhibited a number of identical but also some divergent cyanobacteria of which two key taxa were specific for each of the two microhabitats (Nostoc and Pseudoscytonema at Sites 1 and 2 respectively). Both cyanobacterial communities characterised, by the cloning of 16S rDNA showed a dominance of mostly unknown and partly divergent filamentous cyanobacteria assigned to the order of Synechococcales. Our microhabitat study of alpine crenal calcites highlights the rather divergent biotic responses of cyanobacteria within spring tufa deposits.

Dataset Information

Distribution and abundance of nontoxic mutants of cyanobacteria in lakes of the Alps.

Publications

Distribution and abundance of nontoxic mutants of cyanobacteria in lakes of the Alps.

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