The intriguing impact of physical mixing processes on species interactions has always fascinated ecologists. Here, we exploit recent advances in plankton models to develop competition theory that predicts how changes in turbulent mixing affect competition for light between buoyant and sinking phytoplankton species. We compared the model predictions with a lake experiment, in which the turbulence structure of the entire lake was manipulated using artificial mixing. Vertical eddy diffusivities were calculated from the measured temperature microstructure in the lake. Changes in turbulent mixing of the lake caused a dramatic shift in phytoplankton species composition, consistent with the predictions of the competition model. The buoyant and potentially toxic cyanobacterium Microcystis dominated at low turbulent diffusivity, whereas sinking diatoms and green algae dominated at high turbulent diffusivity. These findings warn that changes in the turbulence structure of natural waters, for instance driven by climate change, may induce major shifts in the species composition of phytoplankton communities.

The cyanobacterium Microcystis can produce microcystins, a family of toxins that are of major concern in water management. In several lakes, the average microcystin content per cell gradually declines from high levels at the onset of Microcystis blooms to low levels at the height of the bloom. Such seasonal dynamics might result from a succession of toxic to nontoxic strains. To investigate this hypothesis, we ran competition experiments with two toxic and two nontoxic Microcystis strains using light-limited chemostats. The population dynamics of these closely related strains were monitored by means of characteristic changes in light absorbance spectra and by PCR amplification of the rRNA internal transcribed spacer region in combination with denaturing gradient gel electrophoresis, which allowed identification and semiquantification of the competing strains. In all experiments, the toxic strains lost competition for light from nontoxic strains. As a consequence, the total microcystin concentrations in the competition experiments gradually declined. We did not find evidence for allelopathic interactions, as nontoxic strains became dominant even when toxic strains were given a major initial advantage. These findings show that, in our experiments, nontoxic strains of Microcystis were better competitors for light than toxic strains. The generality of this finding deserves further investigation with other Microcystis strains. The competitive replacement of toxic by nontoxic strains offers a plausible explanation for the gradual decrease in average toxicity per cell during the development of dense Microcystis blooms.Blooms of the cyanobacterium Microcystis can be a major hazard in recreational lakes, drinking water reservoirs, and protected wetland areas (6,47,49). Microcystis often forms dense blooms that may cause anoxia when cells die off massively. Moreover, Microcystis can produce the toxin microcystin. This hepatotoxin poses serious health risks for animals and humans (3, 7). Especially in dense scums, the concentration of microcystins may increase dramatically. Microcystin concentrations up to 25,000 g liter Ϫ1 have been reported (10), exceeding the guideline values for recreational waters of 20 g liter Ϫ1 by more than 3 orders of magnitude (5). Microcystis populations often consist of mixtures of microcystin-producing and non-microcystin-producing strains (10, 23, 48, 52). Interestingly, several studies show that the average microcystin content expressed per cell is typically high at the onset of Microcystis blooms but much lower at the height of these blooms (22,51,53). In other words, with increasing Microcystis biomass, the Microcystis cells become, on average, less toxic. Examples from three Microcystis-dominated Dutch lakes are shown in Fig. 1. This striking seasonal variability in microcystin content of Microcystis blooms exceeds the physiological variability in cellular microcystin content reported for isolated Microcystis strains in laboratory experiments (13,29,54). Thus, it seems that the changes in...

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Assessing and predicting bloom dynamics and toxin production by Microcystis requires analysis of toxic and nontoxic Microcystis genotypes in natural communities. We show that genetic differentiation of Microcystis colonies based on rRNA internal transcribed spacer (ITS) sequences provides an adequate basis for recognition of microcystin producers. Consequently, ecological studies of toxic and nontoxic cyanobacteria are now possible through studies of rRNA ITS genotypic diversity in isolated cultures or colonies and in natural communities. A total of 107 Microcystis colonies were isolated from 15 lakes in Europe and Morocco, the presence of microcystins in each colony was examined by matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS), and they were grouped by rRNA ITS denaturing gradient gel electrophoresis ( There was no indication for geographical restriction of strains, since identical sequences originated from geographically distant lakes. About 28% of the analyzed colonies gave rise to multiple bands in DGGE profiles, indicating either aggregation of different colonies, or the occurrence of sequence differences between multiple operons. Cyanobacterial community profiles from two Dutch lakes from which colonies had been isolated showed different relative abundances of genotypes between bloom stages and between the water column and surface scum. Although not all bands in the community profiles could be matched with isolated colonies, the profiles suggest a dominance of nontoxic colonies, mainly later in the season and in scums.Mass occurrences (blooms) of toxic cyanobacteria from the genus Microcystis constitute a threat to the safety and ecological quality of surface waters worldwide. The most prominent toxin produced by Microcystis is the hepatotoxin microcystin, a cyclic heptapeptide which is formed nonribosomally by peptide and polyketide synthetases (3). Proper assessment of the hazards of Microcystis blooms necessitates rapid and reliable methods for microcystin detection. For predictions of the development of microcystin concentrations, tools and insights for understanding the dynamics of microcystin production are required. Environmental factors may affect microcystin production in Microcystis cultures by a factor of 3 to 4 (24). However, the capability for microcystin production as such is genetically determined. Strains isolated from the same bloom sample are constitutively microcystin producing or nonproducing (15,19,28), and the types and cellular content of microcystins may differ considerably between strains (2, 16, 28). The decisive factors determining the toxicity of a bloom are therefore the ratio of microcystin-producing and non-microcystin-producing genotypes and the amounts and variants of microcystins produced by individual cells (4, 23).Understanding of the community composition and dynamics of microcystin-producing and non-microcystin-producing Microcystis strains in the field is very limited, due to a lack of suitable identification methods. Trad...

For many ecological studies of cyanobacteria, it is essential that closely related species or strains can be discriminated. Since this is often not possible by using morphological features, cyanobacteria are frequently studied by using DNA-based methods. A powerful method for analysis of the diversity and dynamics of microbial populations and for checking the purity and affiliation of cultivated strains is denaturing gradient gel electrophoresis (DGGE). We realized high-resolution discrimination of a variety of cyanobacteria by means of DGGE analysis of sections of the internal transcribed spacer between the 16S and 23S rRNA genes (rRNA-ITS). A forward primer specific for cyanobacteria, targeted at the 3 end of the 16S rRNA gene, was designed. The combination of this primer and three different reverse primers targeted to the rRNA-ITS or to the 23S rRNA gene yielded PCR products of different sizes from cultures of all 16 cyanobacterial genera that were tested but not from other bacteria. DGGE profiles produced from the shortest section of rRNA-ITS consisted of one band for all but one cyanobacterial genera, and those generated from longer stretches of rRNA-ITS yielded DGGE profiles containing one to four bands. The suitability of DGGE for detecting intrageneric and intraspecific variation was tested by using strains of the genus Microcystis. Many strains could be discriminated by means of rRNA-ITS DGGE, and the resolution of this method was strikingly higher than that obtained with previously described methods. The applicability of the developed DGGE assays for analysis of cyanobacteria in field samples was demonstrated by using samples from freshwater lakes. The advantages and disadvantages associated with the use of each developed primer set are discussed.