Picocyanobacterial community structure of freshwater lakes and the Baltic Sea revealed by phylogenetic analyses and clade-specific quantitative PCR.

Phylogenetic relationships among picocyanobacteria from the Syn/Pro clade sensu Sánchez-Baracaldo et al. (2005) were determined using small subunit (ssu) rDNA sequences from novel culture isolates together with environmental samples from the Baltic Sea and seven freshwater lakes. The picocyanobacterial community comprised members of previously identified clades and of two previously undescribed clades. The number of well-supported clades suggests that freshwater picocyanobacterial communities encompass much greater diversity than is found in marine systems. To allow the quantification of community structure and temporal succession, clade-specific ssu rDNA TaqMan assays were designed and implemented. These assays were used to assess picocyanobacterial community structure in two lakes over an annual cycle in 2003/4, and in a small number of Baltic Sea samples collected in July 2003. In the lake-water samples, picocyanobacteria were found to be scarce during most of the year, with members of each clade reaching their peak abundance over a relatively short period during the summer (June to September), although representatives of the Cyanobium clade also developed an autumn peak extending towards the end of October. All four freshwater clades were present in the Baltic Sea, but their distribution was patchy over relatively short spatial scales. The use of molecular tools for describing and quantifying community structures reveals previously unexplored complexity in the phytoplankton and will facilitate the development of a more sophisticated understanding of community dynamics at the base of the food chains in lakes.

Spontaneous mutations in gas vesicle genes of Planktothrix spp. affect gas vesicle production and critical pressure.

Wild-type strains of the cyanobacterium Planktothrix rubescens have a cluster of gas vesicle (gvp) genes with repeats of alternating gvpA and gvpC. The gvpC occurs in three length variants, all with the same 3'-sequence, OmegaC. Spontaneous non-buoyant mutants had lost some of the alternating gvpAC copies and their gvpC genes had a novel 3'-end sequence, PsiC; additional gvpC genes terminating in this sequence were also found in the wild-type and representatives of other GV genotypes. Alleles of gvpC terminating in PsiC occurred only at the downstream ends of the gvpAC clusters investigated; all other gvpCs terminated in OmegaC. Mutants of strains with the GV3 genotype produced only 30-50% of the gas vesicles present in the wild-type; their gas vesicles had lower mean critical pressures (0.70-0.78 MPa) than those in the wild-type (1.05-1.10 MPa).

Allele-specific PCR shows that genetic exchange occurs among genetically diverse Nodularia (cyanobacteria) filaments in the Baltic Sea.

Some cyanobacteria have been shown to exchange genetic information under laboratory conditions, but it has not been clear whether such genetic exchange occurs in the natural environment. To address this, a population genetic study was carried out on the filamentous diazotrophic cyanobacterium Nodularia in the Baltic Sea. Nodularia filaments were collected from 20 widely distributed sampling stations in the Baltic Sea during June and July 1998. Allele-specific PCR (AS-PCR) was used to characterize over 2000 filaments at three loci: a non-coding spacer between adjacent copies of the main structural gas vesicle gene gvpA (gvpA-IGS), the phycocyanin intergenic spacer (PC-IGS) and the rDNA internal transcribed spacer (rDNA-ITS). The three loci were all found to be polymorphic in the 1998 population: two alternative alleles were distinguished at the gvpA-IGS and PC-IGS loci, and three at the rDNA-ITS locus. All 12 possible combinations of alleles were found in the filaments studied, but some were much more common than others. The index of association (I:(A)) for all possible pairwise combinations of isolates was found to differ significantly from zero, which implies that there is some linkage disequilibrium between loci. The I:(A) values for 16 out of 20 individual sampling stations also differed significantly from zero: this shows that the observed linkage disequilibrium is not due to pooling data from genetically distinct subpopulations. Monte-Carlo simulations with random subsets of the data confirmed that some combinations showed significantly more linkage disequilibrium than expected by chance alone. It is concluded that genetic exchange occurs in the natural Nodularia population, but the frequency is not high enough for the loci to be in linkage equilibrium. The distribution of the 12 genotypes across the Baltic Sea was found to be non-random, but did not correlate with temperature, salinity or major nutrient concentrations. A significant relationship was found between the gene diversity among filaments at each station and the distance of the station from the centre of the sampling area: possible reasons for this trend are discussed.