) shows that strains of the GV3 genotype produce gas vesicles with a higher critical pressure than those of GV1 and GV2. A PCR survey of 185 clonal cultures of P. rubescens isolated from Lake Zu $ rich revealed that 3 isolates were of genotype GV1, 73 were of GV2 and 109 were of GV3. The PCR technique was used to distinguish the gas vesicle genotype, and thence the associated critical-pressure phenotype, of single filaments selected from lakewater samples. Sequence analysis of the 16S rDNA and of regions within the operons encoding phycoerythrin, phycocyanin and Rubisco confirmed that these strains of Planktothrix form a tight phylogenetic group.

Phylogenetic relationships among picocyanobacteria from the Syn/Pro clade sensu Sá nchez- 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.

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 noncoding 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.

Airborne survival of two pseudomonads and a reference strain of Escherichia coli (strain MRE 162) was studied outdoors using a modified microthread technique. When cells of E. coli were suspended as clusters, survival was much greater than single cells, particularly outdoors. Culture age had a highly significant effect on survival of Pseudomonas maltophila with survival of 24 h cultures being more than 100-fold higher than 48 h cultures. Survival of Pseudomonas fluorescens was variable and depended also upon the method of culture. Survival of E. coli and Ps. maltophila was studied at three locations differing in air quality and was found to be significantly reduced outdoors, particularly when held in direct daylight. Outdoor survival was not significantly different at the three locations but was reduced at increasing temperatures. There was no apparent effect of wind direction or air quality. Results are discussed with reference to the release of genetically-modified micro-organisms.

Aphanizomenon Morren is an important member of the cyanobacterial community in the Baltic Sea, but studies of this genus have been hampered by the difficulty of growing it in laboratory culture. PCR amplification of DNA from colonies picked directly from water samples has circumvented this problem and made it possible to carry out an analysis of genetic diversity within the Baltic Sea and in two small North American lakes separated by just a few kilometers. The nucleotide sequence of the phycocyanin intergenic spacer and partial flanking coding regions of cpcB and cpcA was determined for 32 colonies of Aphanizomenon, 26 from the Baltic Sea, and 6 from the North American lakes. No variation was detected among the 26 Baltic Sea colonies, but two alleles, differing at 19 nucleotide positions (5.4%), were found in the North American lake colonies. Surprisingly, the two North American types were less closely related to each other than to the Baltic Sea genotype. The Baltic Sea Aphanizomenon is clearly distinct from A. flos‐aquae at both the cpcB–cpcA and 16S rDNA loci, which lends phylogenetic support to their tentative separation based on ultrastructural analysis. We conclude that although there is significant genetic diversity in the genus Aphanizomenon, the population in the Baltic Sea is, in contrast to the Nodularia population, genetically homogeneous.