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Cyanobacteria and their phages are significant microbial components of the freshwater and marine environments. We identified a lytic phage, Ma-LMM01, infecting Microcystis aeruginosa, a cyanobacterium that forms toxic blooms on the surfaces of freshwater lakes. Here, we describe the first sequenced freshwater cyanomyovirus genome of Ma-LMM01. The linear, circularly permuted, and terminally redundant genome has 162,109 bp and contains 184 predicted protein-coding genes and two tRNA genes. The genome exhibits no colinearity with previously sequenced genomes of cyanomyoviruses or other Myoviridae. The majority of the predicted genes have no detectable homologues in the databases. These findings indicate that Ma-LMM01 is a member of a new lineage of the Myoviridae family. The genome lacks homologues for the photosynthetic genes that are prevalent in marine cyanophages. However, it has a homologue of nblA, which is essential for the degradation of the major cyanobacteria light-harvesting complex, the phycobilisomes. The genome codes for a site-specific recombinase and two prophage antirepressors, suggesting that it has the capacity to integrate into the host genome. Ma-LMM01 possesses six genes, including three coding for transposases, that are highly similar to homologues found in cyanobacteria, suggesting that recent gene transfers have occurred between Ma-LMM01 and its host. We propose that the Ma-LMM01 NblA homologue possibly reduces the absorption of excess light energy and confers benefits to the phage living in surface waters. This phage genome study suggests that light is central in the phage-cyanobacterium relationships where the viruses use diverse genetic strategies to control their host's photosynthesis.

The abundance of potentially Microcystis aeruginosa-infectious cyanophages in freshwater is studied by using g91 real-time PCR. The studies have shown that cyanophage dynamics might affect shifts in microcystin-producing and non-microcystin-producing populations.

A cyanophage (Ma-LMM01) that infects a toxic strain of the bloom forming cyanobacterium Microcystis aeruginosa was isolated and phylogenetic analyses revealed that Ma-LMM01 forms a sister group with marine and freshwater cyanobacteria. The studies could be used to control toxic cyanobacterial blooms since M. aeruginosa is highly noxious cyanobacteria that forms dense blooms in eutrophic freshwaters and is responsible for the death of livestock and wildlife.

Temporal changes in hepatotoxin microcystin-producing and non-microcystin-producing Microcystis aeruginosa populations were examined in Lake Mikata, Japan. To monitor the densities of the total M. aeruginosa population and the potential microcystin-producing subpopulation, we used a quantitative real-time PCR assay targeting the phycocyanin intergenic spacer and the microcystin synthetase gene ( mcyA), respectively. During the sampling period, the ratio of the mcyA subpopulation to the total M. aeruginosa varied considerably, from 0.5% to 35%. When surface nitrate concentrations increased, there was a rise in the relative abundance of the mcyA subpopulation. This was a positive correlation with the nitrate concentrations ( r=0.53, P<0.05, n=14); whereas temperature and ortho-phosphate had no significant correlation with the presence of mcyA. Our data suggest that high nitrate loading may be a significant factor promoting the growth of the microcystin subpopulations within M. aeruginosa communities in Lake Mikata. [ABSTRACT FROM AUTHOR]

The aim of the present study was to clarify the bloom dynamics and community composition of hepatotoxin microcystin-producing and non-microcystin-producing Microcystis genotypes in the environment. In Lake Mikata (Fukui, Japan) from April 2003 to January 2004, seasonal variation in the number of cells with microcystin (mcy) genotypes and the genetic diversity of the total population were investigated using quantitative competitive PCR and a 16S rDNA clone library, respectively. Using competitive PCR, cells with mcyA genotypes were quantified in August and October, and the ratio of the number of these mcyA genotypes to colony-forming Microcystis cells was 0.37 and 2.37, respectively. The 16S rDNA clones obtained could be divided into 12 ribotypes: a-l. Sixty-one Microcystis strains isolated from Lake Mikata during the sampling period were subjected to toxicity tests using HPLC and ELISA, PCR-based detection of the mcyA gene, and sequence analysis of the 16S rDNA. All isolates could be differentiated into 11 ribotypes (a, b, d, f, h, i, and m-q). Ribotypes b, f, i, m, n, and p had at least one strain that was a microcystin producer. In natural communities ribotypes b and f accounted for 85% of the 16S rDNA clones in August, and ribotypes b and i accounted for 24% of the clones in October. Thus, in some bloom stages the presence of microcystin genotypes identified using the 16S rDNA clone library correlated with that of mcy genotypes determined using competitive PCR., ((c) 2005 Wiley Periodicals, Inc.)