a b s t r a c tPhenotypic and genotypic changes in Chlamydomonas reinhardtii BafJ5, a starchless mutant, with respect to lipid metabolism was studied in different trophic states under nitrogen (N) sufficient and limited conditions. Interestingly, cellular lipid content increased linearly with input acetate concentration with highest lipid content ($42%) under nitrogen limitation and mixotrophic state. RT-qPCR studies indicate that key fatty acid biosynthesis genes are down-regulated under N limitation but not under mixotrophic state, whereas, ACS2, encoding Acetyl-CoA synthetase, and DGTT4, encoding Diacylglycerol O-acyltransferase, are up-regulated under all conditions. These results collectively indicate that acetate is the limiting factor and central molecule in lipid droplet synthesis. The study also provides further evidence of the presence of a chloroplast pathway for triacylglycerol synthesis in microalgae.

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The relative genetic diversity of microcystin-producing Microcystis in the water and sediment of the Daechung Reservoir, Korea, was investigated over an entire year, including the cyanobacterial bloom season. The cells of potentially toxic Microcystis strains containing mcyJ genotypes and cells containing the genus-specific cpcBA gene were quantified by a real-time PCR. The ratio of cells with mcyJ genotypes to the total Microcystis population in the water body was the highest (68.3%) in August when the cyanobacterial bloom reached its peak and the microcystin concentration in the water began to increase. A denaturing gradient gel electrophoresis profile analysis of the mcyJ genotypes performed to monitor any changes in the toxic Microcystis population showed the appearance of new genotypes and the disappearance of existing genotypes in the reservoir water collected during the summer months, when compared with the profile for the samples collected in spring and autumn. However, very little change was observed over the course of the year as regards the population diversity of the sediment samples.

The detection and prevention of cyanobacterial blooms are important issues in water quality management. As such, the diversity and community dynamics of cyanobacteria during cyanobacterial bloom in the Daechung Reservoir, Korea, were studied by analyzing the intergenic spacer (IGS) region between phycocyanin subunit genes cpcB and cpcA (cpcBA IGS). To amplify the cpcBA IGS from environmental samples, new PCR primers that could cover a wider range of cyanobacteria than previously known primers were designed. In the samples taken around the bloom peak (2 September 2003), seven groups of cpcBA IGS sequences were detected, and none of the amplified cpcBA IGSs was closely related to the cpcBA IGS from chloroplasts. Apart from the Microcystis-, Aphanizomenon (Anabaena)-, Pseudanabaena-, and Planktothrix (Oscillatoria)-like groups, the three other groups of cpcBA IGS sequences were only distantly related to previously reported sequences (<85% similarity to their closest relatives). The most prominent changes during the bloom were the gradual decrease and eventual disappearance of the Aphanizomenon (Anabaena)-like group before the bloom peak and the gradual increase and sudden disappearance of Planktothrix (Oscillatoria)-like groups right after the bloom peak. The community succession profile obtained based on the cpcBA IGS analysis was also supported by a PCR-denaturing gradient gel electrophoresis analysis of the 16S rRNA genes.

Interactions between microalgae and bacteria are often obligatory for harmful algal blooms (HABs). Here, we investigated the specific bacterial communities associated with Alexandrium tamarense and Cochlodinium polykrikoides, which cause ecological and economic damage during their blooms. To this end, the bacterial metagenome was selectively isolated from the two dinoflagellates and subsequently used for 16S rRNA analysis via the Nanopore MinION and Illumina sequencing platforms. Although the full-length 16S rRNA reads from the MinION platform showed high correlation in higher taxonomic ranks to the partial-length 16S rRNA reads from the Illumina platform, there was less correlation at the genus and species levels. MinION reads that are similar in the V3-V4 hypervariable regions with Illumina reads are classified to different taxonomies due to the extra information encoded in the full-length 16S rRNA reads. This indicates that bias arising from the short length Illumina reads can be supplemented by MinION reads. Furthermore, integrated analysis of the Illumina and MinION data showed that A. tamarense was predominantly enriched in the Roseobacter clade and C. polykrikoides was enriched in Gammaproteobacteria and Alphaproteobacteria. These results suggest that the association of different bacterial communities with A. tamarense and C. polykrikoides may be required for HABs.