Use of a Generalized Additive Model to Investigate Key Abiotic Factors Affecting Microcystin Cellular Quotas in Heavy Bloom Areas of Lake Taihu

Lake Taihu is the third largest freshwater lake in China and is suffering from serious cyanobacterial blooms with the associated drinking water contamination by microcystin (MC) for millions of citizens. So far, most studies on MCs have been limited to two small bays, while systematic research on the whole lake is lacking. To explain the variations in MC concentrations during cyanobacterial bloom, a large-scale survey at 30 sites across the lake was conducted monthly in 2008. The health risks of MC exposure were high, especially in the northern area. Both Microcystis abundance and MC cellular quotas presented positive correlations with MC concentration in the bloom seasons, suggesting that the toxic risks during Microcystis proliferations were affected by variations in both Microcystis density and MC production per Microcystis cell. Use of a powerful predictive modeling tool named generalized additive model (GAM) helped visualize significant effects of abiotic factors related to carbon fixation and proliferation of Microcystis (conductivity, dissolved inorganic carbon (DIC), water temperature and pH) on MC cellular quotas from recruitment period of Microcystis to the bloom seasons, suggesting the possible use of these factors, in addition to Microcystis abundance, as warning signs to predict toxic events in the future. The interesting relationship between macrophytes and MC cellular quotas of Microcystis (i.e., high MC cellular quotas in the presence of macrophytes) needs further investigation

From Green to Red: Horizontal Gene Transfer of the Phycoerythrin Gene Cluster between Planktothrix Strains

Horizontal gene transfer is common in cyanobacteria, and transfer of large gene clusters may lead to acquisition of new functions and conceivably niche adaption. In the present study, we demonstrate that horizontal gene transfer between closely related Planktothrix strains can explain the production of the same oligopeptide isoforms by strains of different colors. Comparison of the genomes of eight Planktothrix strains revealed that strains producing the same oligopeptide isoforms are closely related, regardless of color. We have investigated genes involved in the synthesis of the photosynthetic pigments phycocyanin and phycoerythrin, which are responsible for green and red appearance, respectively. Sequence comparisons suggest the transfer of a functional phycoerythrin gene cluster generating a red phenotype in a strain that is otherwise more closely related to green strains. Our data show that the insertion of a DNA fragment containing the 19.7-kb phycoerythrin gene cluster has been facilitated by homologous recombination, also replacing a region of the phycocyanin operon. These findings demonstrate that large DNA fragments spanning entire functional gene clusters can be effectively transferred between closely related cyanobacterial strains and result in a changed phenotype. Further, the results shed new light on the discussion of the role of horizontal gene transfer in the sporadic distribution of large gene clusters in cyanobacteria, as well as the appearance of red and green strains

Unusual cohabitation and competition between Planktothrix rubescens and Microcystis sp. (cyanobacteria) in a subtropical reservoir (Hammam Debagh) located in Algeria

International audienceSuccession in bloom-forming cyanobacteria belonging to distant functional groups in freshwater ecosystems is currently an undescribed phenomenon. However in the Hammam Debagh reservoir (Algeria), P. rubescens and Microcystis sp. co-occur and sometimes proliferate. With the aim of identifying the main factors and processes involved in this unusual cohabitation, water samples were collected monthly from February 2013 to June 2015 at the subsurface at four sampling stations and along the entire water column at one sampling station. In addition, the composition of the cyanobacterial communities was estimated by Illumina sequencing of a 16S rRNA gene fragment from samples collected over one year (October 2013-November 2014). This molecular approach showed that the Hammam Debagh reservoir displays high species richness (89 species) but very low diversity due to the high dominance of Microcystis in this community. Furthermore, it appears that Planktothrix rubescens and Microcystis sp. coexisted (from September to January) but proliferated alternately (Spring 2015 for P. rubescens and Spring 2014 and Autumn 2014/2015 for Microcystis). The main factors and processes explaining these changes in bloom-forming species seem to be related to the variation in the depth of the lake during the mixing period and to the water temperatures during the winter prior to the bloom season in spring