Occurrence and diversity of viruses associated with cyanobacterial communities in a Brazilian freshwater reservoir.

As part of the phytoplankton of marine and freshwater environments around the world, cyanobacteria interact with viruses (cyanophages) that affect their abundance and diversity. Investigations focusing on cyanophages co-occurring with freshwater cyanobacteria are scarce, particularly in Brazil. The aim of this study was to assess the diversity of cyanophages associated with a Microcystis-dominated cyanobacterial bloom in a tropical reservoir. Samples were processed as viral fractions of water and cellular fractions, and temporal fluctuations in the abundance of Ma-LMM01-type cyanophages and their Microcystis hosts were determined by qPCR. We applied shotgun metagenomics to obtain a wider characterization of the cyanophage community. During the study period, Microcystis gene copies were quantified in all cellular fractions, and the copy number of the Ma-LMM01 phage gene tended to increase with host abundance. Metagenomic analysis demonstrated that Caudovirales was the major viral order associated with the cyanophage families Myoviridae (34-88%), Podoviridae (3-42%), and Siphoviridae (6-23%). The metagenomic analysis results confirmed the presence of Microcystis cyanophages in both viral and cellular fractions and demonstrated a high relative abundance of picocyanobacteria-related viruses and Prochlorococcus (36-52%) and Synechococcus (37-50%) phages. For other main cyanobacterial genera, no related cyanophages were identified, which was probably due to the scarce representation of cyanophage sequences in databanks. Thus, the studied reservoir hosted a diverse cyanophage community with a remarkable contribution of phages related to picoplanktonic cyanobacteria. These results provide insights that motivate future sequencing efforts to assess cyanophage diversity and recover complete genomes.

Characterization of Trichodesmium-associated viral communities in the eastern Gulf of Mexico.

Trichodesmium surface aggregations shape the co-occurring microbial community by providing organic carbon and nitrogen and surfaces on which microorganisms can aggregate. Rapid collapse of Trichodesmium aggregations leads to drastic changes in the chemical and physical properties of surrounding waters, eliciting a response from the microbial community and their associated viruses. Three viral metagenomes were constructed from experimentally lysed Trichodesmium collected from two locations in the eastern Gulf of Mexico. Trichodesmium were either treated with mitomycin C to induce potential lysogens or incubated in the absence of mitomycin C. Comparative analyses of viral contiguous sequences indicated that viral composition was responsive to treatment type. Cyanophages were more represented within incubations treated with mitomycin C, while gammaproteobacterial phages were more represented within the untreated incubation. The detection of latent bacteriophage integrases in both the chemically treated and untreated incubations suggests that Trichodesmium death may lead to prophage induction within associated microorganisms. While no single cyanophage-like genotype associated with Trichodesmium lysis could be identified that might point to an infectious Trichodesmium phage, reads resembling Trichodesmium were recovered. These data reveal a diverse consortium of lytic and temperate phages associated with Trichodesmium whose patterns of representation within treated and untreated libraries offer insights into the activities of host and viral communities during Trichodesmium aggregation collapse.