Prokaryotic and viral community structure in the singular chaotropic salt lake Salar de Uyuni

Salar de Uyuni (SdU) is the largest hypersaline salt flat and the highest lithium reservoir on Earth. In addition to extreme temperatures and high UV irradiance, SdU has high concentrations of chaotropic salts which can be important factors in controlling microbial diversity. Here, for the first time we characterize the viral diversity of this hypersaline environment during the two seasons, as well as the physicochemical characteristics and the prokaryotic communities of the analysed samples. Most of the selected samples showed a peculiar physicochemical composition and prokaryotic diversity, mostly different from each other even for samples from locations in close proximity or the same season. In contrast to most hypersaline systems Bacteria frequently outnumbered Archaea. Furthermore, an outstanding percentage of members of Salinibacter sp., likely a species different from the cosmopolitan Salinibacter ruber, was obtained in most of the samples. Viral communities displayed the morphologies normally found in hypersaline environments. Two seasonal samples were chosen for a detailed metagenomic analysis of the viral assemblage. Both viral communities shared common sequences but were dominated by sample-specific viruses, mirroring the differences also observed in physicochemical and prokaryotic community composition. These metaviromes were distinct from those detected in other hypersaline systems analysed to date.     

Morphological Characterization of Viruses in the Stratified Water Column of Alkaline, Hypersaline Mono Lake

Concentrations of viruses and prokaryotes in the alkaline, moderately hypersaline, seasonally stratified Mono Lake are among the highest reported for a natural aquatic environment. We used electron microscopy to test whether viral morphological characteristics differed among the epilimnion, metalimnion, and the anoxic hypolimnion of the lake and to determine how the properties of viruses in Mono Lake compare to other aquatic environments. Viral capsid size distributions were more similar in the metalimnion and hypolimnion of Mono Lake, while viral tail lengths were more similar in the epilimnion and metalimnion. The percentage of tailed viruses decreased with depth and the relative percentages of tailed phage families changed with depth. The presence of large (>125 nm capsid), untailed viruses in the metalimnion and hypolimnion suggests that eukaryotic viruses are produced in these suboxic and anoxic, hypersaline environments. Capsid diameters of viruses were larger on average in Mono Lake compared to other aquatic environments, and no lemon-shaped or filamentous viruses were found, in contrast to other high-salinity or high-altitude lakes and seas. Our data suggest that the physically and chemically distinct layers of Mono Lake harbor different viral assemblages, and that these assemblages are distinct from other aquatic environments that have been studied. Furthermore, we found that filtration of a sample through a 0.22-μm pore-size filter significantly altered the distribution of viral capsid diameters and tail lengths, resulting in a relative depletion of viruses having larger capsids and longer tails. This observation highlights the potential for bias in molecular surveys of viral diversity, which typically rely on filtration through 0.2- or 0.22-μm pore-size membrane filters to remove bacteria during sample preparation.     

Diatoms shape the biogeography of heterotrophic prokaryotes in early spring in the Southern Ocean

The interplay among microorganisms profoundly impacts biogeochemical cycles in the ocean. Culture-based work has illustrated the diversity of diatom–prokaryote interactions, but the question of whether these associations can affect the spatial distribution of microbial communities is open. Here, we investigated the relationship between assemblages of diatoms and of heterotrophic prokaryotes in surface waters of the Indian sector of the Southern Ocean in early spring. The community composition of diatoms and that of total and active prokaryotes were different among the major ocean zones investigated. We found significant relationships between compositional changes of diatoms and of prokaryotes. In contrast, spatial changes in the prokaryotic community composition were not related to geographic distance and to environmental parameters when the effect of diatoms was accounted for. Diatoms explained 30% of the variance in both the total and the active prokaryotic community composition in early spring in the Southern Ocean. Using co-occurrence analyses, we identified a large number of highly significant correlations between abundant diatom species and prokaryotic taxa. Our results show that key diatom species of the Southern Ocean are each associated with a distinct prokaryotic community, suggesting that diatom assemblages contribute to shaping the habitat type for heterotrophic prokaryotes.     

Prokaryotic diversity in one of the largest hypersaline coastal lagoons in the world

Araruama Lagoon is an environment characterized by high salt concentrations. The low raining and high evaporation rates in this region favored the development of many salty ponds around the lagoon. In order to reveal the microbial composition of this system, we performed a 16S rRNA gene survey. Among archaea, most clones were related to uncultured environmental Euryarchaeota. In lagoon water, we found some clones related to Methanomicrobia and Methanothermococcus groups, while in the saline pond water members related to the genus Haloarcula were detected. Bacterial community was dominated by clones related to Gamma-proteobacteria, Actinobacteria, and Synechococcus in lagoon water, while Salinibacter ruber relatives dominated in saline pond. We also detected the presence of Alpha-proteobacteria, Pseudomonas-like bacteria and Verrucomicrobia. Only representatives of the genus Ralstonia were cosmopolitan, being observed in both systems. The detection of a substantial number of clones related to uncultured archaea and bacteria suggest that the hypersaline waters of Araruama harbor a pool of novel prokaryotic phylotypes, distinct from those observed in other similar systems. We also observed clones related to halophilic genera of cyanobacteria that are specific for each habitat studied. Additionally, two bacterioplankton molecular markers with ecological relevance were analyzed, one is linked to nitrogen fixation (nifH) and the other is linked to carbon fixation by bacterial photosynthesis, the protochlorophyllide genes, revealing a specific genetic distribution in this ecosystem. This is the first study of the biogeography and community structure of microbial assemblages in Brazilian tropical hypersaline environments. This work is directed towards a better understanding of the free-living prokaryotic diversity adapted to life in hypersaline waters.     

Interannual dynamics of viriobenthos abundance and morphological diversity in Chesapeake Bay sediments

Despite significant implications of viral activity in sediment ecosystems, there are limited data describing how sediment viral assemblages respond to broader ecosystem changes. To document this, the spatial and temporal dynamics of viral and bacterial abundance (BA) and changes in the morphological distribution of viruses were examined within three salinity regions over 2 years. Viral abundances (VA) ranged from 0.2 to 17 × 10(10) viruses mL(-1) sediment while direct bacterial counts ranged from 3.8 to 37 × 10(8) cells mL(-1) sediment. Peaks and valleys in the abundance of extracted viruses and bacteria from surface sediments occurred simultaneously, with lows in February 2004 and highs in April 2003. Across all samples, viral and BA were positively correlated (P < 0.001). Vertical profiles showed a decrease in viral and BA with depth in sediments. Based on transmission electron microscopy results, viruses with diminutive capsids (20-50 nm) and from the Myoviridae and Podoviridae viral family types were dominant within surface sediments. The most morphologically diverse viral assemblages occurred in autumn samples from the sandy, polyhaline station and spring samples from the mesohaline station. Seasonal changes showed an average 72% decrease in VA from spring to winter. These observations support the view that viriobenthos assemblages are responsive to seasonal environmental changes and that viral processes have significant implications for the biogeochemical processes mediated by bacterial communities within Bay sediments.     

High-Level Diversity of Tailed Phages,Eukaryote-Associated Viruses,and Virophage-Like Elements in the Metaviromes of Antarctic Soils

The metaviromes of two distinct Antarctic hyperarid desert soil communities have been characterized. Hypolithic communities, cyanobacterium-dominated assemblages situated on the ventral surfaces of quartz pebbles embedded in the desert pavement, showed higher virus diversity than surface soils, which correlated with previous bacterial community studies. Prokaryotic viruses (i.e., phages) represented the largest viral component (particularly Mycobacterium phages) in both habitats, with an identical hierarchical sequence abundance of families of tailed phages (Siphoviridae > Myoviridae > Podoviridae). No archaeal viruses were found. Unexpectedly, cyanophages were poorly represented in both metaviromes and were phylogenetically distant from currently characterized cyanophages. Putative phage genomes were assembled and showed a high level of unaffiliated genes, mostly from hypolithic viruses. Moreover, unusual gene arrangements in which eukaryotic and prokaryotic virus-derived genes were found within identical genome segments were observed. Phycodnaviridae and Mimiviridae viruses were the second-most-abundant taxa and more numerous within open soil. Novel virophage-like sequences (within the Sputnik clade) were identified. These findings highlight high-level virus diversity and novel species discovery potential within Antarctic hyperarid soils and may serve as a starting point for future studies targeting specific viral groups.     

Contrasting taxonomic stratification of microbial communities in two hypersaline meromictic lakes

Hypersaline meromictic lakes are extreme environments in which water stratification is associated with powerful physicochemical gradients and high salt concentrations. Furthermore, their physical stability coupled with vertical water column partitioning makes them important research model systems in microbial niche differentiation and biogeochemical cycling. Here, we compare the prokaryotic assemblages from Ursu and Fara Fund hypersaline meromictic lakes (Transylvanian Basin, Romania) in relation to their limnological factors and infer their role in elemental cycling by matching taxa to known taxon-specific biogeochemical functions. To assess the composition and structure of prokaryotic communities and the environmental factors that structure them, deep-coverage small subunit (SSU) ribosomal RNA (rDNA) amplicon sequencing, community domain-specific quantitative PCR and physicochemical analyses were performed on samples collected along depth profiles. The analyses showed that the lakes harbored multiple and diverse prokaryotic communities whose distribution mirrored the water stratification patterns. Ursu Lake was found to be dominated by Bacteria and to have a greater prokaryotic diversity than Fara Fund Lake that harbored an increased cell density and was populated mostly by Archaea within oxic strata. In spite of their contrasting diversity, the microbial populations indigenous to each lake pointed to similar physiological functions within carbon degradation and sulfate reduction. Furthermore, the taxonomy results coupled with methane detection and its stable C isotope composition indicated the presence of a yet-undescribed methanogenic group in the lakes'' hypersaline monimolimnion. In addition, ultrasmall uncultivated archaeal lineages were detected in the chemocline of Fara Fund Lake, where the recently proposed Nanohaloarchaeota phylum was found to thrive.     

Pulsed-field gel electrophoresis analysis of virus assemblages present in a hypersaline environment.

A method for analyzing virus assemblages in aquatic environments was developed and used for studying the highest-salinity ponds (from 13.4 to 35% salinity) from a multi-pond solar saltern in Alicante, Spain. The protocol consisted of a series of concentration and purification steps including tangential flow filtration and ultracentrifugation, followed by the preparation of total viral nucleic acids that were subsequently separated by pulsed-field gel electrophoresis. For every sample analyzed, a characteristic DNA pattern was obtained, whose complexity was related to viral diversity. The comparison of our results with a similar analysis carried out with marine virus assemblages shows that, as expected, the viral diversity corresponding to the analyzed hypersaline environment is considerably lower than that of a marine environment.     

Changes in viral and bacterial communities during the ice-melting season in the coastal Arctic (Kongsfjorden, Ny-Ålesund)

Microbial communities in Arctic coastal waters experience dramatic changes in environmental conditions during the spring to summer transition period, potentially leading to major variations in the relationship between viral and prokaryotic communities. To document these variations, a number of physico-chemical and biological parameters were determined during the ice-melting season in the coastal Arctic (Kongsfjorden, Ny-?lesund, Spitsbergen). The bacterial and viral abundance increased during the spring to summer transition period, probably associated to the increase in temperature and the development of a phytoplankton bloom. The increase in viral abundance was less pronounced than the increase in prokaryotic abundance; consequently, the viral to prokaryotic abundance ratio decreased. The bacterial and viral communities were stratified as determined by Automated Ribosomal Intergenic Spacer Analysis and Randomly Amplified Polymorphic DNA-PCR respectively. Both the bacterial and viral communities were characterized by a relatively low number of operational taxonomic units (OTUs). Despite the apparent low complexity of the bacterial and viral communities, the link between these two communities was weak over the melting season, as suggested by the different trends of prokaryotic and viral abundance during the sampling period. This weak relationship between the two communities might be explained by UV radiation and suspended particles differently affecting the viruses and prokaryotes in the coastal Arctic during this period. Based on our results, we conclude that the viral and bacterial communities in the Arctic were strongly affected by the variability of the environmental conditions during the transition period between spring and summer.     

Virioplankton Community Structure in Tunisian Solar Salterns

The microbial community inhabiting Sfax solar salterns on the east coast of Tunisia has been studied by means of different molecular and culture-dependent tools that have unveiled the presence of novel microbial groups as well as a community structure different from that of other coastal hypersaline environments. We have focused on the study of the viral assemblages of these salterns and their changes along the salinity gradient and over time. Viruses from three ponds (C4, M1, and TS) encompassing salinities from moderately hypersaline to saturated (around 14, 19, and 35%, respectively) were sampled in May and October 2009 and analyzed by transmission electron microscopy (TEM) and pulsed-field gel electrophoresis (PFGE). Additionally, for all three October samples and the May TS sample, viral metagenomic DNA was cloned in fosmids, end sequenced, and analyzed. Viral concentration, as well as virus-to-cell ratios, increased along the salinity gradient, with around 10¹⁰ virus-like particles (VLPs)/ml in close-to-saturation ponds, which represents the highest viral concentration reported so far for aquatic systems. Four distinct morphologies could be observed with TEM (spherical, tailed, spindled, and filamentous) but with various proportions in the different samples. Metagenomic analyses indicated that every pond harbored a distinct viral assemblage whose G+C content could be roughly correlated with that of the active part of the microbial community that may have constituted the putative hosts. As previously reported for hypersaline metaviromes, most sequences did not have matches in the databases, although some were conserved among the Sfax metaviromes. BLASTx, BLASTp, and dinucleotide frequency analyses indicated that (i) factors additional to salinity could be structuring viral communities and (ii) every metavirome had unique gene contents and dinucleotide frequencies. Comparison with hypersaline metaviromes available in the databases indicated that the viral assemblages present in close-to-saturation environments located thousands of kilometers apart presented some common traits among them in spite of their differences regarding the putative hosts. A small core metavirome for close-to-saturation systems was found that contained 7 sequences of around 100 nucleotides (nt) whose function was not hinted at by in silico search results, although it most likely represents properties essential for hyperhalophilic viruses.     

Isolation and Genetic Analysis of Haloalkaliphilic Bacteriophages in a North American Soda Lake

Mono Lake is a meromictic, hypersaline, soda lake that harbors a diverse and abundant microbial community. A previous report documented the high viral abundance in Mono Lake, and pulsed-field gel electrophoresis analysis of viral DNA from lake water samples showed a diverse population based on a broad range of viral genome sizes. To better understand the ecology of bacteriophages and their hosts in this unique environment, water samples were collected between February 2001 and July 2004 for isolation of bacteriophages by using four indigenous bacterial hosts. Plaque assay results showed a differential seasonal expression of cultured bacteriophages. To reveal the diversity of uncultured bacteriophages, viral DNA from lake water samples was used to construct clone libraries. Sequence analysis of viral clones revealed homology to viral as well as bacterial proteins. Furthermore, dot blot DNA hybridization analyses showed that the uncultured viruses are more prevalent during most seasons, whereas the viral isolates (Aφ and φ2) were less prevalent, confirming the belief that uncultured viruses represent the dominant members of the community, whereas cultured isolates represent the minority species.     

Effects of environmental variation and spatial distance on Bacteria,Archaea and viruses in sub-polar and arctic waters

We investigated the influence of environmental parameters and spatial distance on bacterial, archaeal and viral community composition from 13 sites along a 3200-km long voyage from Halifax to Kugluktuk (Canada) through the Labrador Sea, Baffin Bay and the Arctic Archipelago. Variation partitioning was used to disentangle the effects of environmental parameters, spatial distance and spatially correlated environmental parameters on prokaryotic and viral communities. Viral and prokaryotic community composition were related in the Labrador Sea, but were independent of each other in Baffin Bay and the Arctic Archipelago. In oceans, the dominant dispersal mechanism for prokaryotes and viruses is the movement of water masses, thus, dispersal for both groups is passive and similar. Nevertheless, spatial distance explained 7–19% of the variation in viral community composition in the Arctic Archipelago, but was not a significant predictor of bacterial or archaeal community composition in either sampling area, suggesting a decoupling of the processes regulating community composition within these taxonomic groups. According to the metacommunity theory, patterns in bacterial and archaeal community composition suggest a role for species sorting, while patterns of virus community composition are consistent with species sorting in the Labrador Sea and suggest a potential role of mass effects in the Arctic Archipelago. Given that, a specific prokaryotic taxon may be infected by multiple viruses with high reproductive potential, our results suggest that viral community composition was subject to a high turnover relative to prokaryotic community composition in the Arctic Archipelago.     

Prokaryotic and viral community of the sulfate-rich crust from Peñahueca ephemeral lake,an astrobiology analogue

Peñahueca is an athalassohaline hypersaline inland ephemeral lake originated under semiarid conditions in the central Iberian Peninsula (Spain). Its chemical composition makes it extreme for microbial life as well as a terrestrial analogue of other planetary environments. To investigate the persistence of microbial life associated with sulfate-rich crusts, we applied cultivation-independent methods (optical and electron microscopy, 16S rRNA gene profiling and metagenomics) to describe the prokaryotic community and its associated viruses. The diversity for Bacteria was very low and was vastly dominated by endospore formers related to Pontibacillus marinus of the Firmicutes phylum. The archaeal assemblage was more diverse and included taxa related to those normally found in hypersaline environments. Several ‘metagenome assembled genomes’ were recovered, corresponding to new species of Pontibacillus, several species from the Halobacteria and one new member of the Nanohaloarchaeota. The viral assemblage, although composed of the morphotypes typical of high salt systems, showed little similarity to previously isolated/reconstructed halophages. Several putative prophages of Pontibacillus and haloarchaeal hosts were identified. Remarkably, the Peñahueca sulfate-rich metagenome contained CRISPR-associated proteins and repetitions which were over 10-fold higher than in most hypersaline systems analysed so far.     

Links between viruses and prokaryotes throughout the water column along a North Atlantic latitudinal transect

Viruses are an abundant, diverse and dynamic component of marine ecosystems and have a key role in the biogeochemical processes of the ocean by controlling prokaryotic and phytoplankton abundance and diversity. However, most of the studies on virus–prokaryote interactions in marine environments have been performed in nearshore waters. To assess potential variations in the relation between viruses and prokaryotes in different oceanographic provinces, we determined viral and prokaryotic abundance and production throughout the water column along a latitudinal transect in the North Atlantic. Depth-related trends in prokaryotic and viral abundance (both decreasing by one order of magnitude from epi- to abyssopelagic waters), and prokaryotic production (decreasing by three orders of magnitude) were observed along the latitudinal transect. The virus-to-prokaryote ratio (VPR) increased from ∼19 in epipelagic to ∼53 in the bathy- and abyssopelagic waters. Although the lytic viral production decreased significantly with depth, the lysogenic viral production did not vary with depth. In bathypelagic waters, pronounced differences in prokaryotic and viral abundance were found among different oceanic provinces with lower leucine incorporation rates and higher VPRs in the North Atlantic Gyre province than in the provinces further north and south. The percentage of lysogeny increased from subpolar regions toward the more oligotrophic lower latitudes. Based on the observed trends over this latitudinal transect, we conclude that the viral–host interactions significantly change among different oceanic provinces in response to changes in the biotic and abiotic variables.     

Seasonal and spatial variations in microbial community structure and diversity in the acid stream draining across an ongoing surface mining site

This study examined the microbial community in an acidic stream draining across the Yun-Fu pyrite mine (Guangdong, China), where extremely acidic mine water is a persistent feature due to the intensive surface mining activities. Analysis of terminal restriction fragment length polymorphism (TRFLP) of 16S rRNA gene sequences showed that microbial populations varied spatially and seasonally and correlated with geochemical and physical conditions. After the stream moves from underground to the surface, the microbial community in the acidic water rapidly evolves into a distinct community close to that in the downstream storage pond. Comparisons of TRFLP peaks with sequenced clone libraries indicated that bacteria related to the recently isolated iron-oxidizer Ferrovum myxofaciens dominated the acidophilic community throughout the year except for the samples collected in spring from the storage pond, where Ferroplasma acidiphilum -like archaea represented the most abundant group. Acidithiobacillus ferrooxidans -affiliated organisms increased along the acid stream and remained common over the year, whereas Leptospirillum ferrooxidans -like bacteria were negligible or even not detected in the analyzed samples. The data indicate that changes in environmental conditions are accompanied by significant shifts in community structure of the prokaryotic assemblages at this opencast mining site.     

Transporting Ocean Viromes: Invasion of the Aquatic Biosphere

Studies of marine viromes (viral metagenomes) have revealed that DNA viruses are highly diverse and exhibit biogeographic patterns. However, little is known about the diversity of RNA viruses, which are mostly composed of eukaryotic viruses, and their biogeographic patterns in the oceans. A growth in global commerce and maritime traffic may accelerate spread of diverse and non-cosmopolitan DNA viruses and potentially RNA viruses from one part of the world to another. Here, we demonstrated through metagenomic analyses that failure to comply with mid-ocean ballast water exchange regulation could result in movement of viromes including both DNA viruses and RNA viruses (including potential viral pathogens) unique to geographic and environmental niches. Furthermore, our results showed that virus richness (known and unknown viruses) in ballast water is associated with distance between ballast water exchange location and its nearest shoreline as well as length of water storage time in ballast tanks (voyage duration). However, richness of only known viruses is governed by local environmental conditions and different viral groups have different responses to environmental variation. Overall, these results identified ballast water as a factor contributing to ocean virome transport and potentially increased exposure of the aquatic bioshpere to viral invasion.     

Proportion of prokaryotes enumerated as viruses by epifluorescence microscopy

It is well known that there are prokaryotes small in size (e.g. ultra-microprokaryotes) that pass through a 0.2-μm filter. As bacterial and viral abundances are determined by epifluorescence microscopy and the differentiation between them is based on particle size, some bacteria can be erroneously enumerated as viruses, namely in marine waters where bacteria are small. However, there is no information on the proportion of prokaryotes that could be misidentified as viruses by epifluorescence microscopy. In this work, we assessed, in water samples collected in the estuarine system Ria de Aveiro (Portugal), the proportion of prokaryotes that could be counted as viruses by the current widespread epifluorescence microscopy and, for the first time, by fluorescence in situ hybridization (FISH). The total number of particles was determined on membranes of 0.2 and 0.02 μm after staining with 4′,6-diamidino-2-phenylindole (DAPI), and the number of prokaryotes (Bacteria and Archaea) was determined by FISH for both pore size membranes. The results show that, in the marine zone of the estuarine system, 28 % of particles enumerated as virus-like particles were prokaryotes, but, in the brackish water zone, only 13 % of the particles counted as viruses were actually prokaryotic cells. Epifluorescence microscopy overestimates viral abundance, and also the ratio viruses:prokaryotes, and this error must be taken into consideration because it can vary significantly within a system. In fact, in the marine zone of an estuarine system, the overestimation of viral abundance can be twice as high as in the brackish water zone.     

Analysis of dsDNA and RNA viromes in methanogenic digesters reveals novel viral genetic diversity

Although viruses are not the key players of the anaerobic digestion process, they may affect the dynamics of bacterial and archaeal populations involved in biogas production. Until now viruses have received very little attention in this specific habitat; therefore, as a first step towards their characterization, we optimized a virus filtration protocol from anaerobic sludge. Afterwards, to assess dsDNA and RNA viral diversity in sludge samples from nine different reactors fed either with waste water, agricultural residues or solid municipal waste plus agro‐food residues, we performed metagenomic analyses. As a result we showed that, while the dsDNA viromes (21 assigned families in total) were dominated by dsDNA phages of the order Caudovirales, RNA viruses (14 assigned families in total) were less diverse and were for the main part plant‐infecting viruses. Interestingly, less than 2% of annotated contigs were assigned as putative human and animal pathogens. Our study greatly extends the existing view of viral genetic diversity in methanogenic reactors and shows that these viral assemblages are distinct not only among the reactor types but also from nearly 30 other environments already studied, including the human gut, fermented food, deep sea sediments and other aquatic habitats.     

Distinctive patterns in prokaryotic community composition in response to viral lysis and flagellate grazing in freshwater microcosms

相似文献   

Viral distribution and life strategies in the Bach Dang Estuary, Vietnam

Although the structure and dynamics of planktonic viruses in freshwater and seawater environments are relatively well documented, little is known about the occurrence and activity of these viruses in estuaries, especially in the tropics. Viral abundance, life strategies, and morphotype distribution were examined in the Bach Dang Estuary (Vietnam) during the dry season in 2009. The abundance of both viruses and their prokaryotic hosts decreased significantly from upstream to downstream, probably as the result of nutrient dilution and osmotic stress faced by the freshwater communities. The antibiotic mitomycin-C revealed that the fraction of lysogenic cells was substantially higher in the lower seawater part of the estuary (max 27.1%) than in the upper freshwater area where no inducible lysogens were observed. The question of whether there is a massive, continuous induction of marine lysogens caused by the mixing with freshwater is considered. Conversely, the production of lytic viruses declined as salinity increased, indicating a spatial succession of viral life strategies in this tropical estuary. Icosahedral tailless viruses with capsids smaller than 60?nm dominated the viral assemblage throughout the estuary (63.0% to 72.1% of the total viral counts), and their distribution was positively correlated with that of viral lytic production. Interestingly, the gamma-proteobacteria explained a significant portion of the variance in the <60?nm and 60 to 90?nm tailless viruses (92% and 80%, respectively), and in the Myoviridae (73%). Also, 60% of the variance of the tailless larger viruses (>90?nm) was explained by the beta-proteobacteria. Overall, these results support the view that the environment, through selection mechanisms, probably shapes the structure of the prokaryotic community. This might be in turn a source of selection for the virioplankton community via specific affiliation favoring particular morphotypes and life strategies.