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The upper limit of the Bathyal Zone lies at the continental shelf-break, defined by physical, geological and biological characteristics. The Circalittoral, the deepest faunal zone on the continental shelf, extends down to the lower limit of multicellular algae, that is also the upper limit of the Bathyal.

Although of primary importance, because of major gradient changes, the shelf-edge sector remains a poorly known environment because it lies between two distinct zones of interest, the coastal-shelf environment and the deep realms. This limit is independent of that of the phytal - aphytal zones, which is related to the transparency of the waters.

In the same way there is no relation between the upper Bathyal limit and the Exclusive Economic Zone EEZ previously limited at m depth. This depth has no scientific justification, just an economical one. However there is often a confusion about. The Bathyal zone extends along the continental slope until the Abyssal rise at about m.

The bathyal zone or bathypelagic is the pelagic zone that extends from a depth of to meters below the ocean surface. This zone is found from a depth of around m to the bottom of the ocean. Get a new mixed Fun Trivia quiz each day in your email. It's a fun way to start your day! Why would the angle of a river increase as it is measured downstream? I did an experiment, and the results increased as I measured further downstream, which implied that the river flows slightly uphill.

Why would this happen? Manganese balls on the abyssal seafloor are formed around organic material, usually around a shark tooth. All sequence data were denoised and analysed using the standard operating procedure in mothur. We further compared the average composition of deep-sea surface sediment communities to an average community from subsurface sediment 5 samples between 2.

This demonstrates that pelagic, benthic surface and deep-subsurface environments exhibit distinct bacterial community signatures already at broad taxonomic resolution levels.


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At a finer taxonomic resolution, even less overlap was detected, with none of the twenty most abundant OTU 0. The deep-sea sediment core microbiome, here defined as OTU 0. They included many taxa comprising heterotrophic polymer degraders, as is expected for deep-sea sediment communities, where the main source of energy and nutrients is marine detritus [ 3 ]. Only three highly abundant OTU 0. Among these, two OTU 0. Nevertheless, the truly cosmopolitan OTU 0.

They ranged from 0.

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Thus, both types appear to be more sequence-abundant in polar, cold regions e. Antarctic, Arctic , and less abundant in warmer regions e. Overall, sequences of the JTB clade have been reported in a range of local and regional marine benthic studies e. Previously, members of the OM1 clade have been predominantly described from seawater [ 73 , 74 ], and further investigations are needed to address their functional relevance in deep-sea sediments.

Our study supports the hypothesis of a distinct core microbiome in global deep-sea sediments with yet unknown adaptations, as no close relative has been cultivated yet or has had its genomic composition established. This core bacterial community may consist of generalists highly adapted to life in the deep sea, e. In addition, we tested whether the diversification of this core microbiome accounts for a substantial fraction of the observed diversity of surface deep-sea sediments, as shown for the microbiome of cold seeps [ 15 ].

In cold seep communities, endemic taxa closely related to the members of the core microbiome make up a substantial proportion of total richness. This trend could not be confirmed for deep-sea sediments, as those families that contained the 18 most abundant OTU 0. Our data suggest that a substantial fraction of the global diversity of bacteria in deep-sea sediments is endemic. Since deep-sea sediments can be considered as a relatively stable and uniform environment, forming a matrix of fine particles that immobilizes their bacterial inhabitants, dispersal of benthic bacteria in the deep sea is probably limited.

Microbial dispersal may, however, occur via the resuspension of sediments by water currents and faunal activity i. Yet, deep-water currents above the seafloor are usually weak, hence long-distance passive transport of deep-sea sediments probably occurs rarely [ 78 , 79 ].

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Comparing community composition at the broad taxonomic levels of phylum to class, a rather uniform distribution of the sediment microbiota was detected across all oceans Fig 2a—2b , S5 Fig. Differences in community composition appeared at the family and higher taxonomic resolution levels data not shown , as reported from other global microbiome studies of permafrost soils [ 80 ], cryosphere habitats [ 81 ], or other environments see also [ 46 ]. Previous studies of bacterial OTU-distribution have shown that average spatial ranges of OTU can change with environment, latitude and sequence abundance [ 23 , 82 ].

Here, a high degree of endemism was detected; higher than in water column environments [ 82 ]. At the resolution of OTU 0. This observation supports previous findings on global bacterial distribution in other ocean realms [ 23 , 82 ]. In comparison, the Census of the Diversity of Abyssal Marine Life also reported that the majority of deep-sea animals occurred at only one or two sampling sites, at a similar proportion as bacterial taxa in this study [ 17 ].

However, it is not known whether this recurring observation points to severe undersampling of the deep sea, or indeed to rarity and limited dispersal [ 17 , 83 ]. We looked at another group of rare OTU 0. These OTU 0. The majority of these OTU 0. Such a high turnover of OTU 0. In contrast, sequence-abundant OTU 0. Consequently, the more sequence-abundant an OTU is, the more likely it is to be found in samples located much further away. These results indicate that, despite our assumption of very slow rates of dispersal in the deep-sea environment, it is still possible to observe truly ubiquitous, cosmopolitan taxa, at the level of their 16S V6 gene signature.

Future studies should direct effort to the question of their identities, traits and functions, to better understand the evolution of core microbiota in the deep-sea realm and on Earth in general. Dashed lines indicate linear models for range-abundance relationships: a Adj. The analysis of how relative sequence abundance changes with geographic range either defined as the number of common samples or the maximum distance an OTU 0.

Comparable patterns have been reported for microbial eukaryotes in the deep sea, where the majority of taxa were regionally restricted, and only a small percentage maintained cosmopolitan distributions [ 18 ]. Positive range-abundance relationships for bacterial types have also been observed in a variety of other microbial realms, including soil and the pelagic realm [ 19 — 24 ].

While methods based on sequencing of 16S rRNA genes do not fully reflect the true abundance of organisms in the environment [ 85 ], a plausible ecological explanation for the observed positive range-abundance relationship would be that higher local population sizes—as approximated by high sequence abundances here—enable a larger organismal pool to be further passively dispersed, higher colonization and lower extinction rates mass-effect of metapopulation dynamics as described in e. In addition other mechanisms may generate positive range-abundance relationships, such as resource breadth and availability, also proposed previously [ 86 , 87 ].

As aforementioned, undersampling is most likely one reason for the pattern observed here, as we still miss—despite the use of high-throughput sequencing—very low abundant types in some samples S3 Fig and therefore underestimate their distribution ranges. Significant distance-decay relationships for bacterial communities have been reported in a global study of pelagic and seafloor environments [ 82 ], in soil [ 88 — 90 ], woodland [ 91 ], and saltmarsh sediments [ 92 ], suggesting this relationship to hold true across different ecosystems. We found that community similarity based on the proportion of shared OTU 0.

The proportion of shared OTU 0. Dotted lines are linear model fits. We also tested whether the distance-decay relationship holds true when considering water paths around continents instead of straight distances earth surface between sampling locations Fig 4b.

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A connectivity of microbial populations via deep-water currents has been suggested for sediment and deep-water communities, and for benthic thermophilic endospores [ 93 — 95 ]. This dispersal mechanism would be similar to what has been proposed for larval stages of benthic deep-sea fauna [ 1 ]. In the future, more advanced sampling schemes and models [ 96 ] should be applied to test for the effect of deep-water transport speed, direction on bacterial deep-sea communities. The distance-decay relationship observed for bacterial communities may arise from multiple mechanisms, involving environmental filtering, neutral processes, and isolation by distance, which is a complex product of limited dispersal, ecological drift, and speciation processes [ 97 ].

To shed more light on the mechanisms generating the observed distance-decay relationship, we also considered changes in community similarity as a function of differences in water depth, latitudinal distance, and longitudinal distance Fig 4c—4e. Increasing water depth is a general indicator of decreasing particle flux as key energy source for deep-sea bacteria [ 3 ]. A relatively weak, but significant relationship was observed for community changes along water depth Fig 4. This confirmed that even below m, bacterial communities are structured by changes in biological or physical parameters that are correlated with water depth, especially the dynamics in particulate organic matter flux that represent the main source of energy and carbon [ 31 , 98 ].

The range of investigated water depths itself did not explain a significant fraction of community variation when other variables, such as geographic distance or organic carbon content, were considered S8 Fig. Latitudinal distance correlates with climatic regions of the surface ocean, and previous studies have reported correlations between bacterial community richness and latitude for communities from the pelagic [ 21 , 99 ], and from terrestrial realms [ , ] for controversial findings see [ 93 , ].

But, according to the physical stability of the deep sea, latitudinal distances were neither a good predictor of community similarity Fig 4d , nor of richness in deep-sea surface sediment communities S9 Fig. However, a trend analysis based on LOESS curve fitting Fig 4d indicated that community similarity increased towards both polar regions, as detected already for epipelagic marine bacteria [ 24 ]. But interestingly, changes in community similarity with geographic distance appeared to be mainly due to changes with longitude Fig 4c.

On the one hand, geographic features like mid-ocean ridges, and deep-water currents [ 83 ], but also land masses, may present barriers to dispersal along longitudinal axes. However, this pattern may also result from changes in productivity regimes with proximity to the productive ocean margins.

We further tested how other environmental parameters may account for changes in bacterial community composition based on relative sequence abundances with geographic distance. For example, the role of surface productivity, particle flux, and of other biological factors in the structuring of benthic communities have previously been suggested [ 6 , 98 ]. Productivity indices based on biogeochemical provinces defined by Longhurst et al.

Discrepancies between surface productivity and total organic carbon availability at the seafloor may be explained by biological processes or hydrographic features altering vertical particle flux, or by a lateral input of organic material. The effect of organic matter availability on benthic communities is in agreement with general trends reported for different benthic size classes in the deep sea [ 1 , 98 , — ].

The effects of these factors on bacterial community structure and distribution will need to be further explored for the deep seafloor at the global scale. Future studies should aim at integrating different spatial scales and at measuring a large range of environmental parameters, e. By investigating the composition and distribution of benthic deep-sea bacterial communities at the global scale, we show that bacterial communities of deep-sea surface sediments are distinct from those of the pelagic or the subsurface seafloor biosphere, and this already at the class level.

Deep-sea sediments are inhabited by a core community of few cosmopolitan, sequence-abundant bacterial OTU which are affiliated with the JTB marine benthic group class Gammaproteobacteria , order Xanthomonadales , and the OM1 clade class Actinobacteria , order Acidimicrobiales , but which still lack representative genomes and cultured organisms.

At the same time, our study revealed a high degree of endemism and isolation, hence a significant part of bacterial communities in deep-sea surface sediments appears to be geographically restricted. We found evidence that the relative sequence-abundance of a taxon and the size of its geographic range are positively related to each other.

Bathyal Zone

We also detected that deep-sea sediment bacterial community similarity decreases with increasing geographic distance, most likely due to isolation-by-distance processes especially along longitudes. Bacterial communities mostly changed with indicators of productivity regimes, such as TOC content of sediments. Standard deviations for richness are indicated in black. Water depth of each sample is displayed in red right y axis. Colors in a mark the taxonomic categories phylum: white, class: red, order: orange, family: yellow.

The boxplots show a summary of permutations, calculated with random subsampling, including absolute singletons for comparison. Relative abundances were averaged across samples and oceans. Error bars indicate standard deviations when considering samples from one oceanic region. Non-metric multidimensional scaling plots for community composition at the class a-b and OTU 0. Samples originating from a same oceanic region are connected by a coloured line, as follows: black: South Pacific; red: North Pacific St.

Total number of samples considered is Total number of sequences in the dataset is , Total number of samples considered is 5. Total number of sequences in the dataset is 72, We thank the Editor and two anonymous reviewers for helpful comments on an earlier draft of this manuscript. Performed the experiments: CB. Analyzed the data: CB LZ. Browse Subject Areas? Click through the PLOS taxonomy to find articles in your field. Kellogg, U. Potential contaminants in sequencing data Betaproteobacteria, especially affiliated to Burkholderiales and Ralstonia , have been reported from deep-sea sediments and also for the terrestrial deep subsurface [ 16 ] and references therein.

Statistical analyses Observed richness i. The microbiome of deep-sea surface sediments Marine sediments characteristically show a dominance of Proteobacteria [ 6 , 34 ]. Download: PPT. Fig 1. Table 1. Most common OTU 0. Deep-sea sediment bacteria endemism, cosmopolitanism and positive range-abundance relationship Since deep-sea sediments can be considered as a relatively stable and uniform environment, forming a matrix of fine particles that immobilizes their bacterial inhabitants, dispersal of benthic bacteria in the deep sea is probably limited. Fig 2. Proportions of unique and cosmopolitan OTU between oceanic regions and individual samples at the class a, b and OTU 0.

Distance-decay and predictors for variation in bacterial surface sediment communities Significant distance-decay relationships for bacterial communities have been reported in a global study of pelagic and seafloor environments [ 82 ], in soil [ 88 — 90 ], woodland [ 91 ], and saltmarsh sediments [ 92 ], suggesting this relationship to hold true across different ecosystems. Fig 4. Distance-decay and geographic patterns of bacterial deep-sea sediment communities.

Effects of spatial and environmental parameters on seafloor bacterial community composition We further tested how other environmental parameters may account for changes in bacterial community composition based on relative sequence abundances with geographic distance. Conclusion By investigating the composition and distribution of benthic deep-sea bacterial communities at the global scale, we show that bacterial communities of deep-sea surface sediments are distinct from those of the pelagic or the subsurface seafloor biosphere, and this already at the class level.

Supporting Information. S1 Fig.

S2 Fig. S3 Fig.


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Species accumulation curves based on different bacterial taxonomic categories: phylum to family a , genera b , and OTU 0. S4 Fig.

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S5 Fig. Differences in bacterial community composition between oceanic regions.