Supplementary Material The supplementary information contains four figures S1-S4. These figures depict the abundance, richness, and phylum level taxa between the seamounts analysed in this manuscript. In addition, we have a supplementary figure graphically showing the error output from the Multivariate Regression Trees run on the microbial and environmental data. from Microbe biogeography tracks water-masses in a dynamic oceanic frontal system

Dispersal limitation, not just environmental selection, plays an important role in microbial biogeography. The distance–decay relationship is thought to be weak in habitats where dispersal is high, such as in the pelagic environment, where ocean currents facilitate microbial dispersal. Most studies of microbial community composition to date have observed little geographical heterogeneity on a regional scale (100 km).We present a study of microbial communities across a dynamic frontal zone in the South West Indian Ocean and investigate the spatial structure of the microbes with respect to the different water masses separated by these fronts.We collected 153 samples of free-living microorganisms from five seamounts located along a gradient from subtropical to subantarctic waters and across three depth layers, (i) the sub-surface chlorophyll maximum (approx. 40 m), (ii) the bottom of the euphotic zone (approx. 200 m) and (iii) the benthic boundary layer (300–2000 m). Diversity and abundance of microbial operational taxonomic units (OTUs) was assessed by amplification and sequencing of the 16S rRNA gene on an Illumina MiSeq platform.Multivariate analyses showed that microbial communities were structured more strongly by depth than by latitude, with similar phyla occurring within each depth stratum across seamounts. The deep layer was homogeneous across the entire survey area, corresponding to the spread of Antarctic intermediate water. However, within both the sub-surface layer and the intermediate depth stratum there was evidence for OTU turnover across fronts. The microbiome of these layers appears to be divided into three distinct biological regimes corresponding to the subantarctic surface water, the convergence zone and subtropical. We show that microbial biogeography across depth and latitudinal gradients is linked to the water-masses the microbes persist in, resulting in regional patterns of microbial biogeography, that correspond to the regional scale physical oceanography

Read 2

Read 2 of Illumina Miseq run on 16S rDNA sequences

Read 1

Read 1 of Illumina Miseq run on 16S rDNA sequences

metadata

This metadata matches all the sequencing data from Read 1 and Read 2 in the same package

Selection of the multivariate regression tree for the global datasets of vent species.

<p>The datasets are species data from Bachraty et al. <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1001234#pbio.1001234-Bachraty1" target="_blank">[8]</a> (red/filled circles/solid line) and the same dataset with Southern Ocean vent sites added (blue/open circles/dashed line). Top panel: Frequency plot of the optimal tree size for 1,000 multiple cross-validations. The most common optimal tree size was five or seven provinces for the Bachraty et al. <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1001234#pbio.1001234-Bachraty1" target="_blank">[8]</a> dataset and 11 provinces for the combined dataset. Bottom panel: The cross-validated relative error indicates that predictive power is similar for a wide range of tree sizes. Vertical bars indicate ± one standard error, and the horizontal lines indicate one standard error above the minimum cross-validated relative error.</p

Dominant fauna at East Scotia Ridge vents E2 and E9.

<p>All identifications are putative and await detailed taxonomic and molecular analysis.</p

Maps of the position and geophysical setting of the ESR vents.

<p>(A) The Scotia Sea showing the ESR in relation to the Scotia Plate (SCO), South Sandwich Plate (SAN), South American Plate (SAM), the Antarctic Plate (ANT), the Antarctic Peninsula (AP), and the South Sandwich Trench (SST). Oceanographic features shown include the Polar Front (PF), the Sub-Antarctic Front (SAF), and the southern Antarctic Circumpolar Current Front (SACCF). The sites E2 and E9 are indicated by red arrows. (B) Ship-based swath bathymetry of the vent sites at E2 showing the axial summit graben. The black circle indicates the sites of main venting. (C and D) ROV-based 3-D swath bathymetry of E2 (C) and high-resolution swath bathymetry of the major steep-sided fissure that runs north–south through the centre of the site, between longitude 30° 19.10′W and 30° 19.15′W (D). Dog's Head vent site is indicated. White arrows indicate vent sites not mentioned in text. (E) Ship-based swath bathymetry of the vent sites at E9 showing the axial fissures and the collapsed crater called the Devil's Punchbowl. The black spot indicates the sites of main venting. (F) ROV-based 3-D swath bathymetry of the vent sites at E9. The vent sites Ivory Tower, Car Wash, and Black and White are indicated. Other vent sites are indicated by white arrows.</p

Collage of frame grabs of high-definition video to show fauna dispersion on the E9 vent site Ivory Tower.

<p>The vertical chimneys are covered with the anomuran <i>Kiwa</i> n. sp., and the area between the chimneys is occupied primarily by an undescribed peltospiroid gastropod (Dive 142, 2,398 m depth, ROV heading 090°). Scale bar: 1 m for foreground. Collage created by L. M.</p

Results of geographically constrained clustering using multivariate regression trees.

<p>An 11-province model based on the combined dataset was the most frequent optimal model when using multiple cross-validations. Vent provinces are resolved comprising the Mid-Atlantic Ridge, the ESR, the northern, central, and southern East Pacific Rise, a further province located south of the Easter Microplate, four provinces in the western Pacific, and a further Indian Ocean province.</p

Chemical composition of the vent fluid end-member at E2 and E9 vent fields.

<p>Data from the Nibelungen vent field on the Mid-Atlantic Ridge <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1001234#pbio.1001234-Schmidt1" target="_blank">[26]</a>, Kairei on the Central Indian Ridge <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1001234#pbio.1001234-Gamo1" target="_blank">[25]</a>, the 17.5°S site on the South East Pacific Rise <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1001234#pbio.1001234-Charlou1" target="_blank">[24]</a>, and sites in the Lau and Pacmanus back-arc basins <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1001234#pbio.1001234-Fouquet1" target="_blank">[27]</a>,<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1001234#pbio.1001234-Craddock1" target="_blank">[28]</a> are provided for comparison. These represent the closest known mid-ocean ridge vent sites to E2 and E9 and geologically comparable back-arc basin sites.</p