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. 2015 Mar 17;9(4):990-1002.
doi: 10.1038/ismej.2014.197.

Oceanographic structure drives the assembly processes of microbial eukaryotic communities

Adam Monier  1 , Jérôme Comte  2 , Marcel Babin  3 , Alexandre Forest  3 , Atsushi Matsuoka  3 , Connie Lovejoy  4
Affiliations

Oceanographic structure drives the assembly processes of microbial eukaryotic communities

Adam Monier et al. ISME J. .

Abstract

Arctic Ocean microbial eukaryote phytoplankton form subsurface chlorophyll maximum (SCM), where much of the annual summer production occurs. This SCM is particularly persistent in the Western Arctic Ocean, which is strongly salinity stratified. The recent loss of multiyear sea ice and increased particulate-rich river discharge in the Arctic Ocean results in a greater volume of fresher water that may displace nutrient-rich saltier waters to deeper depths and decrease light penetration in areas affected by river discharge. Here, we surveyed microbial eukaryotic assemblages in the surface waters, and within and below the SCM. In most samples, we detected the pronounced SCM that usually occurs at the interface of the upper mixed layer and Pacific Summer Water (PSW). Poorly developed SCM was seen under two conditions, one above PSW and associated with a downwelling eddy, and the second in a region influenced by the Mackenzie River plume. Four phylogenetically distinct communities were identified: surface, pronounced SCM, weak SCM and a deeper community just below the SCM. Distance-decay relationships and phylogenetic structure suggested distinct ecological processes operating within these communities. In the pronounced SCM, picophytoplanktons were prevalent and community assembly was attributed to water mass history. In contrast, environmental filtering impacted the composition of the weak SCM communities, where heterotrophic Picozoa were more numerous. These results imply that displacement of Pacific waters to greater depth and increased terrigenous input may act as a control on SCM development and result in lower net summer primary production with a more heterotroph dominated eukaryotic microbial community.

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Figures

Figure 1
Figure 1
Location of the Beaufort Sea stations sampled in August 2009. White arrows indicate the Mackenzie River main outlets. The Mackenzie Trough is marked with a dashed line and the shaded contours denotes the bathymetry retrieved from the International Bathymetric Chart of the Arctic Ocean (IBCAO v.3; Jakobsson et al., 2012). Station latitude and longitude coordinates are: St 430 (71.217,−136.720); St 460 (70.678,−136.047); St 540 (70.751,−137.892); St 620 (70.680,−139.627); St 670 (69.798,−138.441); St 760 (70.554,−140.798).
Figure 2
Figure 2
Profiles from the Beaufort Sea selected stations. (a) Chlorophyll a concentration, from calibrated in situ fluorescence profiles (mg Chl a m−3). (b) Salinity, which is unitless but often referred to as practical salinity units (PSU). (c) Percentage of surface PAR. (d) Nitrate concentration from calibrated in situ profiles (μM). The red lines indicate weak SCM stations; depth indicated on Y axis.
Figure 3
Figure 3
Water mass and microbial community partitioning across the Beaufort Sea water column. (a) Hierarchical clustering on temperature, salinity and dissolved O2 distances (centered-scaled; z-score scale denoted by color gradient) on 24 seawater samples collected for microbial sampling. The water column partitioned into the PML, PSW and PWW water masses. The station number and depth classifications are indicated by the given symbols. (b) Hierarchical clustering on weighted UniFrac distances (dW5000) of OTUs. Communities were classified into four distinct clusters: Surface (brown; support, 65% of 1000 jackknife replicates), SCM (cyan, 58%), wSCM (pink, 91%) and Deep (lavender, 93%). (c) NMDS ordination on dW5000 distances (2D stress=0.0971). For each community cluster, ellipses represent 95% confidence intervals. Arrows represent best explanatory environmental and biological variables fitted onto the ordination space (environmental factors as in Supplementary Table S3).
Figure 4
Figure 4
Community phylogenetic similarities across the selected Beaufort Sea stations. (a) Distance–decay curves for microbial communities from contiguous water masses (Surface vs SCM or wSCM; Deep vs SCM or wSCM) or from the same water mass (SCM vs wSCM). Each dot represents a pairwise comparison of community phylogenetic similarity (expressed as ln(1−dW5000)) and geographic distance between microbial communities. Lines represent best linear fit, with a gray-shaded 95% confidence interval, for which the slope was significantly different than zero (SCM vs Surface: slope=−0.0003 and Bonferroni adjusted P=0.028; SCM vs wSCM: slope=−0.0008 and adjusted P=0.058). (b) NRI (in units of standard deviation) for each community cluster. NRIs were based on standardized mean pairwise distances from community cluster OTU phylogenetic tree. To generate null models to compute NRIs, tips of this phylogenic tree were shuffled to create randomized OTU phylogenetic relationships. An alternative null model yielded similar results (see Materials and methods).
Figure 5
Figure 5
Taxonomic composition differences among Beaufort Sea microbial communities. Areas colored according to the top legend denote the relative abundances of the main eukaryotic lineages. Markers represent each sample as in Figure 3. Left dendrogram represents the hierarchical clustering output on dW5000, as displayed in Figure 3b.
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