Abstract
We determined the taxonomic composition of the bacterioplankton of the epipelagic zone of the Atlantic Ocean along a latitudinal transect (51°S–47°N) using Illumina sequencing of the V5-V6 region of the 16S rRNA gene and inferred co-occurrence networks. Bacterioplankon community composition was distinct for Longhurstian provinces and water depth. Free-living microbial communities (between 0.22 and 3 μm) were dominated by highly abundant and ubiquitous taxa with streamlined genomes (e.g., SAR11, SAR86, OM1, Prochlorococcus) and could clearly be separated from particle-associated communities which were dominated by Bacteroidetes, Planktomycetes, Verrucomicrobia, and Roseobacters. From a total of 369 different communities we then inferred co-occurrence networks for each size fraction and depth layer of the plankton between bacteria and between bacteria and phototrophic micro-eukaryotes. The inferred networks showed a reduction of edges in the deepest layer of the photic zone. Networks comprised of free-living bacteria had a larger amount of connections per OTU when compared to the particle associated communities throughout the water column. Negative correlations accounted for roughly one third of the total edges in the free-living communities at all depths, while they decreased with depth in the particle associated communities where they amounted for roughly 10% of the total in the last part of the epipelagic zone. Co-occurrence networks of bacteria with phototrophic micro-eukaryotes were not taxon-specific, and dominated by mutual exclusion (~60%). The data show a high degree of specialization to micro-environments in the water column and highlight the importance of interdependencies particularly between free-living bacteria in the upper layers of the epipelagic zone.
Highlights
Microorganisms play an essential role in all terrestrial and aquatic ecosystems
In silico test of primers coverage (F807 and R1050) (Bohorquez et al, 2012; Klindworth et al, 2013) showed a good coverage (86.4%) of bacterial lineages included in the SILVA database
Taxonomic classification of the bacterial sequences showed that the majority of the OTUs (59–69%) could be affiliated at the order level, between 35 and 42% of OTUs could be affiliated to a family, and 16–24% were assigned to a genus (Table S6)
Summary
Microorganisms play an essential role in all terrestrial and aquatic ecosystems. Their activity directly influences biogeochemical cycles of essential elements like carbon, nitrogen, and sulfur (Azam and Malfatti, 2007; Falkowski et al, 2008; Zehr and Kudela, 2011; Moran et al, 2012), and has a tremendous effect on the whole planet. Prokaryotes (bacteria and archaea) are the most abundant living organisms, with an average of 5 × 105 cells per milliliter of sea water on the continental shelf and in the upper 200 m of the oceanic water column (Whitman et al, 1998). This number refers only to the domains of bacteria and archaea without taking into account eukaryotic microorganisms (Caron et al, 2012; de Vargas et al, 2015), viruses (Suttle, 2005; Brum et al, 2015) or the ultra-small bacteria discovered recently (Brown et al, 2015). The high number of microorganisms (archaea, bacteria, eukaryotes) and viruses per milliliter of seawater and their long evolutionary history (Cavalier-Smith, 2006) naturally forces the coexistence, interaction, and possibly co-evolution of microorganisms in the same spatial niche
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