Distinctive Microbial Community Structure in Highly Stratified Deep-Sea Brine Water Columns

Atlantis II and Discovery are two hydrothermal and hypersaline deep-sea pools in the Red Sea rift that are characterised by strong thermo-halo stratification and temperatures steadily peaking near the bottom. We conducted comprehensive vertical profiling of the microbial populations in both pools and highlighted the influential environmental factors. Pyrosequencing of the 16S ribosomal RNA genes (rRNA genes) revealed shifts in community structures vis-à-vis depth. High diversity and low abundance were the features of the deepest convective layers despite the low cell density. Surprisingly, the brine interfaces were significantly higher in cell count, compared with the overlying deep-sea water, yet they were lowest in diversity. Vertical stratification of the bacterial populations was apparent as we move from the Alphaproteobacteria-dominated deep-sea to the Planctomycetacia or Deferribacteres-dominated interfaces to the Gammaproteobacteria-dominated brine layers. The archaeal marine group I was dominant in the deep-sea water and interfaces; whilst several euryarchaeotic groups increased in the brine. Across sites, microbial phylotypes and abundances varied substantially in the brine interface of Discovery compared with Atlantis II; despite the near-identical populations in the overlaying deep-sea waters. The lowest convective layers harboured interestingly similar microbial communities, even though temperature and heavy metal concentrations were very different. Multivariate analysis indicated that temperature and salinity were the major influences shaping the communities. The harsh conditions and the low-abundance phylotypes could explain the observed correlation in the brine pools.