Deep sea environments cover more than 65% of the earth's surface and fulfil a range of ecosystem functions, yet they are also amongst the least known habitats on earth. Whilst the discovery of key geological processes, combined with technological developments, has focused interest onto geologically active areas such as hydrothermal vents, most abyssal biodiversity remains to be discovered (Danovaro et al. 2010). However, as for terrestrial reservoirs of biodiversity, the world's largest biome is under threat from anthropogenic activities ranging from environmental change to the exploitation of minerals and rare-earth elements (Kato et al. 2011). It is therefore important to understand the magnitude, nature and composition of deep sea biological communities to inform us of levels of local adaptation, functionality and resilience with respect to future environmental perturbation. In this issue of Molecular Ecology, Bik et al. utilize 454 Roche metagenetic environmental sequencing to assess microbial metazoan community composition and phylogenetic identity across deep sea depth gradients and between ocean basins. The analyses suggest that although the majority of microbial eukaryotic taxa are regionally restricted, a small percentage might maintain cosmopolitan deep sea distributions, and an even smaller fraction appear to be eurybathic (live across depth gradients).
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