Foraminiferal assemblages are a ubiquitous and abundant component of the deep-sea benthos, even in the deepest ocean trenches. While their distribution seems not constrained over large geographical distance, the current knowledge of foraminifera in trench is solely based on morphological observations. In this study, we document the first DNA metabarcoding dataset from a deep-sea trench focusing specifically on benthic foraminifera. Here we show that, consistent with previous molecular studies of abyssal fauna, trench foraminifera include diverse sequences of yet unknown species captured only by their molecular traces in the sediment. The molecular assemblages of foraminifera significantly differed along a depth gradient of almost 5000 m in the Kuril-Kamchatka trench. The deepest stations at nearly 9500 m were composed of unique phylotypes that were not identified in shallower stations, which means that these assemblages are unlikely the result of a sinking effect from shallower depths. Finally, both sides of the trench harbored very different communities, which could imply that the trench constitutes a physical barrier for the dispersion of some deep-sea foraminiferal species.
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