Spatial patchiness is a natural feature that strongly influences the level of species richness we perceive in surface sediments sampled in the deep-sea. Recent environmental DNA (eDNA) surveys of benthic micro- and meiofauna confirmed this exceptional richness. However, it is unknown to which extent the results of these studies, based usually on few grams of sediment, are affected by spatial patchiness of deep-sea benthos. Here, we analyse the eDNA diversity of Foraminifera in 42 deep-sea sediment samples collected across different scales in the Southern Ocean. At three stations, we deployed at least twice the multicorer and from each multicorer cast, we subsampled 3 sediment replicates per core for 2 cores. Using high-throughput sequencing (HTS), we generated over 2.35 million high-quality sequences that we clustered into 451 operational taxonomic units (OTUs). The majority of OTUs were assigned to the monothalamous (single-chambered) taxa and environmental clades. On average, a one-gram sediment sample captures 57.9% of the overall OTU diversity found in a single core, while three replicates cover at most 61.9% of the diversity found in a station. The OTUs found in all the replicates of each core gather up to 87.9% of the total sequenced reads, but only represent from 12.2% to 30% of the OTUs found in one core. These OTUs represent the most abundant species, among which dominate environmental lineages. The majority of the OTUs are represented by few sequences comprising several well-known deep-sea morphospecies or remaining unassigned. It is crucial to study wider arrays of sample and PCR replicates as well as RNA together with DNA in order to overcome biases stemming from deep-sea patchiness and molecular methods.
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