Abstract
An understanding of the forces controlling community structure in the deep sea is essential at a time when its pristineness is threatened by polymetallic nodule mining. Because abiotically defined communities are more sensitive to environmental change, we applied occurrence- and phylogeny-based metrics to determine the importance of biotic versus abiotic structuring processes in nematodes, the most abundant invertebrate taxon of the Clarion–Clipperton Fracture Zone (CCFZ), an area targeted for mining. We investigated the prevalence of rarity and the explanatory power of environmental parameters with respect to phylogenetic diversity (PD). We found evidence for aggregation and phylogenetic clustering in nematode amplicon sequence variants (ASVs) and the dominant genus Acantholaimus, indicating the influence of environmental filtering, sympatric speciation, affinity for overlapping habitats and facilitation for community structure. PD was associated with abiotic variables such as total organic carbon, chloroplastic pigments equivalents and/or mud content, explaining up to 57% of the observed variability and providing further support of the prominence of environmental structuring forces. Rarity was high throughout, ranging from 64 to 75% unique ASVs. Communities defined by environmental filtering with a prevalence of rarity in the CCFZ suggest taxa of these nodule-bearing abyssal plains will be especially vulnerable to the risk of extinction brought about by the efforts to extract them.
Highlights
The abyssal plains of the Clarion–Clipperton Fracture Zone (CCFZ) in the northeastern Equatorial Pacific contain the densest known aggregation of polymetallic nodules; mineral concretions abundant in commercially important metals (e.g. Ni, Cu, Co)
Because abiotically defined communities are more sensitive to environmental change, we applied occurrence- and phylogeny-based metrics to determine the importance of biotic versus abiotic structuring processes in nematodes, the most abundant invertebrate taxon of the Clarion–Clipperton Fracture Zone (CCFZ), an area targeted for mining
We found evidence for aggregation and phylogenetic clustering in nematode amplicon sequence variants (ASVs) and the dominant genus Acantholaimus, indicating the influence of environmental filtering, sympatric speciation, affinity for overlapping habitats and facilitation for community structure
Summary
The abyssal plains of the Clarion–Clipperton Fracture Zone (CCFZ) in the northeastern Equatorial Pacific (figure 1) contain the densest known aggregation of polymetallic nodules; mineral concretions abundant in commercially important metals (e.g. Ni, Cu, Co). Co-occurring species being more closely related than would be expected by chance (clustering) results from environmental filtering due to one or more shared conserved trait(s), which allow them to persist in that locality in the absence of resource limitation and associated competitive interactions ( see [16]). The observed metric (obsMetric) was compared to that obtained from 999 randomizations (nullMetric) of the assemblage generated using all null models (i.e. taxa.labels, richness, frequency, sample.pool, phylogeny.pool, independentswap, trialswap, see electronic supplementary material, table S12 for full description). And in the event of fewer than three replicates per area (i.e. IOM.C and Desmoscolex ASVs for APEI3), the Wilcoxon signed-rank test was used These metrics have commonly been used to reveal phylogenetic structure in bacterial, protist, plant, bird and mammal assemblages [40,41,42,43,44,45]. A diagram of our sampling design and analysis is provided in electronic supplementary material, figure S1
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