Spatial Variability of Abyssal Nitrifying Microbes in the North-Eastern Clarion-Clipperton Zone

Anita L Hollingsworth,Daniel O B Jones,C Robert Young

doi:10.3389/fmars.2021.663420

Anita L Hollingsworth, Daniel O B Jones + Show 1 more

Open Access

https://doi.org/10.3389/fmars.2021.663420

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Abstract

Abyssal microbes drive biogeochemical cycles, regulate fluxes of energy and contribute to organic carbon production and remineralization. Therefore, characterizing the spatial variability of benthic microbes is important for understanding their roles in benthic environments and for conducting baseline assessments of areas of the seabed that might be targeted by commercial mining activities. Yet, detailed assessments of the spatial distributions of benthic microbial communities in these regions are still incomplete, and these efforts have not yet considered the influence of seafloor topography and heterogeneity on microbial distributions across a range of scales. In this study, we investigated the composition and spatial variability of benthic microbial assemblages found in sediments and polymetallic nodules collected from the Clarion Clipperton Zone (CCZ) in the equatorial Pacific (4000–4300 m water depth). We used 16S rRNA gene sequences to characterize these communities. The upper 20 cm of abyssal sediments harbored diverse and distinctive microbial communities in both sediments and their associated polymetallic nodules, with high similarity across topographical areas of the seabed. Assemblage composition differed vertically through the sediment, by habitat and across small to mesoscales. Potential carbon-fixing microbes formed more than 25% relative abundance of sediment assemblages, which were dominated by ammonia-oxidizing ArchaeaNitrosopumilus. Non-photosynthetic Cyanobacteria were more frequent in the deeper sediment layers and nodules. Sediment communities had a higher abundance of taxa involved in nitrogen cycling, such asNitrosopumilus,Nitrospina,Nitrospira,AqS1(Nitrosococcaceae), and methanogenswb1-A12(NC10 phylum). In contrast, nodules were more enriched in Alphaproteobacteria, Gammaproteobacteria, Planctomycetes, Acidobacteria, Bacteroidetes, Nanoarchaeaeota, andCalditrichaeota. Microbes related to potential metal-cycling (Magnetospiraceae andKiloniellaceae), organic carbon remineralization (Woeseia), and sulfur-oxidizing Thiohalorhabdaceae were also more enriched in nodules. Our results indicate that benthic microbial community composition is driven by sediment profile depth and seafloor heterogeneity at small and mesoscales. The most abundant microbial taxa within the sediments were nitrifying and putative carbon-fixing microbes, and may have key ecological roles in mediating biogeochemical cycles in this habitat.

Highlights

The high abundances of polymetallic nodules and the possibility of deep-sea mining has heightened scientific and commercial interest in the environment of the Clarion Clipperton Zone (CCZ) in the eastern Pacific (Wedding et al, 2015)
Samples were obtained from 20 megacores (Bowers and Connelly type; Barnett et al, 1984) that were deployed in the south western part of APEI6 (Sampling area centered: 17◦ 10 N, 122◦ 75 W) and 2 megacores deployed in the UK-1 Exploration Contract Area, ∼750 km away from Areas of Particular Environmental Interest (APEIs)-6 (13◦ 28 N, 116◦ 35 W) (Table 1)
Of these amplicon sequence variants (ASVs) from APEI-6, 19% were classified as Archaea and 81% as Bacteria

Summary

Introduction

The high abundances of polymetallic nodules and the possibility of deep-sea mining has heightened scientific and commercial interest in the environment of the Clarion Clipperton Zone (CCZ) in the eastern Pacific (Wedding et al, 2015). The ISA have designated nine Areas of Particular Environmental Interest (APEIs), protected from mining activities and these conservation areas surround the belt of mining contract areas (Lodge et al, 2014) The aim of these conservation areas is to form a network of sites that represent the range of habitats and communities of the CCZ. The process of nodule removal would remove the top layers of sediment and generate plumes near the seabed from the action of the mining collector vehicle as well as in the water column from discharge of sedimentladen water from shipboard dewatering of nodules (Jones et al, 2018) This activity could lead to widespread ecological disturbance on the seafloor. As a result of a low POC flux, the soft sediments of the CCZ are oxygenated down to 300 cm below seafloor (cmbsf: Mewes et al, 2014; Volz et al, 2018; Menendez et al, 2019)

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