Microbial Diversity and Connectivity in Deep-Sea Sediments of the South Atlantic Polar Front.

Gilda Varliero,Antje Boetius,Christina Bienhold,Massimiliano Molari,Florian Schmid

doi:10.3389/fmicb.2019.00665

Abstract

Ultraslow spreading ridges account for one-third of the global mid-ocean ridges. Their impact on the diversity and connectivity of benthic deep-sea microbial assemblages is poorly understood, especially for hydrothermally inactive, magma-starved ridges. We investigated bacterial and archaeal diversity in sediments collected from an amagmatic segment (10°–17°E) of the Southwest Indian Ridge (SWIR) and in the adjacent northern and southern abyssal zones of similar water depths within one biogeochemical province of the Indian Ocean. Microbial diversity was determined by 16S ribosomal RNA (rRNA) gene sequencing. Our results show significant differences in microbial communities between stations outside and inside the SWIR, which were mostly explained by environmental selection. Community similarity correlated significantly with differences in chlorophyll a content and with the presence of upward porewater fluxes carrying reduced compounds (e.g., ammonia and sulfide), suggesting that trophic resource availability is a main driver for changes in microbial community composition. At the stations in the SWIR axial valley (3,655–4,448 m water depth), microbial communities were enriched in bacterial and archaeal taxa common in organic matter-rich subsurface sediments (e.g., SEEP-SRB1, Dehalococcoida, Atribacteria, and Woesearchaeota) and chemosynthetic environments (mainly Helicobacteraceae). The abyssal stations outside the SWIR communities (3,760–4,869 m water depth) were dominated by OM1 clade, JTB255, Planctomycetaceae, and Rhodospirillaceae. We conclude that ultraslow spreading ridges create a unique environmental setting in sedimented segments without distinct hydrothermal activity, and play an important role in shaping microbial communities and promoting diversity, but also in connectivity among deep-sea habitats.

Highlights

The deep seafloor beyond the shelf break comprises about 67% of the Earth’s lithosphere (Jørgensen and Boetius, 2007), making it the largest ecological realm worldwide
The stations inside the ridge were selected based on environmental data and visual observations: Area 1 (A1) had highest heat flow values; at Area 2 a signature for a hydrothermal plume had previously been reported by Bach et al (2002), but only based on turbidity maxima in the water column, with stronger turbidity plumes at station A2m than at station A2 (Figure 1B); Area 3 represents the axial valley of the ridge, with station A3 sampled in the central part and station A3m located close to the site where a vesicomyid clam, a typical inhabitant of reduced chemosynthetic habitats, was discovered (Supplementary Figure S6)
In order to better investigate the effect of the geographical distance and the Southwest Indian Ridge (SWIR) on benthic microbial diversity and connectivity in the South Atlantic Polar Front, the microbial communities were investigated in sediments from two stations (N1 and N2) sampled during the Polarstern cruise PS79 in 2012 (Ruff et al, 2014), and located northwest of the SWIR segment investigated here (Figure 1)

Summary

Introduction

The deep seafloor beyond the shelf break comprises about 67% of the Earth’s lithosphere (Jørgensen and Boetius, 2007), making it the largest ecological realm worldwide. Drift, dispersal and mutation are the four evolutionary and ecological interplay processes that shape the microbial biogeography (Hanson et al, 2012). Whilst environmental selection has been shown to play an important role in shaping deep-sea benthic microbial communities (e.g., Bienhold et al, 2012, 2016), the physical mechanisms responsible for dispersal limitation (e.g., currents and seafloor geomorphology) are poorly understood (Zinger et al, 2014). Patterns of deep-sea bacterial biogeography observed at the global scale suggest that seafloor geomorphology (i.e., mid-ocean ridges and oceanic trenches), deep-water masses and landmasses may represent barriers to dispersal (Nunoura et al, 2015; Bienhold et al, 2016; Salazar et al, 2016; Wenzhöfer et al, 2016)

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Discussion

Conclusion