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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 degrees-17 degrees 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
Glade, 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.