Relationship of bacterial richness to organic degradation rate and sediment age in subseafloor sediment

Abstract

Subseafloor sediment hosts a large, taxonomically rich, and metabolically diverse microbial ecosystem. However, the factors thatcontrol microbial diversity in subseafloor sediment have rarely been explored. Here, we show that bacterial richness varies withorganic degradation rate and sediment age. At three open-ocean sites (in the Bering Sea and equatorial Pacific) and one continentalmargin site (Indian Ocean), richness decreases exponentially with increasing sediment depth. The rate of decrease in richnesswith increasing depth varies from site to site. The vertical succession of predominant terminal electron acceptors correlateswith abundance-weighted community composition but does not drive the vertical decrease in richness. Vertical patterns of richnessat the open-ocean sites closely match organic degradation rates; both properties are highest near the seafloor and declinetogether as sediment depth increases. This relationship suggests that (i) total catabolic activity and/or electron donor diversityexerts a primary influence on bacterial richness in marine sediment and (ii) many bacterial taxa that are poorly adapted for subseafloorsedimentary conditions are degraded in the geologically young sediment, where respiration rates are high. Richnessconsistently takes a few hundred thousand years to decline from near-seafloor values to much lower values in deep anoxic subseafloorsediment, regardless of sedimentation rate, predominant terminal electron acceptor, or oceanographic context.