Abstract
Microbial communities living in deeply buried sediment may be adapted to long-term energy limitation as they are removed from new detrital energy inputs for thousands to millions of years. However, sediment layers near the underlying oceanic crust may receive inputs from below that influence microbial community structure and/or activity. As part of the Census of Deep Life, we used 16S rRNA gene tag pyrosequencing on DNA extracted from a spectrum of deep sediment-basement interface samples from the subsurface of the Juan de Fuca Ridge flank (collected on IODP Expedition 327) to examine this possible basement influence on deep sediment communities. This area experiences rapid sedimentation, with an underlying basaltic crust that hosts a dynamic flux of hydrothermal fluids that diffuse into the sediment. Chloroflexi sequences dominated tag libraries in all sediment samples, with variation in the abundance of other bacterial groups (e.g., Actinobacteria, Aerophobetes, Atribacteria, Planctomycetes, and Nitrospirae). These variations occur in relation to the type of sediment (clays versus carbonate-rich) and the depth of sample origin, and show no clear connection to the distance from the discharge outcrop or to basement fluid microbial communities. Actinobacteria-related sequences dominated the basalt libraries, but these should be viewed cautiously due to possibilities for imprinting from contamination. Our results indicate that proximity to basement or areas of seawater recharge is not a primary driver of microbial community composition in basal sediment, even though fluids diffusing from basement into sediment may stimulate microbial activity.
Highlights
Over the past decade, several studies have expanded our understanding of the composition and function of “deep biosphere” microbial communities in deep marine sediments in both organicrich (D’Hondt et al, 2004; Inagaki et al, 2006; Biddle et al, 2006, 2008; Lever et al, 2010) and organic-poor environments (D’Hondt et al, 2009; Kallmeyer et al, 2012; Røy et al, 2012)
Microbial abundance in the sediment and basalt samples was analyzed in a variety of ways: direct cell counting using two different staining approaches, and quantitative assessment of 16S rRNA gene copy number in DNA extracts
Interior sediment core samples were analyzed for cell density measurements following procedures developed for low biomass deep sediment samples (Kallmeyer et al, 2008)
Summary
Several studies have expanded our understanding of the composition and function of “deep biosphere” microbial communities in deep marine sediments in both organicrich (D’Hondt et al, 2004; Inagaki et al, 2006; Biddle et al, 2006, 2008; Lever et al, 2010) and organic-poor environments (D’Hondt et al, 2009; Kallmeyer et al, 2012; Røy et al, 2012). Another study confirmed the persistence of microbial groups from initial deposition through burial down to 2.5 km below seafloor millions of years later, and documented strong decreases in microbial abundance at 40–60◦C (Inagaki et al, 2015), supporting the notion that increased biomolecule damage combined with severe energy limitation may cause the upper temperature limit of life in subsurface sediments to be lower than in energy-rich surface environments (Lever et al, 2015) While most of these studies have focused on what happens to microbial life as it is buried beneath increasing layers of sediment, several recent studies have highlighted the impact that the underlying basement can have on stimulating microbial activity from below. The influence of these basement sources on the structure of the microbial community is not currently resolved
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