Abstract
The Mexican region of the Perdido Fold Belt (PFB), in northwestern Gulf of Mexico (GoM), is a geological province with important oil reservoirs that will be subjected to forthcoming oil exploration and extraction activities. To date, little is known about the native microbial communities of this region, and how these change relative to water depth. In this study we assessed the bacterial community structure of surficial sediments by high-throughput sequencing of the 16S rRNA gene at 11 sites in the PFB, along a water column depth gradient from 20 to 3,700 m, including five shallow (20–600 m) and six deep (2,800–3,700 m) samples. The results indicated that OTUs richness and diversity were higher for shallow sites (OTUs = 2,888.2 ± 567.88; H′ = 9.6 ± 0.85) than for deep sites (OTUs = 1,884.7 ± 464.2; H′ = 7.74 ± 1.02). Nonmetric multidimensional scaling (NMDS) ordination revealed that shallow microbial communities grouped separately from deep samples. Additionally, the shallow sites plotted further from each other on the NMDS whereas samples from the deeper sites (abyssal plains) plotted much more closely to each other. These differences were related to depth, redox potential, sulfur concentration, and grain size (lime and clay), based on the environmental variables fitted with the axis of the NMDS ordination. In addition, differential abundance analysis identified 147 OTUs with significant fold changes among the zones (107 from shallow and 40 from deep sites), which constituted 10 to 40% of the total relative abundances of the microbial communities. The most abundant OTUs with significant fold changes in shallow samples corresponded to Kordiimonadales, Rhodospirillales, Desulfobacterales (Desulfococcus), Syntrophobacterales and Nitrospirales (GOUTA 19, BD2-6, LCP-6), whilst Chromatiales, Oceanospirillales (Amphritea, Alcanivorax), Methylococcales, Flavobacteriales, Alteromonadales (Shewanella, ZD0117) and Rhodobacterales were the better represented taxa in deep samples. Several of the OTUs detected in both deep and shallow sites have been previously related to hydrocarbons consumption. Thus, this metabolism seems to be well represented in the studied sites, and it could abate future hydrocarbon contamination in this ecosystem. The results presented herein, along with biological and physicochemical data, constitute an available reference for further monitoring of the bacterial communities in this economically important region in the GoM.
Highlights
Microorganisms are well recognized as key drivers of biogeochemical cycles in marine environments (Webster et al, 2003; Santos et al, 2011)
In the Gulf of Mexico (GoM), changes in bacterial community structure have been mainly related with depth in the water column, and likely result from differences in temperature, dissolved oxygen and suspended particles (King et al, 2013) which occur across these depth differences
Samples were collected with a Hessler-Sandia MK-II boxcore (40 × 40 cm) from which three different surficial (0–5 cm) subsamples were taken: (1) sterile 100 mL plastic containers immediately frozen at −20 ◦C on board for further molecular analysis; (2) a 2 inch core extracted to determine total sulfur concentration (TS) and redox potential on board, with a sulfide ion selective electrode and potentiometer (Bricker, 1982; Brassard, 1997); and (3) approximately 400 g of sediment stored in high density polyethylene bags at 4 ◦C until the determination of total organic matter (TOM), total organic carbon (TOC), and grain size in the laboratory
Summary
Microorganisms are well recognized as key drivers of biogeochemical cycles in marine environments (Webster et al, 2003; Santos et al, 2011). In the Gulf of Mexico (GoM), changes in bacterial community structure have been mainly related with depth in the water column, and likely result from differences in temperature, dissolved oxygen and suspended particles (King et al, 2013) which occur across these depth differences. In marine sediments from the GoM, bacterial community composition has been determined at different depths below the seafloor (from sediment cores). It has been proposed that the bacterial community composition of these sediments likely results from the interaction between the water column and a benthic microbial population limited to the upper layer of the sediments (Reese et al, 2013). The microbial diversity present in different sediment depths from seep systems in the GoM have been directly related to the composition and magnitude of hydrocarbon seepage (e.g., natural oils, methane, and non-methane hydrocarbons) (Orcutt et al, 2010), as well as to the presence of overlying microbial mats (Mills et al, 2004)
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