Investigating the Differences in the Total and Active Microbial Community of Mid-Atlantic Ridge Sediments

Morgan S Sobol1, Laura A Zinke2, Beth Orcutt3, Heath Jordan Mills4, Katrina J Edwards2, Peter R Girguis5 and Brandi Kiel Reese1, (1)Texas A&M University - Corpus Christi, Life Sciences, Corpus Christi, TX, United States, (2)University of Southern California, Los Angeles, CA, United States, (3)Bigelow Laboratory for Ocean Science, East Boothbay, ME, United States, (4)University of Houston Clear Lake, Houston, TX, United States, (5)Harvard University, Cambridge, MA, United States
Abstract:
Microbes in the marine deep subsurface are key mediators of many geochemical cycles. It is important to understand how microbial communities and the diversity of those communities impacts geochemical cycling. Sediment cores were collected from IODP (Integrated Ocean Drilling Program) Expedition 336 to the western flank of the mid-Atlantic ridge also referred to as North Pond. The dissolved oxygen concentration decreased with depth for 60-70 mbsf, followed by a sharp increase in oxygen until it terminated at the basement. The 16S rRNA genes (DNA) and transcripts (RNA) were extracted simultaneously using a method designed by Reese et al. (2013) to differentiate between the total and active microbial community structures, respectively, as well as correlate the putative metabolism with the geochemistry. We observed many differences between the active and total communities. Sequences most closely related to Cyanobacteria were found to dominate the total community at both sites, but were found in small numbers in the active community. The most abundant phyla in the active community were Alphaproteobacteria, which suggests that they may have high activity even though the abundance was not as great in the total community. This suggests that, even in small numbers, bacteria are capable of contributing greatly to their environment. Principal Component Analysis (PCA) and Singular Value Decomposition (SVD) showed that iron-reducing bacteria in the active (RNA) community correlated strongly with solid phase iron oxides. SVD also showed that the putative nitrate reducers in the active community were found in greater abundance where porewater NO3- and NO2– total concentrations were elevated. Overall, the active (RNA) community correlated significantly with the geochemistry whereas the total (DNA) community did not. Therefore, RNA analysis yields a more accurate representation of how microbial communities impact geochemical cycling.
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