The extent of fractionation of sulfur isotopes by sulfate‐reducing microbes is dictated by genomic and environmental factors. A greater understanding of species‐specific fractionations may better inform interpretation of sulfur isotopes preserved in the rock record. To examine whether gene diversity influences net isotopic fractionation in situ, we assessed environmental chemistry, sulfate reduction rates, diversity of putative sulfur‐metabolizing organisms by 16S rRNA and dissimilatory sulfite reductase (dsrB) gene amplicon sequencing, and net fractionation of sulfur isotopes along a sediment transect of a hypersaline Arctic spring. In situ sulfate reduction rates yielded minimum cell‐specific sulfate reduction rates < 0.3 × 10−15 moles cell−1 day−1. Neither 16S rRNA nor dsrB diversity indices correlated with relatively constant (38‰–45‰) net isotope fractionation (ε34Ssulfide‐sulfate). Measured ε34S values could be reproduced in a mechanistic fractionation model if 1%–2% of the microbial community (10%–60% of Deltaproteobacteria) were engaged in sulfate respiration, indicating heterogeneous respiratory activity within sulfate‐reducing populations. This model indicated enzymatic kinetic diversity of Apr was more likely to correlate with sulfur fractionation than DsrB. We propose that, above a threshold Shannon diversity value of 0.8 for dsrB, the influence of the specific composition of the microbial community responsible for generating an isotope signal is overprinted by the control exerted by environmental variables on microbial physiology.
Between March 25 and 29, 2019, I attended the ECORD Training Course at the MARUM, in Bremen, Germany, supported by the USSSP and a C-DEBI exchange award. The purpose of my participation was to acquire a greater understanding of IODP science planning and shipboard logistics and analyses, in order to facilitate realistic IODP proposal writing. Specific labs provided opportunities to make measurements of physical properties, pore water analyses, biostratigraphy, and magnetostratigraphy from archived cores. Classroom time was allocated to lecturers describing the structure of IODP and group proposal writing exercises. This experience was an important stepping stone in co-authoring my first IODP pre-proposal (submitted in April, 2019), and hopefully will improve the likelihood for my first application to sail to be approved. The IODP is the greatest tool for marine subsurface microbial science, and a program I plan to be involved in for the duration of my science career.
Microbial evolution is driven by environmental and ecological pressures and realized by selection for adaptive traits within affected populations. In recent decades, increased understanding of the abundance, diversity and activity of microbial inhabitants of marine subsurface sediments has illuminated ecological interactions and environmental variations in the deep biosphere. Upon this foundation, subsurface microbial evolutionary processes are ripe for exploration. To examine the interplay between phenotypic adjustment and evolutionary adaptation in the subsurface, and tradeoffs between metabolic capacity and growth in these energy-limited environments, a novel laboratory platform will be employed for the study of in vivo spatially resolved microbial experimental evolution. The platform is a large growth plate set up to mimic a subsurface sedimentary environment with sustained anoxia and progressive limitations in the abundance and quality of organic carbon. An isoclonal inoculum of sulfate reducing Desulfobacterium autotrophicum will be introduced at one end of the plate and monitored visually as the culture encounters and adapts to abrupt changes in organic carbon availability. Successful strains will be characterized for growth characteristics, adaptive mutations and isotopic fractionation. These experiments further C-DEBI Theme III and inform the tempo, mode and synonymous nature of microbial evolution for a substantial portion of Earth’s biomass.