Biology subjects

O'Loughlin, E. J.

Publications and source records attributed to O'Loughlin, E. J..

5 recordsLinked to original sources

bioRxiv2024

Effects of Soluble Electron Shuttles on Microbial Iron Reduction and Methanogenesis

In many aquatic and terrestrial ecosystems, iron (Fe) reduction by microorganisms is a key part of biogeochemical cycling and energy flux. The presence of redox active electron shuttles in the environment potentially enables a phylogenetically diverse group of microbes to use insoluble iron as a terminal electron acceptor. We investigated the impact that different electron shuttles had on respiration, microbial physiology, and microbial ecology. We tested eight different electron shuttles, seven quinones and riboflavin, with redox potentials between 0.217 V and -0.340 V. Fe(III) reduction coupled to acetate oxidation was observed with all shuttles. Once Fe(III) reduction began to plateau, a rapid increase in acetate consumption was observed and coincided with the onset of methane production, except in the incubations with the shuttle 9,10-anthraquinone-2-carboxylic acid (AQC). The rates of iron reduction, acetate consumption, methanogenesis, and the microbial communities varied significantly across the different shuttles independent of redox potential. In general, shuttles appeared to reduce the overall diversity of the community compared to no shuttle controls, but certain shuttles were exceptions to this trend. Geobacteraceae were the predominant taxonomic family in all enrichments except in the presence of AQC or 1,2,-dihydroxyanthraquinone (AQZ), but each shuttle enriched a unique community significantly different from the no shuttle control conditions. This suggests that the presence of different redox-active electron shuttles can have a large influence on the microbial ecology and total carbon flux in the environment. ImportanceIron is the fourth most abundant element in the Earths crust and the reduction of iron by microbes is an important component of global biogeochemical cycles. A phylogenetically diverse group of microbes are capable of conserving energy with oxidized iron as a terminal electron acceptor, but the environmental conditions favoring certain taxonomic clades in iron-reducing environments is unclear. One complicating factor often overlooked in small-scale enrichments is the influence of soluble, redox-active electron shuttles on the rate and microbial ecology of iron reduction. We tested the effects of eight different electron shuttles on the microbial physiology and ecology in iron-reducing enrichments derived from a local wetland. Each electron shuttle varied the microbial activity and enriched for a microbial community distinct from the no shuttle control condition. Therefore, in complex subsurface environments with many redox-active compounds present, we propose electron shuttles as a reason for the coexistence of multiple clades of iron-reducing bacteria.

microbiology↗

bioRxiv2023

Microbial Reduction of Geogenic and Synthetic Goethite and Hematite

The microbial reduction of Fe(III) is a major component of Fe cycling in terrestrial and aquatic environments and is affected by the Fe(III) mineralogy of the system. The majority of the research examining the bioreduction of Fe(III) oxides by Fe(III)-reducing bacteria (IRB) has focused on the reduction of poorly crystalline Fe(III) phases, primarily ferrihydrite; however, crystalline Fe(III) oxides like goethite (-FeOOH) and hematite (-Fe2O3) comprise the majority of Fe(III) oxides in soils. This study examined the bioreduction of goethite and hematite of geogenic and synthetic origin by Shewanella putrefaciens CN2, a well-studied model IRB, in laboratory incubations. Overall, the rate and extent of Fe(II) production were greater for goethite than for hematite, and for geogenic Fe(III) oxides relative to their synthetic analogs. Although there was substantial production of Fe(II) (i.e., > 5 mM Fe(II)) in many of the systems, X-ray diffraction analysis of the solids at the end of the incubation did not indicate the formation of any Fe(II)-bearing secondary minerals (e.g., magnetite, siderite, green rust, etc.). The results of this study demonstrate the variability in the extent of bioreduction of geogenic goethite and hematite, and furthermore, that synthetic goethite and hematite may not be good analogs for the biogeochemical behavior of Fe(III) oxides in aquatic and terrestrial environments.

microbiology↗

bioRxiv2023

Groundwater redox dynamics across the terrestrial-aquatic interface of Lake Erie coastal ecosystems

Groundwater biogeochemistry in coastal areas is spatially and temporally dynamic because fluctuations in groundwater level may cause alternate redox between distinct hydrological conditions. Recent studies have proposed connections between biogeochemistry and large-scale hydrological processes, specifically focusing on the role of redox-active compounds in changing the oxidation state during flooding and draining events. While water saturation generally results in a shift of redox-active compounds from electron donors to acceptors, the specific mechanisms underlying the transition of groundwater between oxidizing and reducing conditions in response to water level fluctuations are uncertain. To determine the effects of groundwater levels on redox dynamics, we monitored groundwater redox potential across the terrestrial-aquatic interface in Lake Erie coastal areas throughout the high and low-water seasons. In contrast to previously observed responses to flooding in soils, our results revealed patterns of oxidizing redox potentials during high-water and reducing during low-water periods. Furthermore, short-term fluctuations in water table levels significantly impacted the redox potential of groundwater when dissolved oxygen increased, and redox dynamics displayed voltage hysteresis in most events. Based on these findings, we propose that for improved predictions of microbial functions and biogeochemical cycles, redox-informed models should incorporate the antagonistic changes in groundwater redox balance compared to soils and consider the time lags in redox fluctuations. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=92 SRC="FIGDIR/small/544684v1_figu1.gif" ALT="Figure 1"> View larger version (26K): org.highwire.dtl.DTLVardef@4d7a19org.highwire.dtl.DTLVardef@8b2b92org.highwire.dtl.DTLVardef@d9bc11org.highwire.dtl.DTLVardef@ca3e13_HPS_FORMAT_FIGEXP M_FIG Graphical AbstractConceptual diagram of groundwater redox fluctuations in coastal ecosystems. Large redox fluctuations are derived by dissolved oxygen inputs and smaller more frequent redox fluctuations are led by redox sensitive species leaching from topsoil. C_FIG

ecology↗

bioRxiv2022

Inhibition of Microbial Methane Oxidation by 2-Chloro-6-Methylpyridine

Several pyridine derivatives including the pesticide nitrapyrin [2-chloro-6-(trichloromethyl) pyridine] are strong inhibitors of methane monooxygenase, a key enzyme of aerobic methane (CH4) oxidation. In this study we examined the effects of 2-chloro-6-methylpyridine (2C6MP) concentration on aerobic CH4 oxidation and the development of populations of putative methanotrophs in sediment from Old Woman Creek, a freshwater estuary in Huron Co., Ohio. Experimental systems were prepared in serum bottles containing minimal medium with a headspace containing 20% O2 and 10% CH4. The microcosms were spiked with 2C6MP to achieve concentrations of 0, 0.1, 1, or 10 mM and inoculated with sediment. When headspace CH4 concentrations decreased from 10% to < 2%, subsamples were taken for DNA extraction and sequencing of 16S rRNA gene amplicons. There was minimal effect of 2C6MP on CH4 oxidation at concentrations of 0.1, and 1 mM, but complete inhibition for > 20 months was observed at 10 mM. ANOSIM of weighted UniFrac distances between groups of triplicate samples supported a primary distinction of the inoculum relative to the enrichments (R=0.999) and a secondary distinction between bottles containing 2C6MP versus those without (R=0.464 [0.1 mM]; R=0.894 [1 mM]). The inoculum was dominated by members of the Proteobacteria (49.9{+/-}1.5%), and to a lesser extent by Bacteroidetes (8.8{+/-}0.2%), Acidobacteria (8.9{+/-}0.4%), and Verrucomicrobia (4.4{+/-}0.3%). In enrichments with or without 2C6MP, Proteobacteria expanded to comprise 65-70% of the total. In the absence of inhibitor, members of the Methylococcaceae and Methylophilaceae increased in relative abundance from < 0.1% of the inoculum to 8.5{+/-}1.0% and 13.4{+/-}2.3%, of the total community respectively. At both 0.1 and 1 mM concentrations of the inhibitor, the Methylococcaceae were much less abundant, representing 3.3{+/-}0.5% and 2.8{+/-}3.3% respectively. No inhibition of the Methylophilaceae was seen at the lower concentration of 2C6MP, but at the higher concentration this taxon was only 7.8{+/-}1.1% of the total. In contrast, members of the Crenotrichaceae, another group of methane oxidizers, increased in relative abundance with greater amounts of inhibitor, representing 8.6{+/-}3.6% of the total at 0.1 mM and 12.0{+/-}4.5% at 1 mM, compared to only 4.1{+/-}0.4% when no inhibitor was present. These results clearly show changes in the populations of putative aerobic methanotrophs relative to the amount of 2C6MP present.

microbiology↗

bioRxiv2020

Wetland plant evolutionary history influences soil and endophyte microbial community composition

Methane is a microbially derived greenhouse gas whose emissions are highly variable throughout wetland ecosystems. Differences in plant community composition account for some of this variability, suggesting an influence of plant species on microbial community structure and function in these ecosystems. Given that closely related plant species have similar morphological and biochemical features, we hypothesize that plant evolutionary history is related to differences in microbial community composition. To examine species-specific patterns in microbiomes, we selected five monoculture-forming wetland plant species based on the evolutionary distances among them. We detected significant differences in microbial communities between sample types (unvegetated soil, bulk soil, rhizosphere soil, internal root tissues, and internal leaf tissues) associated with these plant species based on 16S relative abundances. We additionally found that differences in plant evolutionary history were correlated with variation in microbial communities across plant species within each sample type. Using qPCR, we observed substantial differences in overall methanogen and methanotroph population sizes between plant species and sample types. Methanogens tended to be most abundant in rhizosphere soils while methanotrophs were the most abundant in roots. Given that microbes influence methane flux and that plants affect methanogen and methanotroph populations, plant species contribute to variable degrees of methane emissions. Incorporating the influence of plant evolutionary history into future modeling efforts may improve predictions of wetland methane emission since microbial community differences correlate with differences in plant evolutionary history.

ecology↗