Duty Cycling Influences Current Generation in Multi-Anode Environmental Microbial Fuel Cells

Improving microbial fuel cell (MFC) performance continues to be the subject of research, yet the role of operating conditions, specifically duty cycling, on MFC performance has been modestly addressed. We present a series of studies in which we use a 15-anode environmental MFC to explore how duty cycling (variations in the time an anode is connected) influences cumulative charge, current, and microbial composition. The data reveal particular switching intervals that result in the greatest time-normalized current. When disconnection times are sufficiently short, there is a striking decrease in current due to an increase in the overall electrode reaction resistance. This was observed over a number of whole cell potentials. Based on these results, we posit that replenishment of depleted electron donors within the biofilm and surrounding diffusion layer is necessary for maximum charge transfer, and that proton flux may be not limiting in the highly buffered aqueous phases that are common among environmental MFCs. Surprisingly, microbial diversity analyses found no discernible difference in gross community composition among duty cycling treatments, suggesting that duty cycling itself has little or no effect. Such duty cycling experiments are valuable in determining which factors govern performance of bioelectrochemical systems and might also be used to optimize field-deployed systems

Box plot showing expression of target genes in the trophosome compared to the plume as determined by qPCR.

<p>The fold expression level differences, of 18 target genes, in the trophosome were compared to those of the plume. The upper and lower ends of the boxes indicate the 25th and 75th percentiles, respectively. The length of the box depicts the interquartile range within which 50% of the values are located. The solid black lines denote the median. Capped error bars represent the minimum and maximum values, excluding outliers (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0038267#pone-0038267-t004" target="_blank">Table 4</a>). The dotted black line represents expression levels in the plume (y = 1), thus genes with medians >1 exhibit higher expression levels in the trophosome compared to the plume, and inversely genes with medians <1 show higher expression levels in the plume. All expression levels are normalized to the expression of actin. The gene abbreviations are as follows: hbB1, hemoglobin B1; HSP70, heat shock protein 70; LITAF, lipopolysaccharide-induced tumor necrosis factor-alpha; PGRPrpi1–5, peptidoglycan recognition protein 1–5; TLR2p, toll-like receptor 2 precursor; A2MRAP, alpha-2 macroglobulin receptor associated protein; CAbr, carbonic anhydrase (branchial plume); CAtr, carbonic anhydrase (trophosome); EF1α, elongation factor 1-alpha; LBPIP, LPS induced bactericidal permeability increasing protein; MMIF, macrophage migration inhibitory factor; MR, macrophage mannose receptor 1-like protein; NF-κBic, NF-κB inhibitor (cactus); ROSm, reactive oxygen species modulator.</p

Reference gene evaluation

Best reference gene: evaluated by NormFinder and GeNor

Nitrate and ammonia concentrations

Concentrations of nitrate and ammonia determined in Ridgeia bloo

Model of host-symbiont interactions between pattern recognition receptors (PRRs) and microbe-associated molecular patterns (MAMPs) in <i>R. piscesae</i>.

<p>The branchial plume (p) of <i>Ridgeia</i> exchanges metabolites with the environment. The plume lacks symbionts but may still use PRRs to interact with microorganisms found in the surrounding vent fluid and seawater. The center of the worm is comprised mainly of one organ, the trophosome (t) made up of lobules that contain bacteriocytes housing the intracellular bacterial symbionts (endosymbionts) as well as blood vessels (bv) that transfer metabolites. Central bacteriocytes (c) harbor healthy and actively dividing endosymbionts. Towards the periphery of the lobules, bacteriocytes and symbionts appear to undergo terminal differentiation and apoptosis with many of the degenerative symbionts undergoing autophagy. We suggest a significantly greater response of the trophosome to MAMPs via PRRs that may trigger signal transduction cascades, ultimately helping to regulate symbiostasis. PRR expression in the trophosome may occur in the bacteriocytes and/or in the surrounding vasculature. Given the densities of endosymbionts, bacteriocytes encounter high concentrations of MAMPs. A constant turnover of bacteriocytes may also release extracellular MAMPs into the trophosome periphery.</p

Targeted gene expression for individual worms.

<p>Values are fold change calculated with the 2^–ΔΔCt method and normalized to actin in trophosome compared to plume. <i>Outliers are shown in italics (</i>see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0038267#s2" target="_blank">Materials and Methods</a><i>).</i></p

Isotopic analysis

δ15N of environmental nitrate and ammonia, as well as long-skinny and short-fat Ridgeia tubeworm

Summary of transcriptome sequencing of the trophosome (EST) and the plume (454 pyrosequencing) from <i>Ridgeia piscesae.</i>

*<p>analysis of Trimmed Unique ESTs.</p

DNA sequences amplified by qPCR primers

DNA sequences were generated by sanger sequencing of amplicons from qPCR primer

Characterizing Microbial Community and Geochemical Dynamics at Hydrothermal Vents Using Osmotically Driven Continuous Fluid Samplers

Microbes play a key role in mediating aquatic biogeochemical cycles. However, our understanding of the relationships between microbial phylogenetic/physiological diversity and habitat physicochemical characteristics is restrained by our limited capacity to concurrently collect microbial and geochemical samples at appropriate spatial and temporal scales. Accordingly, we have developed a low-cost, continuous fluid sampling system (the Biological OsmoSampling System, or BOSS) to address this limitation. The BOSS does not use electricity, can be deployed in harsh/remote environments, and collects/preserves samples with daily resolution for >1 year. Here, we present data on the efficacy of DNA and protein preservation during a 1.5 year laboratory study as well as the results of two field deployments at deep-sea hydrothermal vents, wherein we examined changes in microbial diversity, protein expression, and geochemistry over time. Our data reveal marked changes in microbial composition co-occurring with changes in hydrothermal fluid composition as well as the temporal dynamics of an enigmatic sulfide-oxidizing symbiont in its free-living state. We also present the first data on in situ protein preservation and expression dynamics highlighting the BOSS’s potential utility in meta-proteomic studies. These data illustrate the value of using BOSS to study relationships among microbial and geochemical phenomena and environmental conditions