https://www.frontiersin.org/article/10.3389/fmicb.2019.02997 Some acetogenic bacteria are capable of using solid electron donors, such as a cathode or metallic iron [Fe(0)]. Acetogens using a cathode as electron donor are of interest for novel applications such as microbial electrosynthesis, while microorganisms using Fe(0) as electron donor cause detrimental microbial induced corrosion. The capacity to use solid electron donors strongly differs between acetogenic strains, which likely relates to their extracellular electron transfer (EET) mechanism. Different EET mechanisms have been proposed for acetogenic bacteria, including a direct mechanism and a H dependent indirect mechanism combined with 2 extracellular hydrogenases catalyzing the H evolution reaction on the cathode or Fe(0) surface. Interestingly, 2 low H partial pressures often prevail during acetogenesis with solid electron donors. Hence, an additional 2 mechanism is here proposed: the maintenance of low H partial pressures by microbial H consumption, which 2 2 thermodynamically favors the H evolution reaction on the cathode or Fe(0) surface. This work elaborates how 2 the H partial pressure affects the H evolution onset potential and the H evolution rate on a cathode, as well as 2 2 2 the free energy change of the anoxic corrosion reaction. In addition, the H consumption characteristics, i.e., H 2 2 threshold (thermodynamic limit for H consumption) and H consumption kinetic parameters, of acetogenic 2 2 bacteria are reviewed and evidence is discussed for strongly different H consumption characteristics. Different 2 acetogenic strains are thus expected to maintain different H partial pressures on a cathode or Fe(0) surface, 2 while those that maintain lower H partial pressures (lower H threshold, higher H affinity) more strongly 2 2 2 increase the H evolution reaction. Consequently, I hypothesize that the different capacities of acetogenic 2 bacteria to use solid electron donors are related to differences in their H consumption characteristics. The focus 2 of this work is on acetogenic bacteria, but similar considerations are likely also relevant for other hydrogenotrophic microorganisms.

Phillips, E., Gilevska, T., Horst, A., Manna, J., Seger, E., Lutz, E.J., Norcross, S., Morgan, S.A., West, K.A., Mack, E.E., Dworatzek, S., Webb, J., Sherwood Lollar, B., 2020. Transformation of chlorofluorocarbons investigated via stable carbon compound-specific isotope analysis. Environmental Science &Technology 54, 870-878.

https://doi.org/10.1021/acs.est.9b05746 Compound-specific isotope analysis (CSIA) is a valuable tool in contaminant remediation studies. Chlorofluorocarbons (CFCs) are ozone-depleting substances previously thought to be persistent in groundwater under most geochemical conditions but more recently have been found to (bio)transform in some laboratory experiments. To date, limited applications of CSIA to CFCs have been undertaken. Here, biotransformation- associated carbon isotope enrichment factors, ε for CFC- 113 (ε = -8.5 ± 0.4‰) and CF C- 11 (ε = C,bulk C,bulk C,bulk 13 -14.5 ± 1.9‰), were determined. δ C signatures of pure-phase CFCs and hydrochlorofluorocarbons were measured to establish source signatures. These findings were applied to investigate potential in situ CFC transformation in groundwater at a field site, where carbon isotope fractionation of CFC-11 suggests naturally occurring biotransformation by indigenous microorganisms. The maximum extent of CFC-11 transformation is 13 estimated to be up to 86% by an approximate calculation using the Rayleigh concept. CFC- 113 δ C values in contrast were not resolvably different from pure-phase sources measured to date, demonstrating that CSIA can aid in identifying which compounds may, or may not, be undergoing reactive processes at field sites. Science and public attention remains focused on CFCs, as unexplained source inputs to the atmosphere have been recently reported, and the potential for CFC biotransformation in surface and groundwaters remains unclear. 13 This study proposes δ C CSIA as a novel application to study the fate of CFCs in groundwater.

Phrampus, B.J., Lee, T.R., Wood, W.T., 2020. A global probabilistic prediction of cold seeps and associated SEAfloor FLuid ExpulsionAnomalies (SEAFLEAs). Geochemistry, Geophysics, Geosystems 21, e2019GC008747.

https://doi.org/10.1029/2019GC008747 A newly compiled, open-source database of focused fluid flow sites (e.g., cold seeps) and associated SEAfloor FLuid Expulsion Anomalies (SEAFLEAs) reveals a variable distribution of anomalies across global continental margins. The SEAFLEA distribution is heavily biased toward North American continental margins, with most observations between 100- and 200-m water depth globally, and with an equal distribution between active and