The reduction of environmentally abundant iron oxides by the methanogen Methanosarcina barkeri

Shaken not stirred - effect of different mixing modes during the cultivation of methanogenic pure cultures

Numerous cultivation techniques for aerobic microorganisms have been extensively investigated in the field of microbiology. Optimisation of these techniques is important for scientific and economic reasons. Methanogenic archaea, however, are obligate anaerobic microorganisms requiring different cultivation techniques than aerobic organisms due to the fundamental differences in physiology. Mixing of aerobic cultures is generally considered as very important as it provides organisms with essential oxygen; however, for anaerobic microorganisms lacking the ability to grow with oxygen, this point in cultivation was widely neglected. This work aimed at investigating the effect of different mixing modes on cultures of the methanogenic archaea Methanomethylovorans thermophila, Methanosarcina acetivorans, Methanosarcina thermophila and Methanococcus vannielii by cultivating them anaerobically in the modes standing/lying, shaken/unshaken and large/small serum flask in order to analyse their impact on the methane and biomass production. This study showed that a shaken incubation mode had a positive impact on methane production and resulted in its accelerated production, especially in hydrogenotrophic cultures; however, higher methane production did not necessarily lead to higher biomass production.

Cometabolism of ferrihydrite reduction and methyl-dismutating methanogenesis by Methanosarcina mazei

Recent discoveries have shown that some Methanosarcina species can reduce Fe(III), reshaping our understanding of Methanosarcina ecophysiology. However, the specific minerals reduced, the products formed, and the underlying metabolic mechanisms remain elusive. Here, we report on the cometabolic process of Fe(III) reduction and methylotrophic methanogenesis in Methanosarcina mazei zm-15. Biogeochemical and mineralogical analyses were conducted to investigate Fe(III) reduction from three mineral preparations-ferrihydrite, goethite, and hematite. The results revealed that 38% of the 6 mM Fe(III) in ferrihydrite was reduced within 4 days, and this percentage increased to 75% with the addition of 100 µM anthraquinone-2,6-disulfonate (AQDS). Active Fe(III) reduction occurred immediately and preceded rapid methanogenesis. The addition of ferrihydrite and AQDS together significantly enhanced the maximal CH₄ production rate. However, Fe(III) reduction did not occur in goethite or hematite, even with the addition of 100 µM AQDS. Vivianite was identified as the major product from ferrihydrite reduction. Transcriptomic analysis revealed that gene expression related to the oxidation branch of the methyl-dismutating pathway and the membrane-associated electron transport chain (ETC) was significantly upregulated, whereas the expressions of genes associated with the reduction branch of the methyl-dismutating pathway were downregulated. In conclusion, M. mazei zm-15 demonstrates a strong ability to reduce poorly crystalline ferrihydrite, but not highly crystalline goethite and hematite. During the cometabolism of Fe(III) reduction and CH₄ production from methanol, the methyl-oxidation and membrane ETC pathways are enhanced, while the methyl-reduction pathway is downregulated. The mechanism of electron relay from cells to ferrihydrite, however, remains unclear and warrants further investigation.IMPORTANCEThe recent discovery that certain Methanosarcina species can grow by reducing Fe(III) challenges the traditional understanding of methanogens. However, the underlying metabolic mechanisms remain largely unexplored. Using a combination of biogeochemical, mineralogical, and microbiological approaches, we investigated the ability of Methanosarcina mazei zm-15. It exhibited a strong capacity to reduce poorly crystalline ferrihydrite but not highly crystalline goethite and hematite. The formation of vivianite from ferrihydrite reduction is likely due to the high rate of Fe(III) reduction and the presence of excess phosphorus in incubations. During the cometabolism of Fe(III) reduction and CH4 production from methanol, the methyl-oxidation and membrane electron transport pathways are upregulated, while the methyl-reduction pathway is downregulated. Our research uncovers a differential regulation of metabolic pathways during the cometabolism of Fe(III) reduction and CH4 production from methanol. The findings shed new light on the adaptive strategies employed by M. mazei in environments with the presence of Fe(III) and suggestthat Methanosarcina can play a significant role in methane production and iron cycling in natural environments.

MmcA is an electron conduit that facilitates both intracellular and extracellular electron transport in Methanosarcina acetivorans

Methanogens are a diverse group of Archaea that obligately couple energy conservation to the production of methane. Some methanogens encode alternate pathways for energy conservation, like anaerobic respiration, but the biochemical details of this process are unknown. We show that a multiheme c-type cytochrome called MmcA from Methanosarcina acetivorans is important for intracellular electron transport during methanogenesis and can also reduce extracellular electron acceptors like soluble Fe3+ and anthraquinone-2,6-disulfonate. Consistent with these observations, MmcA displays reversible redox features ranging from -100 to -450 mV versus SHE. Additionally, mutants lacking mmcA have significantly slower Fe3+ reduction rates. The mmcA locus is prevalent in members of the Order Methanosarcinales and is a part of a distinct clade of multiheme cytochromes that are closely related to octaheme tetrathionate reductases. Taken together, MmcA might act as an electron conduit that can potentially support a variety of energy conservation strategies that extend beyond methanogenesis.