The air in subterranean karst cavities is often depleted in methane (CH₄) relative to the atmosphere. Karst is considered a potential sink for the atmospheric greenhouse gas CH₄ because its subsurface drainage networks and solution-enlarged fractures facilitate atmospheric exchange. Karst landscapes cover about 14% of earth’s continental surface, but observations of CH₄ concentrations in cave air are limited to localized studies in Gibraltar, Spain, Indiana (USA), Vietnam, Australia, and by incomplete isotopic data. To test if karst is acting as a global CH₄ sink, we measured the CH₄ concentrations, δ¹³C_CH4, and δ²H_CH4 values of cave air from 33 caves in the USA and three caves in New Zealand. We also measured CO₂ concentrations, δ¹³C_CO2, and radon (Rn) concentrations to support CH₄ data interpretation by assessing cave air residence times and mixing processes. Among these caves, 35 exhibited subatmospheric CH₄ concentrations in at least one location compared to their local atmospheric backgrounds. CH₄ concentrations, δ¹³C_CH4, and δ²H_CH4 values suggest that microbial methanotrophy within caves is the primary CH₄ consumption mechanism. Only 5 locations from 3 caves showed elevated CH₄ concentrations compared to the atmospheric background and could be ascribed to local CH₄ sources from sewage and outgassing swamp water. Several associated δ¹³C_CH4 and δ²H_CH4 values point to carbonate reduction and acetate fermentation as biochemical pathways of limited methanogenesis in karst environments and suggest that these pathways occur in the environment over large spatial scales. Our data show that karst environments function as a global CH₄ sink.

Ruminant mammals extract nutrients from plant-based food through fermentation in the rumen; fiber and starch are pre-digested by microorganisms and methane is produced as a by-product, which released into the atmosphere acts as a potent greenhouse gas. In an effort to reduce enteric methanogenesis, dietary additives for ruminants have been investigated, and marine macroalgae have proven particularly promising, e.g., the inclusion of 0.2% dry matter of the red alga A. taxiformis into cow feed decreased in vivo methane production by up to 98%. Thus, if globally applied, the addition of algae in ruminant diets could revolutionize the management of greenhouse gas emissions across the livestock sector. However, the ozone-depleting nature of halogen compounds produced in Asparagopsis sp. and the reported adverse health impacts on humans, along with impracticability issues and the difficulty to produce, commercialize and distribute algae widely, has sown some doubt on the feasibility of using macroalgae as methane mitigation instruments. To circumvent such obstacles, and taking into account the paradigm that eukaryotic hosts cannot be understood without considering interactions with their associated microbiome, the exploration of marine algae associated microorganisms is anticipated. Following the notion that in the close and intimate relationships between algae-hosts and their microbiota the origin of chemical response mechanisms is often unclear, and that compounds initially assigned to algae have previously been shown to stem from host-associated microbes, it is not unreasonable to think that these may be involved in the antimethanogenic effects of marine algae in the rumen. Once identified, such microorganisms could lead to antimethanogenic feed additives, and reduce enteric methanogenesis from livestock ruminants substantially. This review is three-fold: it provides a brief, historic overview of macroalgae as feed supplements for ruminants, sums up the difficulties related to using whole-macroalgae as large-scale antimethanogenic feed additives, and describes the macroalga microbiome, including its potential to serve as an antimethanogen for enteric fermentation.

This paper analyzes the mitigation of enteric methane (CH4) emissions from ruminants with the use of feed additives inhibiting of rumen methanogenesis to limit global temperature increase to 1.5 °C. A mathematical simulation conducted herein predicted that pronounced inhibition of rumen methanogenesis with pure chemicals or bromoform-containing algae can contribute to limit global temperature increase by 2050 to 1.5 °C only if widely adopted at a global level and considering an efficacy higher than obtained in most studies. Currently, the most important limitations to the adoption of antimethanogenic feed additives are probably increased feeding cost without a consistent return in production efficiency, and achieving sustained delivery of inhibitors to the rumens of non-supplemented, extensively ranging animals. Economic incentives, and changes in rumen microbial metabolism caused by inhibiting methanogenesis, could potentially be used to make the methanogenesis inhibition intervention cost effective. Also, the composition of the methanogenic community, and rate of disappearance of inhibitors of methanogenesis in the rumen can influence the effective dose of the inhibitors, and hence the cost of their adoption. Possible means for sustained delivery of antimethanogenic compounds to extensively grazing animals are discussed. Limitations and knowledge gaps of these approaches, and future research directions, are examined.

Methane (CH4) has attracted attention as not only synthetic natural gas, but also one of the hy-drogen carriers in terms of energy density. On the other hand, there exist bacterial ecosystems in nature that can decompose organic compounds to produce CH4 and CO2. In this study, Clostridi-um cellulovorans was first cultivated with pig manure (PM) as an unused biomass. Regarding the measurement of organic acids by high-performance liquid chromatography (HPLC), acetate and butyrate were increased in the C. cellulovorans medium containing 0.5% PM, while formate and lactate were decreased in it. Next, in comparison with carbon sources such as glucose, cellobiose, and acetate, cocultivation of C. cellulovorans and Methanosarcina mazei or microbial flora of me-thane production (MFMP) was performed in the C. cellulovorans medium. These results revealed that 0.5% acetate as the sole carbon source produced CH4 only by cocultivating C. cellulovorans and MFMP. Furthermore, MFMP was only cultivated with 1% acetate or 1% methane as a carbon source after precultivated with 0.5% glucose medium for 12 h. As a result, methane productivity of MFMP with 1% methanol medium was approximately eight times higher than that with 1% acetate medium. Finally, next-generation sequencing (NGS) analysis of MFMP after cultivation with 1% acetate or 1% methane was carried out. Interestingly, Methanofollis (0.211%) belonging to H2/CO2 -using methanogens (CO2 reduction pathway) was dominant in the 1% acetate medium for 72 h cultivation, whereas Methanosarcina siciliae (1.178%), M. barkeri (0.571%), and Methano-follis (0.490%) were major species in 1% methanol medium for 72 h cultivation. Since Methano-sarcina spp. are belonging to acetoclasts (acetoclastic pathway), methanol could promote to grow Methanosarcina spp. rather than acetate. Therefore, it seemed Methanosarcina spp. may play a key methanogenesis in MFMP. Thus, these results will provide important information for low cost biomethane production.

This study deals with the computational analysis of the kinetic processes of a microbial electrolysis cell assisted-anaerobic digestion (MEC-AD) treating raw-waste activated sludge (WAS), comparatively with a conventional AD, as well as the effect of the Organic Loading Rate (OLR) on the system’s performance. The aim was to derive a single kinetic and mathematical model for the study of the MEC-AD, using the ADM1 framework, which can be utilized to extract the effect of an applied potential on the kinetics of AD. The experimental data were obtained from the long-term, continuous operation of two identical reactors (an AD and a MEC-AD), which were operated at different OLRs (1.1, 1.7 and 2.9 gCOD/(L*d)). The results showed that the MEC-AD yielded improved biomass yields, substrate consumption kinetics and 1st order disintegration kinetics, with a predominant contribution to disintegration of complex particulates, comparatively with the AD. Moreover, it enabled operation at higher OLRs (achieving the highest divergence from the AD at the OLR of 4.14 gCOD/(L*d)), therefore accelerating sludge treatment, as well as an improved performance at an increased solids retention time (SRT). These findings can serve as a quantitative guide for the effects of the operating parameters on the raw-WAS-fed MEC-AD performance.

Methane (CH4) has attracted attention as not only synthetic natural gas, but also one of the hy-drogen carriers in terms of energy density. On the other hand, there exist bacterial ecosystems in nature that can decompose organic compounds to produce CH4 and CO2. In this study, Clostridi-um cellulovorans was first cultivated with pig manure (PM) as an unused biomass. Regarding the measurement of organic acids by high-performance liquid chromatography (HPLC), acetate and butyrate were increased in the C. cellulovorans medium containing 0.5% PM, while formate and lactate were decreased in it. Next, in comparison with carbon sources such as glucose, cellobiose, and acetate, cocultivation of C. cellulovorans and Methanosarcina mazei or microbial flora of me-thane production (MFMP) was performed in the C. cellulovorans medium. These results revealed that 0.5% acetate as the sole carbon source produced CH4 only by cocultivating C. cellulovorans and MFMP. Furthermore, MFMP was only cultivated with 1% acetate or 1% methane as a carbon source after precultivated with 0.5% glucose medium for 12 h. As a result, methane productivity of MFMP with 1% methanol medium was approximately eight times higher than that with 1% acetate medium. Finally, next-generation sequencing (NGS) analysis of MFMP after cultivation with 1% acetate or 1% methane was carried out. Interestingly, Methanofollis (0.211%) belonging to H2/CO2 -using methanogens (CO2 reduction pathway) was dominant in the 1% acetate medium for 72 h cultivation, whereas Methanosarcina siciliae (1.178%), M. barkeri (0.571%), and Methano-follis (0.490%) were major species in 1% methanol medium for 72 h cultivation. Since Methano-sarcina spp. are belonging to acetoclasts (acetoclastic pathway), methanol could promote to grow Methanosarcina spp. rather than acetate. Therefore, it seemed Methanosarcina spp. may play a key methanogenesis in MFMP. Thus, these results will provide important information for low cost biomethane production.