Responses of mixed methanotrophic consortia to variable Cu2+/Fe2+ ratios

Metal(loid) speciation and transformation by aerobic methanotrophs

Manufacturing and resource industries are the key drivers for economic growth with a huge environmental cost (e.g. discharge of industrial effluents and post-mining substrates). Pollutants from waste streams, either organic or inorganic (e.g. heavy metals), are prone to interact with their physical environment that not only affects the ecosystem health but also the livelihood of local communities. Unlike organic pollutants, heavy metals or trace metals (e.g. chromium, mercury) are non-biodegradable, bioaccumulate through food-web interactions and are likely to have a long-term impact on ecosystem health. Microorganisms provide varied ecosystem services including climate regulation, purification of groundwater, rehabilitation of contaminated sites by detoxifying pollutants. Recent studies have highlighted the potential of methanotrophs, a group of bacteria that can use methane as a sole carbon and energy source, to transform toxic metal (loids) such as chromium, mercury and selenium. In this review, we synthesise recent advances in the role of essential metals (e.g. copper) for methanotroph activity, uptake mechanisms alongside their potential to transform toxic heavy metal (loids). Case studies are presented on chromium, selenium and mercury pollution from the tanneries, coal burning and artisanal gold mining, respectively, which are particular problems in the developing economy that we propose may be suitable for remediation by methanotrophs. Video Abstract.

Environmental and Microbial Interactions Shape Methane-Oxidizing Bacterial Communities in a Stratified Lake

In stratified lakes, methane-oxidizing bacteria (MOB) are strongly mitigating methane fluxes to the atmosphere by consuming methane entering the water column from the sediments. MOB communities in lakes are diverse and vertically structured, but their spatio-temporal dynamics along the water column as well as physico-chemical parameters and interactions with other bacterial species that drive the community assembly have so far not been explored in depth. Here, we present a detailed investigation of the MOB and bacterial community composition and a large set of physico-chemical parameters in a shallow, seasonally stratified, and sub-alpine lake. Four highly resolved vertical profiles were sampled in three different years and during various stages of development of the stratified water column. Non-randomly assembled MOB communities were detected in all compartments. We could identify methane and oxygen gradients and physico-chemical parameters like pH, light, available copper and iron, and total dissolved nitrogen as important drivers of the MOB community structure. In addition, MOB were well-integrated into a bacterial-environmental network. Partial redundancy analysis of the relevance network of physico-chemical variables and bacteria explained up to 84% of the MOB abundances. Spatio-temporal MOB community changes were 51% congruent with shifts in the total bacterial community and 22% of variance in MOB abundances could be explained exclusively by the bacterial community composition. Our results show that microbial interactions may play an important role in structuring the MOB community along the depth gradient of stratified lakes.

Proteinaceous methanotrophs for feed additive using biowaste as carbon and nutrients source

To achieve a sustainable production of food and feed production, inexpensive carbon and nutrient sources are needed. In the present study, biologically upgraded biogas is coupled with electrochemically extracted nitrogen from digested biowaste to cultivate mixed methanotrophs as protein source. Results showed that an increase from less than 5 μgCu²⁺/L to 100 μgCu²⁺/L increased the biomass production by 41%. Microbial analysis revealed that the dominated Methylomonas spp. followed by Methylophilus spp. created a specialized community for high CH₄ assimilation. Moreover, duplicate semi-continuous fermenters run for 120 days validating the efficiency of alternative carbon and nitrogen feedstocks at long-term operation. As for dry cell weight (DCW) production, more than 2.5 g-DCW/L were produced using biologically upgraded biogas and electrochemically extracted nitrogen. Furthermore, the protein content and amino acid profile (>50% of DCW) demonstrated that the microbial biomass pose the characteristics to be used as animal feed additive.

Methane oxidising bacteria to upcycle effluent streams from anaerobic digestion of municipal biowaste

Conventional microbial protein production relies on the usage of pure chemicals and gases. Natural gas, which is a fossil resource, is the common input gas for bacterial protein production. Alternative sources for gas feedstock and nutrients can sufficiently decrease the operational cost and environmental impact of microbial protein production processes. In the present study, the effluents streams of municipal biowaste anaerobic digestion, were used to grow methane oxidising bacteria which can be used as protein source. Results demonstrated that a 40:60 CH₄:O₂ (v/v) gas feeding resulted in microbial biomass production of 0.95 g-DM/L by a Methylophilus dominated community. When raw biogas was used as input for methane corresponding to the same initial methane partial pressure as before, instead of pure methane, the growth was partially hindered (0.61 g-DM/L) due to the presence of H₂S (IC₅₀: 1376 ppm). Hence, desulfurization is suggested before using biogas for microbial protein production. At semi-continuous mode, results showed that the produced biomass had relatively high protein content (>40% of dry weight) and the essential amino acids lysine, valine, leucine and histidine were detected at high levels.