Figures (2)  Tables (5)

    • Figure 1. 

      Metabolic pathways of methanotrophs (adapted from Park & Kim[32]).

    • Figure 2. 

      Pathways for high-value product production by (a) methanotrophs, (b) yields of primary high-value products, and (c) yields of key high-value products with large-scale production potential at common reaction temperatures (G3P: Glyceraldehyde 3-phosphate; MEP: Methyl-erythritol Phosphate; FPP: Farnesyl Pyrophosphate; PHB: Poly-3-hydroxybutyrate; OAA: Oxaloacetate, FA: Fatty acid. Data are sourced from the literature[111], and Tables 3[112−119], 4[85,120−124], and 5[106,125−131]).

    • Type Genera Species Representative strains Separation source Characteristics Ref.
      Type I Methylococcus Methylococcus geothermalis IM1 A geothermal spring Thermophilic (48 °C) [40]
      Methylomonas Methylomonas methanica MC09 Coastal seawater Halotolerant (seawater) [41]
      Methylomonas koyamae Fw12E-Y A rice paddy field Methanol-utilizing [42]
      Methylobacter Methylobacter tundripaludum SV96 Arctic wetland soil Nitrogen-fixing (nifH) [43]
      Methylovulum Methylovulum miyakonense HT12 Forest soil Formaldehyde-assimilating [44]
      Methylovulum psychrotolerans Sph1 Low-temperature terrestrial environments Psychrotolerant (2 °C) [45]
      Methylosoma Methylosoma difficile Lc 2 Lake sediment Nitrogen-fixing (nifH) [46]
      Methylothermus Methylothermus thermalis MYHT A hot spring Thermophilic (67 °C) [47]
      Methylothermus subterraneus HTM55 Subsurface hot aquifer Thermophilic (65 °C) [48]
      Methylogaea Methylogaea oryzae E10 A rice paddy field Nitrogen-fixing (nifH) [49]
      Methylohalobius Methylohalobius crimeensis 10Ki Hypersaline lakes Extremely halophilic
      (15% NaCl)      		 [50]
      Methylomarinum Methylomarinum vadi IT-4 Marine environment Obligate marine [51]
      Methyloprofundus Methyloprofundus sedimenti WF1 Marine sediment Nitrogen-fixing (nifH) [52]
      Methylotenera Methylotenera versatilis 301 Lake sediment Multiple substrate utilization [53]
      Type II Methylocystis Methylocystis hirsuta CSC1 A groundwater aquifer Special surface structure [54]
      Methylocella Methylocella silvestris BL2 An acidic forest cambisol Multiple substrate utilization [55]
      Methylocapsa Methylocapsa aurea KYG A forest soil Multiple substrate utilization [56]
      Methyloferula Methyloferula stellata AR4 Acidic Sphagnum peat bogst Acidophilia (pH = 3.5) [57]
      Methylorubrum Methylorubrum rhodesianum MB200 A household biodigester Multiple substrate utilization [58]
      Methylobrevis Methylobrevis albus L22 Freshwater lake sediment Oxidase and catalase production [59]
      Type X Methylacidiphilum Methylacidiphilum fumariolicum SolV Volcanic region Hydrogenase-possessing [60]
      Methylacidiphilum infernorum V4 A geothermal field Hyperthermophilic (60 °C) [61]
      Methylacidimicrobium Methylacidimicrobium fagopyrum 3C Volcanic soil Acidophilia (pH = 0.6) [62]
      Methylacidimicrobium tartarophylax 4AC Volcanic soil Acidophilia (pH = 0.5)
      Methylacidimicrobium cyclopophantes 3B Volcanic soil Acidophilia (pH = 3.6)
      Candidatus Methylacidithermus Candidatus Methylacidithermus pantelleriae PQ17 Volcanic environments Sulfur-fixing (cysD/C/H) [63]
      Methylotrophs Methylophaga Methylophaga marina ATCC 35842 Sea water Fructose and methylamine utilization [64]
      Methylophaga thalassica ATCC 33146 Sea water Fructose and methylamine utilization
      Methylotenera Methylotenera mobilis JLW8 Lake sediment Methylamine-utilizing [65]
      Hyphomicrobium Hyphomicrobium denitrificans TK 0415 − Anaerobic denitrification [66]
      Paracoccus Paracoccus denitrificans Stanier 381 Garden soil Hydrogen-utilizing [67]
      Methyloversatilis Methyloversatilis universalis FAM5 Freshwater wetlands Multiple substrate utilization [68]
      Methylopila Methylopila capsulata IM1 Soil Multiple substrate utilization [69]

      Table 1. 

      Representative genera and characteristics of methanotrophic communities

    • Production Methanol SCP PHA
      Biosynthesis pathway Central metabolic pathway Multiple carbon assimilation pathways Serine carbon assimilation pathway
      Producers Type I, II, and X capable Type I dominant, Type II, and X applicable Primarily type II
      Product value Moderate Relatively low Relatively high
      Commercialisation status Not yet commercialised Large-scale commercialisation Small-scale commercialisation
      Carbon conversion challenges Methanol is a metabolic intermediate that is readily oxidized, leading to low accumulation. The production requires maximized carbon flux toward biomass and suppression of complete oxidation. The production is typically induced under nutrient imbalance, creating a growth-synthesis trade-off.
      Applications Chemical feedstocks, fuel, bioplastic precursors Animal feed, food additives, nutrient supplements Biodegradable plastics, biomedical materials

      Table 2. 

      Comparison of high-value resource products of methanotrophs[4,15,111]

    • Production Output Production condition Corresponding producer Ref.
      Methanol 52.9 mM 30% CH4, 30 °C, NMS medium, immobilized on coconut coir, eight repeated batch conditions Methylocystis bryophila, Methyloferula stellata, Methylocella tundrae [112]
      Methanol 25.75 mM CH4 : CO2 = 2:1, 30 °C, NMS medium, immobilized on chitosan, eight repeated batch conditions Methylocystis bryophilla [113]
      Methanol 24.36 mM CH4 : CO2 = 4:1, 30% CH4, 30 °C, NMS medium, immobilized on chemically modified chitosan, eight repeated batch conditions Methylomicrobium album, Methylocystis bryophila, Methyloferula stellata [114]
      Methanol 5.34 mM Cultivation in biogas containing CH4, 25 °C, AMS medium, six repeated batch conditions Primarily Methylobacter and Methylosarcina [115]
      Methanol 64.6 mM 30% CH4, 15% H2, 30 °C, NMS medium, six repeated batch conditions Metholosinus sporium, Methylocystis bryophila [116]
      Methanol 16.4 mM 30% CH4, 15% CO2, 30 °C, NMS medium, immobilized on synthetic precursor solution, ten repeated batch conditions Methyloferula stellata, Methylocystis bryophila [117]
      Methanol 8.59 mM CH4 : air = 1:4, 37 °C, NMS medium, 500 ppm H2S Methylocaldum sp. [118]
      Methanol 5.37 mM 30% CH4, 30 °C, NMS medium, immobilized on polyvinyl alcohol, five repeated batch conditions Methylocystis bryophila, Methyloferula stellata [119]

      Table 3. 

      Cases of methanol production by methanotrophs

    • Production Content Production condition Corresponding producer Ref.
      SCP 56.10% ± 10.99% CH4 : O2 = 1:2, NMS medium, 2,973 ppm H2S Primarily Methylocystis and Terrimonas [14]
      SCP 73% ± 5% CH4 : O2 = 2:3, 30 °C, NMS medium, 1,500 ppm H2S Primarily Methylocystis spp. and Chryseobacterium spp. [85]
      SCP 59.2% ± 3.6% CH4 : CO2 = 70:30 or 50:50, CH4 : O2 = 2:3, 30 °C,
      AMS medium, 4,000 ppm H2S      		 Primarily Methylocystis spp. and Chryseobacterium spp. [120]
      SCP 41% ± 2.0% CH4 : O2 = 1:2, 25 °C, dAMS medium Primarily Methylophilus sp.1 and Methylomonas sp.1 [121]
      SCP 45% 60% CH4, 30% O2, 10% CO2, 37 °C, cultivation in
      wastewater containing NH4+      		 Methylococcus capsulatus [122]
      SCP 52.3% 60% CH4, 40% CO2, 27 °C, AMS medium Primarily Methylosinus and Methylococcus [123]
      SCP 67% CH4 : O2 = 1:4, 25 °C, AMS medium Primarily Methylomonadaceae and Methylococcaceae [124]
      SCP 50.2% Primarily CH4 : O2 : CO2 = 1:2:0.05, NMS medium Primarily Methylococcus and Methylotenera [138]
      dAMS: dilute ammonium mineral salt.

      Table 4. 

      Cases of SCP production by methanotrophs

    • Production Content Production condition Corresponding producer Ref.
      PHA 12.6% ± 2.4% 20% CH4, 30 °C, AMS medium Primarily Methylocystis [106]
      PHB 48.7% ± 1.2% CH4 : O2 = 1:1, 30 °C, NFMS medium Primarily Methylophilus and Methylocella [125]
      PHB 59.4% ± 4.5% CH4 : O2 = 1:1, 25 °C, AMS medium, recycle PHB producers after accumulation Primarily Methylocystis and Pseudomonas [126]
      PHBV 41.9% 30% CH4, 30 °C, NMS medium Methylocystis sp. MJC1 [127]
      Mutiple PHA 50% ± 4% to 56% ± 4% CH4 : O2 = 2:3, 30 °C, JM2 medium (modified AMS medium) Methylocystis parvus OBBP [128]
      PHB 22.20% CH4 : O2 = 1:1, 30 °C, NMS medium Primarily Methylocystis [129]
      PHBV 35% 0.5 atm CH4, 0.33 atm O2, 38 °C, AMS medium Methylosinus thricosporum OB3b [130]
      PHB 52.9% ± 4% CH4 : O2 = 1:1, 25 °C, AMS medium Mutiple methanotrophs [131]
      NFMS: nitrate free mineral salt.

      Table 5. 

      Cases of PHA production by methanotrophs