- Volume 55, Issue 3, 2005

Strain TL 2T was isolated on mineral medium with thiosulfate from a near-shore sulfidic hydrothermal area in Matupi Harbour on the island of New Britain, Papua New Guinea. The cells varied from long filaments with swollen ends, often aggregated, to short rods, depending on the growth conditions. The bacterium was obligately aerobic and grew autotrophically with thiosulfate as energy source or heterotrophically with organic acids and sugars. In thiosulfate-limited continuous culture, μ max and Y max for autotrophic growth were 0·1 h−1 and 3 g protein mol−1, respectively. From the various reduced sulfur compounds tested, only thiosulfate and sulfide supported active respiration. Inorganic carbon was assimilated via the Calvin cycle. Presence of the ‘green’-type of form I RubisCO gene was detected. Growth was possible from 15 to 47 °C with an optimum at 35 °C, pH 6·5–8·5 with an optimum at pH 8·0, and between 10 and 90 g NaCl l−1 with an optimum at 35 g l−1. Phylogenetic analysis based on 16S rRNA and cbbL gene sequences demonstrated that strain TL 2T forms a separate lineage within the alpha-3 subdivision of the Proteobacteria, distantly related to the genera Rhodovulum and Rhodobacter. On the basis of these results, a novel genus and species, Thioclava pacifica gen. nov., sp. nov., is proposed to accommodate strain TL 2T (=DSM 10166T=UNIQEM 229T).

The taxonomic position of a Pseudomonas-like strain, designated BN9T, was investigated. This strain had previously been isolated as a 5-aminosalicylate-degrading organism from a 6-aminonaphthalene-2-sulphonate-degrading mixed bacterial culture. Previously, detection of ubiquinone Q-8, a polyamine pattern with putrescine, spermidine and 2-hydroxyputrescine as the major polyamines, and partial 16S rRNA gene sequencing had suggested that strain BN9T belongs to the ‘Betaproteobacteria’. This was supported by sequencing the 16S rRNA gene, which demonstrated 94–96 % sequence similarity to different species of the genera Achromobacter, Alcaligenes and Bordetella, and suggested that strain BN9T represents a member of the family Alcaligenaceae. Different methods for the construction of phylogenetic dendrograms placed the strain separately from all other genera within the Alcaligenaceae. Fatty acid analysis demonstrated the presence of high concentrations of C19 : 0 cyclo ω8c. On the basis of low 16S rRNA gene sequence similarity to other members of the Alcaligenaceae, fatty acid and polar lipid profiles, and other unique phenotypic properties of strain BN9T, the creation of a new genus and species with the name Pusillimonas noertemannii gen. nov., sp. nov. is proposed. The type strain is BN9T (=DSM 10065T=NCIMB 14020T).

A novel group of aerobic anoxygenic phototrophic bacteria was isolated from marine dinoflagellates, and two strains were characterized in detail. Cells were Gram-negative cocci or ovoid rods and were motile by means of a single, polarly inserted flagellum. They were obligate aerobes requiring 1–7 % salinity. The optimal pH range for growth was 6·5–9·0 and the temperature optimum was 33 °C. The bacteria contained bacteriochlorophyll a and spheroidenone as the only carotenoid. The in vivo absorption spectrum displayed two maxima in the infrared region at 804 and 868 nm. The distinct 804 nm band indicates the presence of light-harvesting system 2. Various organic carbon sources were assimilated, including many carboxylic acids, glucose and glycerol, but not butyrate, ethanol or methanol. Dissimilatory nitrate reduction was found for both strains. The physiological characteristics of the new strains resembled those of Roseobacter denitrificans, but there were differences in the lipid composition. Based on 16S rRNA gene sequence analysis the new strains are relatively distant from other recognized species, with the closest relatives Jannaschia helgolandensis, Ruegeria atlantica and Rhodobacter veldkampii showing 94·1–93·4 % similarity. Similarity to Roseobacter denitrificans was only 92·2 %, in line with numerous other species of the Roseobacter group. Therefore, it is proposed to classify the strains in a new genus and species within the Roseobacter clade, Dinoroseobacter shibae gen. nov., sp. nov. The type strain is DFL 12T (=DSM 16493T=NCIMB 14021T).

During a study in the Argentinian region of Chaco (Córdoba), some strains were isolated from the rhizosphere of grasses growing in semi-desertic arid soils. Two of these strains, one isolated from the rhizospheric soil of Chloris ciliata (strain CH01T) and the other from Pappophorum caespitosum (strain PA01), were Gram-negative, strictly aerobic rods, which formed yellow round colonies on nutrient agar. They produced a water-insoluble yellow pigment, and a fluorescent pigment was also detected. A polyphasic taxonomic approach was used to characterize the strains. Comparison of the 16S rRNA gene sequences showed a similarity of 99·3 % between them, and phylogenetic analysis revealed that the strains belong to the genus Pseudomonas, within the γ-subclass of the Proteobacteria. The closest related species is Pseudomonas straminea IAM 1598T (similarity of 99·0 % to strain CH01T and 98·8 % to strain PA01), clustering in a separate branch with the various methods of tree building used. Strains CH01T and PA01 both had a single polar flagellum, like other yellow pigment-producing pseudomonads related to them. Both strains produced catalase and oxidase. Similar to P. straminea, they did not hydrolyse gelatin or casein. The G+C DNA contents determined were 57·5 mol% for CH01T and 58·0 mol% for PA01. DNA–DNA hybridization results showed 81 % relatedness between them, and only 40–44 % relatedness with respect to the type strain of P. straminea. These results, together with other phenotypic characteristics, support the conclusion that both isolates belong to the same species, and should be described as representing a novel species within the genus Pseudomonas, for which the name Pseudomonas argentinensis sp. nov. is proposed. The type strain is CH01T (=LMG 22563T=CECT 7010T).

A taxonomic study was performed on strain YIM 31775T, which was isolated from a soil sample collected from Yunnan Province, China. The isolate was chemo-organotrophic, aerobic and Gram-negative. Cells were short rods and motile, with one or more polar flagella. Growth temperature and pH ranged from 4 to 55 °C and 6·5 to 12·0, respectively; the optimum growth temperature and pH were 28–37 °C and 7·0–9·0, respectively. Q-8 was the predominant respiratory lipoquinone. The major fatty acids were C16 : 1 ω7c (42·4 %) and C16 : 0 (28·1 %). The DNA G+C content was 62·4±0·3 mol%. Phylogenetic analysis based on the 16S rRNA gene sequence indicated that strain YIM 31775T should be placed within the family ‘Oxalobacteraceae’, in which it formed a distinct lineage. Based on the high 16S rRNA gene sequence divergence and phenotypic characteristics, it is proposed that strain YIM 31775T should be classified as representing a novel member of the family ‘Oxalobacteraceae’, for which the name Naxibacter alkalitolerans gen. nov., sp. nov. is proposed. The type strain is YIM 31775T (=CCTCC AA 204003T=KCTC 12194T).

The taxonomic positions of Lysobacter species with validly published names and a novel strain Ko07T, which was newly isolated from an upflow anaerobic sludge blanket reactor treating wastewater from a brewery, were (re)estimated on the basis of results obtained by using a polyphasic taxonomy approach. Phylogenetic inference based on 16S rRNA gene sequences showed that strain Ko07T and all Lysobacter species with validly published names clustered together in a phylogenetic branch within the class ‘Gammaproteobacteria’. The sequence similarity of strain Ko07T to the type strains of established Lysobacter species was in the range 94·9–96·7 %. Ubiquinone Q-8 and branched fatty acids, C11 : 0 iso, C15 : 0 iso, C16 : 0 iso, iso C17 : 1 ω9c and C11 : 0 iso 3OH, predominantly appeared in strain Ko07T as well as in all type strains of the recognized Lysobacter species. The DNA–DNA hybridization values of strain Ko07T with those of recognized Lysobacter species were estimated to be 2–20 %. Despite sharing common taxonomic features in important phenotypic characteristics, such as gliding movement, long-rod shape and proteolytic activity, strain Ko07T could be distinguished from the Lysobacter species with validly published names by its low DNA–DNA hybridization value, a comparatively low DNA G+C content (63·8 mol%), substrate utilization and some physiochemical characteristics. On the basis of the results obtained in this study, it is proposed that strain Ko07T should be classified as representing a novel member of the genus Lysobacter, for which the name Lysobacter concretionis sp. nov. is proposed. The type strain is Ko07T (=KCTC 12205T=DSM 16239T).

Members of the species Agrobacterium ferrugineum were isolated from marine environments. The type strain of this species (=LMG 22047T=ATCC 25652T) was recently reclassified in the new genus Pseudorhodobacter, in the order ‘Rhodobacterales’ of the class ‘Alphaproteobacteria’. Strain LMG 128 (=ATCC 25654) was also initially classified as belonging to the species Agrobacterium ferrugineum; however, the nearly complete 16S rRNA gene sequence of this strain indicated that it does not belong within the genus Agrobacterium or within the genus Pseudorhodobacter. The closest related organism, with 95·5 % 16S rRNA gene similarity, was Aquamicrobium defluvii from the family ‘Phyllobacteriaceae’ in the order ‘Rhizobiales’. The remaining genera from this order had 16S rRNA gene sequence similarities that were lower than 95·1 % with respect to strain LMG 128. These phylogenetic distances suggested that strain LMG 128 belonged to a different genus. The major fatty acid present in strain LMG 128 was mono-unsaturated straight chain 18 : 1ω7c. The G+C content of the DNA was 53·1 mol%. Strain LMG 128 grew at 4 °C but not at 40 °C, and tolerated up to 5 % NaCl. The pH range for growth was 6–8. It produced urease and β-galactosidase, and hydrolysed aesculin. Denitrification was negative. Growth was observed with many carbohydrates as the only carbon source. The data from this polyphasic study indicate that this strain belongs to a new genus of the family ‘Phyllobacteriaceae’, and therefore it is proposed that strain LMG 128T should be reclassified as representing a novel species within the new genus Hoeflea gen. nov., for which the name Hoeflea marina sp. nov. is proposed.

A Gram-negative, non-spore-forming, yellow-pigmented, slightly halophilic bacterial strain, SW-109T, was isolated from a tidal flat of the Yellow Sea in Korea, and subjected to a polyphasic taxonomic study. This isolate did not produce bacteriochlorophyll a and contained ubiquinone-10 as the predominant respiratory lipoquinone and C18 : 1 ω7c as the major fatty acid. The DNA G+C content was 60·3 mol%. Phylogenetic analyses based on 16S rRNA gene sequences showed that strain SW-109T is phylogenetically affiliated to the genus Erythrobacter of the family Sphingomonadaceae. Strain SW-109T exhibited levels of 16S rRNA gene sequence similarity to the type strains of Erythrobacter species of 94·0–96·3 %, making it possible to categorize strain SW-109T as a species that is separate from previously recognized Erythrobacter species. On the basis of its phenotypic properties and phylogenetic distinctiveness, SW-109T (=KCTC 12311T=JCM 12599T) was classified as the type strain of a novel Erythrobacter species, for which the name Erythrobacter luteolus sp. nov. is proposed.

Two bacterial strains, B-5T and NO1A, were isolated from the surface water of the Bohai Sea and deep-sea sediment of the east Pacific Ocean, respectively. Both strains were halophilic, aerobic, Gram-negative, non-spore-forming, catalase- and oxidase-positive motile rods. They grew on a restricted spectrum of organic compounds, including some organic acids and alkanes. On the basis of 16S rRNA gene sequence similarity, strains B-5T and NO1A were shown to belong to the γ-Proteobacteria. Highest similarity values were found with Alcanivorax venustensis (95·2 %), Alcanivorax jadensis (94·6 %) and Alcanivorax borkumensis (94·1 %). Principal fatty acids of both strains were C16 : 0, C16 : 1 ω7c and C18 : 1 ω7c. The chemotaxonomically characteristic fatty acid C19 : 0 cyclo ω8c was also detected. On the basis of the above, together with results of physiological and biochemical tests, DNA–DNA hybridization, comparisons of 16S–23S internal transcribed spacer sequences and comparisons of the partial deduced amino acid sequence of alkane hydroxylase, both strains were affiliated to the genus Alcanivorax but were differentiated from recognized Alcanivorax species. Therefore, a novel species, Alcanivorax dieselolei sp. nov., represented by strains B-5T and NO1A is proposed, with the type strain B-5T (=DSM 16502T=CGMCC 1.3690T).

A Gram-negative, non-sporulating, rod-shaped, motile bacterium, with a single polar flagellum, designated strain PsJNT, was isolated from surface-sterilized onion roots. This isolate proved to be a highly effective plant-beneficial bacterium, and was able to establish rhizosphere and endophytic populations associated with various plants. Seven related strains were recovered from Dutch soils. Based on 16S rRNA gene sequence data, strain PsJNT and the Dutch strains were identified as representing a member of the genus Burkholderia, as they were closely related to Burkholderia fungorum (98·7 %) and Burkholderia phenazinium (98·5 %). Analysis of whole-cell protein profiles and DNA–DNA hybridization experiments confirmed that all eight strains belonged to a single species. Strain PsJNT had a DNA G+C content of 61·0 mol%. Only low levels of DNA–DNA hybridization to closely related species were found. Qualitative and quantitative differences in fatty acid composition between strain PsJNT and closely related species were identified. The predominant fatty acids in strain PsJNT were 16 : 0, 18 : 1ω7c and summed feature 3 (comprising 16 : 1ω7c and/or iso-15 : 0 2-OH). Isolate PsJNT showed high 1-aminocyclopropane-1-carboxylate deaminase activity and is therefore able to lower the ethylene level in a developing or stressed plant. Production of the quorum-sensing signal compound 3-hydroxy-C8-homoserine lactone was detected. Based on the results of this polyphasic taxonomic study, strain PsJNT and the seven Dutch isolates are considered to represent a single, novel species, for which the name Burkholderia phytofirmans sp. nov. is proposed. The type strain is strain PsJNT (=LMG 22146T=CCUG 49060T).

A Gram-negative, motile, slightly halophilic bacterial strain, SW-150T, was isolated from sea water of the Yellow Sea, Korea, and was characterized by a polyphasic taxonomic approach. Strain SW-150T grew optimally at 25–30 °C and in the presence of 2 % (w/v) NaCl. The isolate could be distinguished from other Sphingopyxis species in producing an orange pigment. It contained ubiquinone-10 as the predominant respiratory lipoquinone and C18 : 1 ω7c and C17 : 1 ω6c as the major fatty acids. No 3-hydroxy fatty acids were detected. Major polar lipids were sphingoglycolipid, diphosphatidylglycerol, phosphatidylglycerol and phosphatidylethanolamine. The DNA G+C content was 63 mol%. Comparative 16S rRNA gene sequence analyses showed that strain SW-150T was phylogenetically affiliated to the genus Sphingopyxis of the family Sphingomonadaceae. Similarity values between the 16S rRNA gene sequences of strain SW-150T and the type strains of Sphingopyxis species ranged from 91·6 to 94·2 %, making it possible to categorize strain SW-150T as a species that is separate from previously described Sphingopyxis species. On the basis of phenotypic properties and phylogenetic distinctiveness, SW-150T (=KCTC 12231T=DSM 16222T) should be classified as the type strain of a novel Sphingopyxis species, for which the name Sphingopyxis baekryungensis sp. nov. is proposed.

A novel aromatic-compound-degrading bacterium, strain T3-B9T, was isolated from sediment of Taihu Lake, Jiangsu Province, south-east China. This bacterial isolate assimilated several aromatic compounds such as phenol, aniline, nitrobenzene, 4-chloronitrobenzene and phenanthrene. The taxonomy of strain T3-B9T was studied by polyphasic methods. The organism showed a range of phenotypic and chemotaxonomic properties consistent with those of the genus Novosphingobium. The 16S rRNA gene sequence similarity of strain T3-B9T to members of the genus Novosphingobium ranged from 91·6 to 97·5 %, and this isolate clustered phylogenetically with members of genus Novosphingobium. The DNA–DNA relatedness values of strain T3-B9T to the most phylogenetically related species, Novosphingobium subterraneum DSM 12447T, Novosphingobium aromaticivorans ATCC 700278T and Novosphingobium stygium ATCC 700280T, were 31, 33 and 14 %, respectively. The combined genotypic and phenotypic data show that strain T3-B9T represents a novel species of the genus Novosphingobium, for which the name Novosphingobium taihuense sp. nov. is proposed. The type strain is T3-B9T (=AS 1.3432T=JCM 12465T).

Five strains of free-living diazotrophs isolated from rice were characterized by using a polyphasic approach. The strains were found to be very closely related, with 99–100 % 16S rRNA gene sequence similarity and DNA–DNA hybridization values greater than 70 %, suggesting that they represent a single species. When compared with other recognized species, they showed not more than 93 and 89 % similarity for the 16S rRNA and nifH gene sequences, respectively. Phylogenetic distances showed that these isolates were distinct from other taxa within the α-Proteobacteria. Chemotaxonomic characteristics of these isolates included the DNA G+C content (62·1–63·1 mol%), the major quinone system (Q-10), predominant fatty acids (18 : 1ω7c, cyclo 19 : 0ω8c and 16 : 0) and major hydroxy fatty acids (14 : 0 3-OH, 18 : 0 3-OH and 16 : 0 3-OH). Based on phylogenetic and phenotypic analyses, these isolates are considered to represent a novel genus and species, for which the name Pleomorphomonas oryzae gen. nov., sp. nov. is proposed. The type strain is F-7T (=IAM 15079T=ATCC BAA-940T=DSM 16300T).

A novel stalked bacterium, designated strain T3-B7T, was isolated from sediment of Taihu Lake, Jiangsu Province, China, and its taxonomy was studied by using a polyphasic approach. Cell morphology, physiological and biochemical properties, and polar lipids indicated that strain T3-B7T represented a member of the genus Asticcacaulis. Based on 16S rRNA gene sequence similarity analysis, strain T3-B7T was found to be phylogenetically related to Asticcacaulis biprosthecium DSM 4723T (98·5 %) and Asticcacaulis excentricus DSM 4724T (95·0 %), but could be differentiated from these two species on the basis of the number and position of prosthecae, assimilation of sugars, nitrate reduction and tolerance to NaCl. Levels of DNA–DNA relatedness of strain T3-B7T to A. biprosthecium DSM 4723T and A. excentricus DSM 4724T were 37·1 and 18·0 %, respectively. The G+C content of strain T3-B7T was 59 mol% (T m). It is concluded that strain T3-B7T represents a novel species of the genus Asticcacaulis, for which the name of Asticcacaulis taihuensis sp. nov. is proposed. The type strain is T3-B7T (=AS 1.3431T=JCM 12463T).

A new bacterial isolate from a methylamine enrichment culture is described, representing a novel species of facultatively methylotrophic bacteria. The non-motile bacterium is Gram-negative, replicates by budding and does not form endospores. The isolate utilizes methylated amines, as well as a variety of monosaccharides, disaccharides, amino acids, organic acids, aromatic compounds and alcohols as substrates, but does not utilize methanol. Growth factors are not required, although yeast extract stimulates growth. The major components of the fatty acid profile are C18 : 1 ω7c, C19 : 0 cyclo and C16 : 0. The dominant cellular phospholipids are phosphatidyl acid, phosphatidylcholine and phosphatidylethanolamine. The G+C content of the DNA is 65·7±0·3 mol%. 16S rRNA gene-based phylogenetic analysis revealed that the novel isolate belongs to the α-Proteobacteria and is closely related to the only representative of the genus Labrys, Labrys monachus (97·4 % sequence similarity). However, the level of DNA–DNA relatedness with L. monachus is less than 3 %, justifying the placement of this isolate into a novel species of the genus Labrys. The name Labrys methylaminiphilus sp. nov. is proposed (type strain JLW10T=ATCC BAA-1080T=DSM 16812T).

Earthworms emit nitrous oxide (N2O) via the activity of bacteria in their gut. Four N2O-producing facultative aerobes, ED1T, ED5T, MH21T and MH72, were isolated from the gut of the earthworm Aporrectodea caliginosa. The isolates produced N2O under conditions that simulated the microenvironment of the earthworm gut. ED1T and ED5T were Gram-negative, motile rods that carried out complete denitrification (i.e. the reduction of nitrate to N2) and contained membranous c-type cytochromes. ED1T grew optimally at 30 °C and pH 7. ED1T oxidized organic acids and reduced (per)chlorate, sulfate, nitrate and nitrite. The closest phylogenetic relative of ED1T was Dechloromonas agitata. ED5T grew optimally at 25 °C and pH 7. ED5T grew mainly on sugars, and nitrate and nitrite were used as alternative electron acceptors. The closest phylogenetic relatives of ED5T were Flavobacterium johnsoniae and Flavobacterium flevense. MH21T and MH72 were motile, spore-forming, rod-shaped bacteria with a three-layered cell wall. Sugars supported the growth of MH21T and MH72. Cells of MH21T grew in chains, were linked by connecting filaments and contained membranous b-type cytochromes. MH21T grew optimally at 30–35 °C and pH 7·7, grew by fermentation and reduced low amounts of nitrite to N2O. The closest phylogenetic relatives of MH21T were Paenibacillus borealis and Paenibacillus chibensis. Based on morphological, physiological and phylogenetic characteristics, ED1T (=DSM 15892T=ATCC BAA-841T), ED5T (=DSM 15936T=ATCC BAA-842T) and MH21T (=DSM 15890T=ATCC BAA-844T) are proposed as type strains of the novel species Dechloromonas denitrificans sp. nov., Flavobacterium denitrificans sp. nov. and Paenibacillus anaericanus sp. nov., respectively. MH72 is considered a new strain of Paenibacillus terrae.

A Gram-negative bacterium, designated LA31BT, was isolated from water collected from a hypersaline lagoon on Laysan Atoll in the north-western Hawaiian Islands. Single cells of LA31BT were slightly curved but became helical as their length increased. Preliminary characterization based on 16S rRNA gene sequence analysis showed that LA31BT shared 96·0 % identity with an Arcobacter sp. isolated from a cyanobacterial mat in hypersaline Lake Sinai, and 94 % identity with Arcobacter nitrofigilis, the type species of the genus Arcobacter. A polyphasic taxonomic study was conducted and confirmed the phylogenetic affiliation of strain LA31BT to the genus Arcobacter. However, LA31BT was found to be distinct from all recognized Arcobacter species, by a comprehensive biochemical test analysis, whole-cell fatty acid profiling, DNA G+C content (35 mol% in LA31BT) and degree of DNA–DNA reassociation. Most notably, LA31BT was found to be an obligate halophile, a hitherto undescribed feature among recognized Arcobacter species. These data indicate that LA31BT should be considered to represent a novel species in the genus Arcobacter, for which the name Arcobacter halophilus sp. nov. is proposed. This is the first obligately halophilic member of the genus. The type strain is LA31BT (=ATCC BAA-1022T=CIP 108450T).

Two obligate anaerobes, TB8106T and WZH410, which degraded propionate in syntrophic association with methanogens, were isolated from two upflow anaerobic sludge blanket reactors, one treating brewery wastewater and the other bean curd wastewater. The strains were Gram-negative, non-spore-forming and non-motile. Cells were egg-shaped, with a size of 1·0–1·3×1·8–2·2 μm. Growth was observed at 20–48 °C and pH 6·2–8·8. Both strains converted propionate to acetate and methane in co-culture with methanogens, and grew on propionate plus sulfate in pure culture, with a doubling time of 52–55 h at 37 °C. Sulfate and thiosulfate both served as electron acceptors for propionate degradation. The DNA G+C contents of the two strains were 58·5 and 58·7 mol%, respectively. Phylogenetic analysis based on 16S rRNA gene sequences revealed that the strains were closely related to a propionate-oxidizing syntrophic bacterium, Syntrophobacter fumaroxidans DSM 10017T (94·7 % similarity). However, the novel strains could not ferment fumarate, and grew at a more alkaline pH range than Syntrophobacter fumaroxidans. Moreover, the novel isolates had obviously higher growth rates on propionate plus sulfate (0·12 day−1) than Syntrophobacter fumaroxidans DSM 10017T (0·024 day−1). Therefore, a novel species, Syntrophobacter sulfatireducens sp. nov., is proposed, with strain TB8106T (=AS 1.5016T=DSM 16706T) as the type strain.

The genus Marinobacter was described with a single species, Marinobacter hydrocarbonoclasticus Gauthier et al. 1992 , to place a hydrocarbon-degrading marine bacterium belonging to the γ-subclass of the Proteobacteria. Marinobacter aquaeolei Nguyen et al. 1999 , the second described species of the genus, was proposed for a strain isolated from an oil-producing well on an offshore platform in southern Vietnam, on the basis of differences in the whole-cell protein pattern and lipopolysaccharide composition, although its phenotypic and genotypic characteristics were very similar to those of the type species, M. hydrocarbonoclasticus. In the present paper, literature data concerning the two species were reviewed. Fatty acid composition, G+C content and DNA–DNA hybridization studies were performed in order to clarify the taxonomic positions of these two species. Based on the results obtained in this study and phenotypic and phylogenetic traits available from the literature, it is proposed that the two species should be united under the same name; according to the rules of priority, M. hydrocarbonoclasticus, the first described species, is the earlier synonym and M. aquaeolei is the later synonym.