Table 2:

Informative Pfam domains with high H^΄ and low std; novel proposed molecular marker domains in metagenomic data of variable MSL

Pfam ID (Suli ocurrences)	H΄ mean	H΄ std	Description
PF12139 58/161	1.2	0.01	Adenosine-5΄-phosphosulfate reductase beta subunit: key protein domain for both sulfur oxidation/reduction metabolic pathways. Has been widely studied in the dissimilatory sulfate reduction metabolism. In all recognized sulfate-reducing prokaryotes, the dissimilatory process is mediated by 3 key enzymes: Sat, Apr, and Dsr. Homologous proteins are also present in the anoxygenic photolithotrophic and chemolithotrophic sulfur-oxidizing bacteria (CLSB, PSB, GSB), in different cluster organizations [35].
PF00374 135/161	1.1	0.09	Nickel-dependent hydrogenase: hydrogenases with S-cluster and selenium containing Cys-x-x-Cys motifs involved in the binding of nickel. Among the homologues of this hydrogenase domain is the alpha subunit of the sulfhydrogenase I complex of Pyrococcus furiosus that catalyzes the reduction of polysulfide to hydrogen sulfide with NADPH as the electron donor [69].
PF01747 103/161	1.03	0.06	ATP-sulfurylase: key protein domain for both sulfur oxidation and reduction processes. The enzyme catalyzes the transfer of the adenylyl group from ATP to inorganic sulfate, producing adenosine 5΄-phosphosulfate (APS) and pyrophosphate, or the reverse reaction [70].
PF02662 62/161	0.82	0.03	Methyl-viologen-reducing hydrogenase, delta subunit: is 1 of the enzymes involved in methanogenesis and encoded in the mth-flp-mvh-mrt cluster of methane genes in Methanothermobacter thermautotrophicus. No specific functions have been assigned to the delta subunit [49].
PF10418 122/161	0.78	0.06	Iron-sulfur cluster binding domain of dihydroorotate dehydrogenase B: among the homologous genes in this family are asrA and asrB from Salmonella enterica enterica serovar Typhimurium, which encode (1) a dissimilatory sulfite reductase, (2) a gamma subunit of the sulfhydrogenase I complex of Pyrococcus furiosus, and (3) a gamma subunit of the sulfhydrogenase II complex of the same organism [12].
PF13247 149/161	0.66	0.06	4Fe-4S dicluster domain: Homologues of this family include: (1) DsrO, a ferredoxin-like protein, related to the electron transfer subunits of respiratory enzymes, (2) dimethylsulfide dehydrogenase β subunit (ddhB), involved in dimethyl sulfide degradation in Rhodovulum sulfidophilum, and (3) sulfur reductase FeS subunit (sreB) of Acidianus ambivalens, involved in sulfur reduction using H2 or organic substrates as electron donors [12].
PF04358 73/161	0.52	0	DsrC like protein: DsrC is present in all organisms encoding a dsrAB sulfite reductase (sulfate/sulfite reducers or sulfur oxidizers). The physiological studies suggest that sulfate reduction rates are determined by cellular levels of this protein. The dissimilatory sulfate reduction couples the 4-electron reduction of the DsrC trisulfide to energy conservation [71]. DsrC was initially described as a subunit of DsrAB, forming a tight complex; however, it is not a subunit, but rather a protein with which DsrAB interacts. DsrC is involved in sulfur-transfer reactions; there is a disulfide bond between the 2 DsrC cysteines as a redox-active center in the sulfite reduction pathway. Moreover, DsrC is among the most highly expressed sulfur energy metabolism genes in isolated organisms and meta-transcriptomes [71].
PF01058 158/161	0.45	0.01	NADH ubiquinone oxidoreductase, 20 Kd subunit: homologous genes are found in the delta subunits of both sulfhydrogenase complexes of Pyrococcus furiosus [12].
PF01568 156/161	0.4	0.05	Molydopterin dinucleotide binding domain: this domain corresponds to the C-terminal domain IV in dimethyl sulfoxide (DMSO) reductase [49].
PF09242 39/161	0.38	0.04	Flavocytochrome c sulphide dehydrogenase, flavin-binding: enzymes found in S-oxidizing bacteria such as the purple phototrophic bacteria Chromatium vinosum [49].
PF04879 151/161	0.37	0.05	Molybdopterin oxidoreductase Fe4S4 domain: is found in a number of reductase/dehydrogenase families, which include the periplasmic nitrate reductase precursor and the formate dehydrogenase alpha chain, i.e., Wolinella succinogenes polysulfide reductase chain, Salmonella typhimurium thiosulfate reductase (gene phsA).
PF08770 45/161	0.35	0.03	Sulphur oxidation protein SoxZ: SoxZ sulfur compound chelating protein, part of the complex known as the Sox enzyme system (for sulfur oxidation) that is able to oxidize thiosulfate to sulfate with no intermediates in Paracoccus parantropus [12].