Table 2. Overrepresented COG groups in the MC metagenomes relative to the CT metagenomes, based on odds ratios (OR) calculated between the copy number of putative gene sequences in the MC and CT metagenomes.

COGa	COG description	Class	Class description	MC	CT	ORMC/CT
Metabolism
0243*	Anaerobic dehydrogenases, typically selenocysteine-containing	C	Energy production and conversion	361	97	2.0
5013*	Nitrate reductase alpha subunit	C	Energy production and conversion	110	6	9.9
1362	Aspartyl aminopeptidase	E	Amino acid transport and metabolism	94	1	50.6
3696*	Putative silver efflux pump	P	Inorganic ion transport and metabolism	938	254	2.0
Information storage and processing
0583*	Transcriptional regulator	K	Transcription	888	182	2.6
Cellular processes and signaling
1291	Flagellar motor component	N	Cell motility	128	10	6.9
0643	Chemotaxis protein histidine kinase andrelated kinases	NT	Cell motility/Signal transduction mechanisms	348	64	2.9
0840	Methyl-accepting chemotaxis protein	NT	Cell motility/Signal transduction mechanisms	442	43	5.5
2804	Type II secretory pathway, ATPase PulE/Tfppilus assembly pathway, ATPase PilB	NU	Cell motility/Intracellular trafficking, secretion,and vesicular transport	551	166	1.9
3419	Tfp pilus assembly protein, tip-associatedadhesin PilY1	NU	Cell motility/Intracellular trafficking, secretion,and vesicular transport	113	11	5.5
5008	Tfp pilus assembly protein, ATPase PilU	NU	Cell motility/Intracellular trafficking, secretion,and vesicular transport	256	21	6.6
0625	Glutathione S-transferase	O	Posttranslational modification, protein turnover, chaperones	352	92	2.1
1391	Glutamine synthetase adenylyltransferase	OT	Posttranslational modification, protein turnover, chaperones/Signal transduction mechanisms	238	46	2.8
0642*	Signal transduction histidine kinase	T	Signal transduction mechanisms	962	343	1.5
0664*	cAMP-binding proteins - catabolite geneactivator and regulatory subunit ofcAMP-dependent protein kinases	T	Signal transduction mechanisms	367	95	2.1
0841	Cation/multidrug efflux pump	V	Defense mechanisms	1693	512	1.8
1566	Multidrug resistance efflux pump	V	Defense mechanisms	376	104	1.9

^aOnly those COG groups that were discussed in the present study or have been reported previously (labeled with asterisks) are shown. A full list is provided in Table S3.