Table 2. Highly detected transport proteins and their substrates.

Substrate	No. of eCDSsa	No. of peptidesb	No. of clustersc	No. of spectrad
Amino acids	480	345	6	775
Carboxylates	673	480	3	861
Nitrate/sulfonate/ bicarbonate	3	5	1	6
Peptides	107	111	2	173
Phosphonate	8	9	1	12
Polyamines	148	118	2	232
Porin	58	37	3	67
Proline/glycine betaine	240	149	1	294
Sugars	400	351	3	742
Unknown symporter	40	7	1	7
Taurine	121	66	1	156
Unknown	463	292	13	502

Abbreviation: eCDSs, protein-coding sequences.

^aTranslated environmental protein-coding sequences with given transport function as determined by BLASTP.

^bUnique peptide sequences detected that mapped to the identified eCDSs.

^cClusters described by Yooseph et al. (2007).

^dMass spectra that matched to peptides of the identified eCDSs.