Contributions
Counter Ion Effects on Metal Fitness Measurements tested whether counter ions, especially chloride delivered by metal salts, confound genome-wide metal-fitness measurements 1. It found that 4,304 of 10,821 metal-important gene records (39.8%) were also NaCl-important, across 19 organisms, 14 metals and 86 organism × metal pairs 1. The project identified 71 NaCl experiments across 25 organisms 1. The report’s novel-contribution summary describes the 39.8% overlap as spanning 25 organisms while the main result specifies 19, and both denominators are recorded without reconciliation 1. Overlap occurred for every metal, with per-metal organism-level mean overlaps (distinct from the pooled gene-record percentage) ranging from 9.2% for molybdenum to 57.6% for manganese 1. Synechococcus elongatus was an outlier at 88.6% shared-stress (565/638), and the report notes that its 12 NaCl experiments made the gene-importance threshold much easier to meet than the 1–6 experiments of other organisms 1. Excluding it lowered the overlap only to 36.7% (3,739/10,183) 1. In Desulfovibrio vulgaris Hildenborough, 73 of 495 unique metal-important genes (14.7%) were shared-stress, and 90.5% of metal-specific genes had SEED annotations compared with 78.1% of shared-stress genes 1. The report reads this as suggesting that the metal-specific set includes well-characterized metal homeostasis functions, an interpretation it did not formally test 1.
The project rejected the predicted dose-dependent relationship between Cl⁻ concentration and NaCl overlap 1. Zinc sulfate, which delivers 0 mM chloride, had 44.6% overlap, higher than cobalt (41.3%), copper (41.0%) and nickel (39.3%) 1. Chloride-delivered metals averaged 41.6% overlap against 37.8% for non-chloride metals 1. In D. vulgaris Hildenborough, whole-genome Pearson correlations with NaCl ranged from zinc r=0.715 down to iron r=0.086, so the ranking did not follow chloride concentration 1. The report proposes that the ranking follows toxicity mechanism and attributes the overlap to shared stress biology, but neither explanation was directly tested 1. The report notes that NaCl is not a pure chloride control, because it also supplies sodium and osmotic stress 1. It also extrapolates, without measuring, that counter-ion effects would be proportionally smaller at the 0.05–2 mM concentrations typical of random barcode transposon sequencing (RB-TnSeq, pooled mutant fitness assays) 1.
The project re-tested the Metal Fitness Atlas, whose metal-important genes were originally 87.4% core with an odds ratio (OR) of 2.08 1. After restricting the analysis to metal-specific genes, core-genome enrichment was preserved for 12 of 14 metals, and the prediction that enrichment would weaken was rejected 1. Enrichment was reported as a delta, the difference between a metal’s core fraction and its baseline, and the narrative counts 7 metals as strengthened, for example molybdenum, whose delta rose from +0.132 to +0.145 1. The narrative reports modest decreases for aluminum (+0.099 → +0.068) and zinc (+0.145 → +0.115) and states that only cadmium reverses (-0.008 → -0.108) 1. The report’s own table conflicts with that wording, because cadmium’s delta stays negative while iron’s changes sign (-0.040 → +0.182) 1. Iron’s increased delta is also not counted among the 7 strengthened metals 1. The report calls both results statistically unreliable, because cadmium rests on n=92 genes and iron on n=9 genes, each from one organism 1.
In psRCH2, the only organism tested with both CuCl₂ and CuSO₄, the project found a cross-salt correlation of r=0.439 1. Within-replicate correlations were r=0.720 for CuCl₂ and r=0.859 for CuSO₄ 1. This comparison is severely confounded, because CuCl₂ was tested anaerobically and CuSO₄ aerobically 1. CuSO₄ correlated more strongly with NaCl than CuCl₂ did (r=0.450 versus r=0.212), despite supplying no chloride 1.