Subsurface Bacillota_B specialization and genome expansion

Deep-clay Bacillota_B differ from soil-baseline Bacillota_B in accessory gene content, genome size, and selected respiratory markers, but interpretation depends on phylogeny and whether genomes represent porewater-associated or rock-attached communities. _{1 2} Genome specialization therefore does not necessarily imply genome reduction: in the reported deep-clay comparison, cultivable Bacillota_B lineages had larger genomes and more orthologous groups (OGs, clusters of evolutionarily related proteins) than soil-baseline lineages, consistent with a self-sufficiency model rather than streamlining. ₁

The cultured deep-clay cohort matched the Bagnoud Mont Terri porewater paradigm rather than demonstrating representation of the rock-attached community. ₂ This finding supports pangenome-integration by showing that within-lineage comparisons can reveal accessory expansion, but also refines the interpretation of subsurface specialization through cohort size, phylogenetic structure, and compartment bias. ₁

The global soil analysis refines the scale at which this specialization should be interpreted. Across 5,441 soil samples, measured stressors did not predict functional gene counts out of sample; the clay-shield test found low-clay cross-validation R² = −0.268 versus high-clay R² = −0.292, difference 0.024, 95% CI: −0.423, 0.161. ₃ Because this confidence interval includes zero, the global result does not support higher predictability in high-clay soils. It does not contradict the deep-clay Bacillota_B genome comparison: the global analysis concerns community-level functional counts and measured stressors, whereas the Bacillota_B studies compare selected genomes and respiratory modules within a lineage and sampled compartment. It does, however, refine the specialization claim by showing that local genome-content differentiation should not be generalized to a globally predictable clay effect. ₃

The SSO subsurface community analysis supports an environmental context for this specialization: hydrogeological zone explained 27.5% of sediment-community variance (F = 4.05, p = 0.0001), and Firmicutes were enriched with depth (Spearman ρ = +0.76). However, this evidence comes from 16S community composition and taxonomy-to-trait inference, not direct Bacillota_B genome measurements; it therefore refines rather than independently confirms the accessory-genome mechanism. ₄

Cohorts and comparative design

The genome-content comparison used 10 deep-clay and 62 soil-baseline Bacillota_B genomes, with eggNOG OG enrichment, genome-content statistics, and corrected multi-heme cytochrome detection. ₁ The deep-clay cohort came from Mont Terri Opalinus borehole and rock-porewater samples plus two Russian Beyelii Yar borehole genomes. The soil-baseline cohort included phylum-matched soil or sediment genomes across Syntrophomonadales, Desulfitobacteriales, Moorellales, Thermacetogeniales, Ammonifexales, Carboxydocellales, Desulfotomaculales, Heliobacteriales, and Thermincolales. ₁

The compartment-focused comparison analyzed 9 deep-confined genomes, 30 shallow-clay genomes, and a phylum-stratified soil baseline of 150 genomes, with 137 remaining after the reported quality filter. ₂ The deep cohort contained 8 Mont Terri Opalinus borehole genomes and 1 bentonite-formation genome; the shallow cohort contained 8 Coalvale silty-clay, 1 Cerrado clay, and 21 agricultural-clay genomes. ₂ Compartment annotations were inferred from isolation-source keywords, so bentonite or “rock” entries could represent porewater or rock-attached material. ₂

The Bacillota_B universe contained 334 genomes in the KBase Data Lakehouse pangenome, versus the v1.1 plan’s estimate of 6,700. ₁ The 10 anchor genomes were clumped among 3 BRH-c8a genomes, 2 BRH-c4a genomes, 2 Desulfosporosinus genomes, Desulforudis, Ch130, and 1 other genome. ₁ The available comparison therefore does not establish that genome expansion is a general property of all subsurface Bacillota_B, and effects supported by anchor counts of 3–5 are primarily descriptive. ₁

The analysis used Firmicutes-level OGs where available and bacteria/root fallbacks otherwise because a Bacillota_B-specific eggNOG tier was unavailable; this heterogeneous OG hierarchy may affect functional resolution. ₁ The anchor cohort was borehole- and porewater-dominated by cultivation bias and could not test whether rock-attached Bacillota_B differ in gene content. ₁ Some enriched OGs may therefore be lineage markers rather than recurrent subsurface-specialization features, linking this interpretation to phylogenetic-confounding-of-pangenome-associations. ₁ Without a per-genus analysis, the enriched OGs cannot be confidently split into genus-specific lineage markers and a signature that recurs across multiple subsurface-specialist genera. With n=10 anchors, such a split is at the limit of statistical resolution. ₁

Accessory functions and anaerobic specialization

The 10-versus-62 comparison tested 14,109 Firmicutes-level eggNOG OGs with per-OG Fisher’s exact tests, Benjamini–Hochberg false-discovery-rate (FDR) correction, a fold-difference threshold of ≥3, and presence in at least 3 anchor genomes. ₁ It found 547 significantly enriched OGs at q<0.05, exceeding the preregistered H1 prediction of ≥10 and providing strong evidence for broad accessory enrichment. ₁ The same screen found 27 anchor-depleted OGs. The top enriched hits had OR=∞, with 7–10 anchor genomes positive versus 0 baseline genomes. ₁

Keyword scanning classified 42 enriched OGs as anaerobic-respiration functions, 24 as sporulation-revival functions, 12 as mineral-attachment or exopolysaccharide functions, 4 as anaerobic regulators, and 3 as osmoadaptation functions; 462 were “other or unannotated.” ₁ Examples included hydrogenase, cytochrome, sulfite, sulfate, nitrate, and fumarate reductases; NADH dehydrogenase, oxidoreductase, menaquinone, spore and germination, biofilm and capsule, sigma-factor, two-component-system, betaine, ectoine, osmoprotectant, and K⁺-uptake functions. ₁

Manual inspection refines this classification. The keyword scanner missed anaerobic-niche signal in the 462-OG “other or unannotated” category, so that category undercounted anaerobic-respiration and electron-transfer functions. The report estimates that manual reclassification would raise the H1-relevant total from the keyword count of 42 to closer to 80–100 of 547 enriched OGs. This is an estimate, not a finalized count. ₁ Selected examples were COG1977, associated with molybdopterin cofactor metabolism, in 10/10 anchors versus 11/62 baselines. The report links this OG to anaerobic respiration because molybdopterin is the cofactor for anaerobic-respiration enzymes such as nitrate reductase, formate dehydrogenase and DMSO reductase; OG 1UIFM, a DsrE/DsrF/DsrH-like family, in 7/10 versus 0/62, which the report reads as extending the clay project’s sulfate-reduction finding to the intracellular sulfite-handling component of the dsr pathway; OG 1VFGN, a 4Fe–4S dicluster associated with 2-oxoglutarate:ferredoxin oxidoreductase, in 8/10 versus 4/62; and OG 1V0WU, a Major Facilitator Superfamily transporter (KEGG K08177), in 9/10 versus 5/62. The report proposes an osmoprotectant or solute-uptake role for OG 1V0WU, but that role is not established. ₁

These enrichments support the hypothesis that expanded deep-clay genomes may encode broader anaerobic persistence and resource-use capacity, including redox flexibility, mineral association, dormancy or revival, regulation, and osmotic stress tolerance. Because the anchor cohort is small and phylogenetically clustered, some OGs may be lineage markers rather than general subsurface-specialization features. ₁ The report also compares these categories with published descriptions of single organisms. These comparisons show agreement with the literature but are not independent tests. Pelosinus HCF1 is cited as an aquifer Firmicute carrying 2 [NiFe] and 4 [FeFe] hydrogenases, a dissimilatory N-oxide reductase, a Cr/Fe reductase and a methylmalonyl-CoA pathway. Desulfosporosinus is cited as spore-forming with broad respiratory versatility. The report states that 3 anchor Desulfosporosinus genomes contributed to the sporulation-OG signal, but its own anchor-composition list gives 2 Desulfosporosinus genomes. This discrepancy within the report is left unresolved here. ₁

The compartment analysis supports an anaerobic-persistence interpretation but refines its phylogenetic and ecological basis. Its anaerobic toolkit combined Wood–Ljungdahl (WL), group 1 [NiFe]-hydrogenase (NiFe), and dissimilatory sulfate-reduction (SR) modules. Mean toolkit scores were 1.889 for anchor_deep (n = 9), 0.033 for anchor_shallow (n = 30), and 0.393 for soil_baseline (n = 140); the proportions containing all three modules were 0.556, 0.000, and 0.021. ₂ The report’s summary gives these as mean toolkit score 1.89 vs 0.39 vs 0.03 of 3 modules for the deep, soil-baseline and shallow-clay cohorts. Later phylogenetic control narrows this habitat-specific signal. The figure h2_toolkit_by_cohort.png shows the stacked-bar toolkit score (0–3) and per-module presence rates by cohort. ₂ The central discoveries digest reports the same unadjusted cohort-level contrast as deep 1.89 versus baseline 0.39. Before phylogenetic control, that contrast looks like a strong deep-clay-versus-soil signal. ₅

The H2 analysis used Fisher tests, a Spearman trend test, and within-phylum Fisher tests on toolkit markers. Against soil baseline, Fisher tests with BH-FDR gave the following results ₂:

• WL: 5/9 deep versus 15/140 baseline genomes, OR = 10.4, p_BH = 0.004. This cohort-level enrichment did not survive within-phylum control. ₂
• NiFe: 7/9 versus 35/140, OR = 10.5, p_BH = 0.004. This cohort-level enrichment also did not survive within-phylum control. ₂
• SR: 5/9 versus 5/140, OR = 33.8, p_BH = 2.5×10⁻⁴. ₂
• Nitrogenase (Nif-complete markers): 4/9 versus 18/140, OR = 5.4, p_BH = 0.035. ₂

The same analysis reported iron reduction (IR) at OR = 0.46, p_BH = 0.69. That IR result used the misidentified original markers, and the revised clay report withdraws the IR-side interpretation (see the respiratory-marker correction below). ₂ Within Bacillota_B, WL occurred in 5/5 deep versus 15/19 baseline genomes, raw p = 0.54 (BH-adjusted 1.0 as reported in the within-phylum table); NiFe occurred in 5/5 versus 14/19, also raw p = 0.54 (BH-adjusted 1.0 as reported); and SR occurred in 5/5 versus 4/19, OR = ∞, p = 0.003, p_BH = 0.044. ₂ The report’s prose gives the same SR result as p_BH=0.04, while its within-phylum table gives 0.044. Both values are kept here as reported. ₂ Thus WL and NiFe primarily tracked Bacillota_B lineage, whereas SR remained associated with the deep cohort after within-phylum control. Of the three modules, only sulfate reduction survived this phylogenetic control. ₂

This within-phylum result refines the cohort-level toolkit enrichment because it exposes a phylogenetic confound. The Bacillota_B phylum contains Desulfosporosinus, BRH-c4a and BRH-c8a, the lineages that dominate the deep anchor cohort. Even in soil samples, the phylum carries WL and [NiFe]-hydrogenase at high background rates, with a Bacillota_B baseline mean toolkit score of 1.65. _2,5 The discoveries digest restates the within-Bacillota_B result. WL (5/5 vs 15/19, p=0.54) and NiFe-hydrogenase (5/5 vs 14/19, p=0.54) are not enriched in deep-clay isolates beyond their phylum congeners. Only sulfate reduction (5/5 vs 4/19, p_BH=0.04) survives the phylogenetic control. The digest draws a methodological warning from this: an apparent habitat signal at cohort level may be a phylum-level signature picked up by biased habitat sampling. That warning links the result to phylogenetic-confounding-of-pangenome-associations. ₅

The strongest compartment-specific result was SR. The deep cohort comprised 9 genomes traceable to clay-confined deep-subsurface biosamples: 8 from Mont Terri Opalinus boreholes and 1 from a bentonite formation. Of these, 5/9 (56%) carried dissimilatory sulfate-reduction markers, versus 1/9 iron-reduction-marker-positive genomes under the original analysis. The comparison null was the Mitzscherling (2023) rock-attached distribution (SRB ~0.2%, IRB ~7% of community). Against it, 5 observed positives versus 0.018 expected of 9 yielded binomial p = 4.0×10⁻¹²; the corresponding iron-reduction comparison was 1 observed versus 0.63 expected, p = 0.87. ₂ This supports a porewater-associated sulfate-reduction signature in the cultured deep cohort, not a complete census of rock-attached metabolism. ₂ The H3 pairwise Fisher tests on SR completeness gave two results. For anchor_deep versus anchor_shallow, OR = ∞ and p = 2×10⁻⁴. For anchor_deep versus soil_baseline, OR = 33.8 and p = 5×10⁻⁵. The second value is a pairwise H3 Fisher p and is reported separately from the BH-adjusted H2 value of p_BH = 2.5×10⁻⁴. ₂ The discoveries digest also restates the result: the anchor_deep clay cohort is 5/9 SR-positive, with binomial p=4×10⁻¹² against the rock-attached null. ₅

The 8 Opalinus genomes traced to BRC-3 or BIC-A1 borehole sources. The report names the deep-cohort genera as follows ₂:

• Desulfosporosinus (×2).
• BRH-c8a (×2), called Peptococcaceae c8a in Bagnoud’s nomenclature.
• BRH-c4a (Desulfotomaculales).
• Lutibacter and BRH-c54 (Bacteroidota).
• Roseovarius (Rhodobacterales).
• Stenotrophomonas, which is the bentonite isolate rather than an Opalinus genome. ₂

The report describes these as the lineages Bagnoud (2016) identified as recurrent across 7 Mont Terri boreholes. Rock-attached Geobacter and Geothrix lineages and CPR/DPANN episymbionts were essentially absent, limiting inference about the complete rock-attached community. ₂

The report attributes to Bagnoud a minimalistic Opalinus food web. In it, Desulfobulbaceae c16a expressed the complete Wood–Ljungdahl pathway, group 1 [NiFe]-hydrogenase, and Sat–AprAB–DsrAB. Three metagenome-assembled genomes (MAGs, genomes reconstructed from metagenomic reads) recurred across seven independent boreholes. ₂ Among the 9 anchor_deep genomes, 5 carried the Sat–AprAB–DsrAB dissimilatory sulfate-reduction module and 7 carried group 1 [NiFe]-hydrogenase markers. These included direct hits on the BRH-c8a and Desulfosporosinus lineages. ₂ This supports gene-content correspondence with the Bagnoud toolkit. However, protein-level expression of the toolkit modules comes from Bagnoud (2016), as the report attributes it. The genomes analyzed here establish that the modules are present, not that they are expressed. ₂

The SSO data refine the environmental interpretation without testing these genomic claims. Inferred denitrification, iron oxidation, nitrification, iron reduction, sulfur oxidation, methanotrophy, and fermentation varied spatially, but genus-level functional annotation covered only 21% of reads and represented literature-linked taxonomic inference rather than direct genomic evidence. ₄ Its inferred redox ladder and plume model are hypotheses awaiting geochemical and metagenomic validation, not evidence that Bacillota_B OGs are expressed or selected in situ. ₄

Genome expansion and biosynthetic interpretation

Deep-clay anchor genomes had mean genome sizes of 4,110,038 bp versus 3,046,124 bp in soil baseline, Cohen’s d=+1.39, Mann–Whitney p=0.025. CheckM-rescaled sizes were 4,323,230 bp versus 3,233,715 bp, d=+1.37, p=0.013. ₁ Mean eggNOG OG counts were 2,630 versus 2,106, d=+1.30, p=0.022; CheckM-rescaled counts were 2,771 versus 2,233, d=+1.32, p=0.009. ₁ Mean GC content was 48.84% versus 47.76%, d=+0.21, p=0.44. ₁ Mean CheckM completeness was 94.7% versus 94.3%, d=+0.08, p=0.93. ₁

Because genome quality was similar while raw and rescaled genome size and OG counts favored anchors, the report concludes that the larger genomes and higher OG counts cannot be explained by a quality artifact. ₁ All four size and OG-count metrics indicated significantly larger genomes, with p≤0.025 and d≥+1.30, rejecting H2’s prediction of smaller deep-clay genomes. ₁ The report summarizes the difference as a 35% mean size difference and ~25% more eggNOG OGs per genome, the opposite of the compactness predicted by H2. ₁ The report also states that anchor genomes are ~1 Mbp larger than baseline. ₁ The central discoveries digest reports the CheckM-rescaled comparison as 4.3 Mbp anchor versus 3.2 Mbp baseline (Cohen’s d = +1.37, Mann–Whitney p=0.013), with the anchor 35% larger. It gives eggNOG OG counts of 2,771 vs 2,233 (d=+1.32, p=0.009) and identical CheckM completeness (94.7 vs 94.3, p=0.93). From this it concludes that the pattern is not a MAG-quality artefact. It also concludes that the Patescibacteria/CPR streamlining pattern documented by Tian et al. 2020 does not generalize to cultivable subsurface Firmicutes. ₅ This supports a self-sufficiency model for sampled cultivable subsurface Bacillota_B. It contrasts with the streamlining that Tian et al. (2020) reported for Patescibacteria/CPR, which have ≤1 Mbp genomes and an episymbiotic lifestyle. The report treats that superphylum as a distinct ecological and phylogenetic group, so the contrast is not a direct test. The report connects the expansion to the Beaver & Neufeld (2024) self-sufficiency hypothesis. It also notes that the anchor cohort includes Ca. Desulforudis audaxviator genome GB_GCA_020725505.1, which Becraft et al. (2021) described as retaining N-fixation, amino-acid biosynthesis and C-fixation. ₁

Two limits remain on interpreting the expansion. First, it is unresolved where the extra DNA lies functionally. The report proposes breaking down the “extra” gene fraction by COG (Clusters of Orthologous Groups) or KEGG-pathway membership to localize the expansion. ₁ Second, it is untested whether the H1 functional-category signal generalizes beyond Bacillota_B or is Bacillota_B-specific. The report proposes follow-up comparisons to test this. ₁

The PHB comparison refines this interpretation by showing that genome size can be a major confounder when accessory metabolic capacity is related to ecological breadth. Across 27,690 GTDB species, PHB-positive species had median genome size of 4.34 Mbp versus 2.44 Mbp for PHB-negative species, while the raw PHB–niche-breadth association changed from rho = 0.106 to partial rho = −0.047 after controlling for genome size. ₆ PHB enrichment nevertheless persisted within all four genome-size quartiles, at 1.4–4.6x enrichment with all p < 1e-11. ₆ This supports treating genome expansion as biologically relevant in the sampled Bacillota_B cohort, but refines any claim that larger genomes alone demonstrate self-sufficiency or broad environmental specialization: genome size may carry both general accessory capacity and lineage- or pathway-specific functions. ₆

The global soil analysis refines this self-sufficiency interpretation. Its negative out-of-sample R² values—Soil & Climate, R² = −0.205 ± 0.197; Geochemical, R² = −0.331 ± 0.071; and Industrial, R² = −0.221 ± 0.042—do not distinguish true biological unpredictability from spatial distributional shift, outlier leverage, or other modelling failure. ₃ They provide no independent evidence that clay-associated communities generally have expanded, predictable functional repertoires; the genome-expansion result remains a direct, cohort-specific genome comparison. This supports retaining the self-sufficiency claim for the sampled Bacillota_B cohort while limiting extrapolation to global clay soils. ₃

The compartment study does not strengthen generalized self-sufficiency. GapMind amino-acid pathway completeness across 18 pathways was 16.22/18 versus 16.66/18 before filtering (p = 0.153, d = −0.17), and 15.50/18 versus 17.14/18 after filtering (p = 0.009, d = −0.84). ₂ This is compatible with accessory expansion beyond the measured amino-acid pathways, and GapMind saturates near 18 for many cultured bacteria. ₂

The environmental depth pattern supports the possibility that subsurface structure sorts lineages but cannot distinguish genome expansion from taxonomic turnover: same-well/different-depth samples had median Bray–Curtis dissimilarity = 0.977, whereas same-depth/different-well samples had 0.835. ₄ It strengthens the case for matched depth, geochemistry, and phylogeny when testing accessory functions.

The GDI, defined as OTU Richness / (Mean Genome Completeness + 1), refines the representation context. At 1° spatial bins, forest had GDI = 902.36 and cropland had GDI = 890.82, while grassland had GDI = 503.42 and wetland had GDI = 525.13; forest and cropland were therefore jointly highest-GDI biomes. ₃ Frontier areas with GDI > 1000 had mean pH = 6.74, compared with mean pH = 5.94 in mapped areas, a +0.8 pH unit gap. ₃ Because GDI can equal 902 even when there are zero genomes and conflates richness with completeness, these results indicate a possible database-representation gap rather than direct evidence of genome expansion or subsurface specialization. ₃

Respiratory-marker correction

The original iron-reduction analysis used K07811, K17324, and K17323, but these are TMAO reductase, glycerol ABC ATP-binding, and glycerol ABC permease genes rather than canonical iron-reduction markers. _{1 2} The discoveries digest gives their KOfam thresholds as 1336, 363, and 318 respectively. It notes that KEGG has no canonical KO for the Geobacter omcS / Shewanella mtr operon multi-heme outer-surface cytochromes. With a corrected triple-signal multi-heme cytochrome detector, the digest reports anchor_deep 56% (was 11%), anchor_shallow 40% (was 50%), and soil_baseline 41% (was 20%). No corrected cohort comparison is significant (all Fisher p ≥ 0.46). The digest calls the shallow-clay rock-attached IR-rich interpretation an artefact. It states that the SR-side H3 finding remains intact, with SR markers correctly identified as K11180/K11181/K00394/K00395/K00958 and p=4×10⁻¹² versus the Mitzscherling rock-attached null. ₅ The corrected detector used PFAM PF02085, PFAM PF22678, and a CXXCH heme-binding motif count of ≥4 in gene-cluster protein sequences, calling a genome positive when any signal was present. ₁

Corrected multi-heme cytochrome rates were 5/9 anchor_deep, or 55.6%, versus 1/9, or 11.1%, under the original markers; 12/30 anchor_shallow, or 40.0%, versus 15/30, or 50.0%, originally; and 61/149 soil_baseline, or 40.9%, versus 30/149, or 20.1%, originally. ₁ All corrected pairwise comparisons were nonsignificant: deep versus shallow OR 1.88, p=0.46; deep versus baseline OR 1.80, p=0.49; shallow versus baseline OR 0.96, p=1.0. ₁ This contradicts the original shallow-enrichment narrative and removes statistical support for a shallow-versus-deep iron-reduction contrast, while retaining the sulfate-reduction signal. _{1 2} The correction is reported as 3 pairwise Fisher tests on corrected multi-heme cytochrome rates and a 5-row cohort-level table comparing original and corrected iron-reduction rates. ₁ The accessory report states that the sulfate-reduction side remains robust: the deep cohort was 5/9 SR-positive versus a Mitzscherling rock-attached null of 0.2%, binomial p=4×10⁻¹², and the SR markers K11180, K11181, K00394, K00395 and K00958 were correctly identified. It calls the iron-reduction side of the clay project’s H3 finding artefactual. When it was written, the clay-project amendment was only planned, and the report cautioned that the correction did not settle whether the original H3 narrative was fully recovered or partly undercut. ₁ The original clay H3 notebook combined two kinds of test: binomial tests against the Mitzscherling rock-attached null, and pairwise cohort Fisher tests on SR versus IR. Its output table holds two binomial tests against the Mitzscherling 2023 rock-attached reference. The notebook was therefore not a validated SR/IR comparison. ₂ The revised clay report has since added a correction notice dated 2026-05-01. The notice explicitly withdraws the IR-side narrative and states that the SR-side H3 finding stands. The porewater-bias headline therefore holds through the SR side alone, and the rock-attached-versus-porewater dichotomy is half-supported on the SR side and half-unsupported on the IR side after marker correction. ₂

Tensions and limits

The 547 enriched OGs provide strong cohort-level differentiation, but the anchor cohort spans four orders while clustering among 3 BRH-c8a, 2 BRH-c4a, 2 Desulfosporosinus, Desulforudis, Ch130, and 1 other genome. ₁ The compartment study likewise had only 9 deep genomes before filtering; within Bacillota_B, only SR remained significant, while WL and NiFe tracked lineage. ₂ Even the 5/5 Bacillota_B SR enrichment rests on an n=5 versus 19 comparison that may hide genus-level granularity: Desulfosporosinus, BRH-c8a and BRH-c4a may differ in which non-SR features they carry. ₂ These studies differ in cohort composition and analytical question, so neither establishes that every anaerobic function is independently selected by depth or compartment. ₂

The genome-expansion result supports self-sufficiency, whereas GapMind found no elevated completeness and found 15.50/18 versus 17.14/18 after filtering. _{1 2} This tension should not be averaged away: OG expansion may capture functions beyond the 18 measured amino-acid pathways, and the cultured cohort may omit extreme self-sufficient lineages represented by MAGs or single-cell genomes. ₂

The PHB evidence adds a related tension rather than resolving this issue: PHB-positive species are larger and remain environmentally enriched after genome-size stratification, but the PHB–niche-breadth effect reverses after genome-size control. ₆ Thus broad genome expansion, pathway-specific accessory enrichment, and ecological breadth are not interchangeable outcomes; the Bacillota_B cohort-specific genome comparison and the cross-clade PHB analysis address different traits and sampling frames. ₆

The global soil study adds a scale-related tension: its clay-shield test found no significant high-clay predictability advantage, with difference 0.024 and 95% CI: −0.423, 0.161, whereas the Bacillota_B comparison found large genome-size and OG-count differences in sampled deep-clay anchors. ₃ The two results address different response variables and sampling frames, so the global null cannot negate the genome comparison; neither can the cohort result establish a general clay effect. The negative global R² values also remain diagnostically incomplete because spatial autocorrelation, train/test distributional shift, high-leverage outliers, and genuine unpredictability were not separated. ₃

The compartment inference is limited by keyword-derived annotations, missing rock-attached lineages, and quality changes from 9 to 6 deep genomes, 30 to 30 shallow genomes, and 150 to 137 baseline genomes after filtering. ₂ With n = 9 before filtering, only large effects, described as Cohen’s d > 0.7 for unfiltered comparisons, are reliably detectable. ₂ The study applies to cultivable porewater-cultured deep-clay isolates, not the full Mont Terri or bentonite communities. ₂

The SSO plume, M5 mixing zone, M6 anaerobic zone, and redox ladder are inferred from 16S composition and trait scores, with direct SSO geochemistry unavailable; they cannot establish that the genomic functions are caused by those gradients. ₄ The sulfate-reduction result was robust to two stricter compartment definitions, but the cohort-level evidence remains a sampled-community result requiring MAG-augmented validation. ₂

The GDI adds a representation limitation: forest and cropland were jointly highest-GDI biomes, but forest GDI = 902.36 versus cropland GDI = 890.82 is a 1.3% difference without bootstrap confidence intervals, and the index can be driven by OTU richness or sampling effort rather than genome recovery. ₃ The +0.8 pH unit gap likewise may reflect fewer sequenced samples from alkaline pH ranges rather than an assembly or annotation barrier. ₃

The PHB cross-clade inference has a parallel limitation: its HGT calls used phylogenetic discordance and core/accessory status as proxies, with 311 potential acquisitions and 278 potential losses, and 60.1% accessory status among discordant PHB-positive species versus 32.3% among all phaC-carrying species. ₆ This supports using accessory enrichment as a hypothesis-generating signal for acquisition, but refines interpretation of the Bacillota_B accessory OGs because neither analysis directly reconstructs gene trees or separates transfer from lineage structure. ₆

Relation to other corpus concepts

This finding extends pangenome-integration by showing that within-lineage comparisons reveal accessory OG enrichment and genome expansion while exposing cohort size, phylogenetic structure, and marker-availability effects. ₁ The PHB analysis supports this perspective by linking pathway prevalence, genome size, environmental category, and core/accessory status across 27,690 GTDB species, while showing that apparent ecological breadth can be genome-size confounded. ₆ The corrected workflow refines functional-marker-validation by replacing mismatched KO markers with PFAM and CXXCH signals. ₁ The SR-rich porewater signature and lineage-dependent WL/NiFe signal connect this concept to phenotype-database-coverage-bias and metabolic-model-gapfilling. ₂

The SSO analysis supports linking this concept to environment-associated community structure but refines the evidentiary boundary: depth-associated Firmicutes and inferred redox gradients motivate genome-resolved tests, not direct claims about Bacillota_B accessory genes. ₄ The global soil analysis further refines that boundary by showing that clay-associated functional predictability is not established at global scale and that spatial structure, sampling effort, and database completeness must be separated from biological specialization. ₃

The expanded-genome interpretation also qualifies interpretations connected to within-species-conservation-between-species-functional-divergence, because observed OG enrichment co-occurs with a large genome-size difference that should be modeled when comparing functional scores. ₁ Evidence is documented in Subsurface Bacillota_B Specialization — What Distinguishes Deep-Clay Lineages from Soil Congeners?, Self-Sufficiency, Anaerobic Toolkit, and Cultivation Bias in Clay-Confined Cultured Bacterial Genomes, SSO Subsurface Community Ecology — Spatial Structure, Functional Gradients, and Hydrogeological Drivers, Soil Microbial Dark Matter and the Clay Shield Null Result, and Polyhydroxybutyrate Granule Formation Pathways — Distribution Across Clades and Environmental Selection. _{1 2 4 3 6}

Open Directions

• Partition the 547 enriched OGs by genus and phylogenetic background using genus-stratified enrichment or a phylogenetically controlled model; decompose functions into respiratory, persistence, mineral-attachment, regulatory, and osmoadaptation modules. ₁
• Reanalyze the 547 enriched OGs with an LLM-based functional scan and manual curation to resolve whether the estimated 80–100 anaerobic-respiration-related OGs form reproducible categories, including standardized reclassification of the 462 “other or unannotated” OGs. ₁
• Apply the framework to additional subsurface comparisons and test whether larger genomes and the accessory profile recur beyond the borehole- and porewater-dominated cohort. ₁
• Add rock-attached Bacillota_B genomes and repeat the pangenome comparison to test whether expansion characterizes porewater-associated isolates specifically or deep-clay Bacillota_B more broadly. ₁
• Use bacdive linkage to add clay-isolated Bacillota_B genomes, enlarging the n=10 anchor so that a genus-level split of the 547 enriched OGs has adequate statistical resolution. ₁
• Apply the corrected multi-heme cytochrome detector to the clay-project branch and compare it with the retained sulfite-reduction signal. ₁
• Test the genome-expansion/self-sufficiency tension with finer-grained amino-acid EC-number completeness from eggNOG and MAG-augmented Mont Terri, Olkiluoto, MX-80 bentonite, and Oak Ridge cohorts. ₂
• Ingest deep-subsurface MAGs from Mont Terri, Olkiluoto, MX-80 bentonite, and Oak Ridge into the pangenome. Then re-run the full H1/H2/H3 framework with the expanded cohort, so that rock-attached and uncultivated lineages are represented. That rerun includes the H2 anaerobic toolkit and sulfate-reduction detection. Any H3 iron-reduction-side rerun should use the corrected multi-heme cytochrome detector. ₂
• Compare BRC-3 porewater and BIC-A1 borehole genomes directly, with 5 + 3 genomes per borehole. This exploratory comparison could test whether the Bagnoud-paradigm SR-rich pattern is borehole-specific or site-wide within Mont Terri. ₂
• Run a within-Bacillota_B genus-level pangenome analysis that could surface a deep-clay-specific accessory genome. ₂
• Link genome presence to Bagnoud’s metaproteomic evidence to distinguish porewater cultivation bias from broader specialization. ₂
• Link genome-predicted sulfate-reduction markers to Bagnoud’s metaproteomic evidence and apply the diagnostic to other subsurface settings. ₂
• Use pathway-level reconstruction and phenotype or growth data to test whether additional genome content improves predicted self-sufficiency under anaerobic subsurface conditions rather than merely reflecting lineage-specific accessory genes. ₁
• Load the 221 SSO geochemistry samples into CORAL and test whether nitrate, pH, and metal concentrations form the predicted northeast-to-southwest gradient; pair them with metagenomics and genome-resolved Bacillota_B analysis. ₄
• Extract pump-test ASVs from Brick 460-462 and compare them with the 16S spatial pattern, especially predicted groundwater enrichment of Rhodanobacter at M5. ₄
• Re-run the soil-frontier NB05 validation with spatial blocking to separate distributional shift, outlier leverage, and genuine unpredictability in the negative R² results before using global clay predictability as a null for local Bacillota_B specialization. ₃
• Recalculate GDI after uniform 16S sequencing-depth correction, report richness and completeness separately, estimate bootstrap 95% CIs for biome rankings, and control the +0.8 pH unit gap for the number of 16S samples per pH bin. ₃
• Reanalyze Bacillota_B genome expansion alongside pathway-specific accessory traits, including PHB, with phylogenetic models and genome-size adjustment to test whether subsurface enrichment reflects broad genome capacity or repeated selection for particular functions. ₆
• Reconstruct gene trees for PHB and Bacillota_B respiratory markers, then compare transfer, loss, core/accessory status, and compartment representation to distinguish horizontal acquisition from lineage-associated retention. ₆