Community genomic analyses constrain the distribution of metabolic traits across the Chloroflexi phylum and indicate roles in sediment carbon cycling

A core of sediment pulled from an aquifer in Rifle, Colorado — grey, anoxic, smelling of sulfur. A microbiologist holds it and asks: what is actually living in here, and what is it doing? The stakes are real. Sediments lock up more organic carbon than any other terrestrial ecosystem, and the microbes inside control whether that carbon stays buried or cycles back through the food web and eventually into the atmosphere. For one of the most abundant bacterial groups in those sediments, the Chloroflexi, scientists had almost no genomic data. Just a name and a sequence tag from a ribosomal gene survey. Hug and colleagues set out to fill that gap. The scale of the problem is worth considering. At the time of this study, public databases held complete genomes for only six of the roughly thirty Chloroflexi classes. The one class anyone understood well was the Dehalococcoidia — organisms that make a living by breathing halogenated solvents, the chlorinated compounds left behind by industrial contamination. Beyond that exception, almost nothing was known about what the rest of the phylum was doing metabolically. Sixteen S ribosomal ribonucleic acid surveys — the standard method for taking a microbial census — kept finding Chloroflexi everywhere, in marine sediments, freshwater sediments, and aquifer sediments. But a census tells you who showed up, not what they eat or how they breathe. Hug and colleagues needed genomes. The approach is called metagenomics: instead of trying to grow microbes in a lab, you extract all the deoxyribonucleic acid from an environmental sample and sequence it directly. The team drilled a sediment core from well D04 at the Rifle Integrated Field Research Challenge site, pulling material from 4, 5, and 6 meters depth. They sampled under anaerobic conditions, kept everything frozen, and performed ten independent deoxyribonucleic acid extractions per depth from roughly 100 grams of sediment equivalent. Illumina HiSeq sequencing produced staggering read counts — nearly 500 million reads from the 5-meter sample alone. To get a high-resolution picture of who was there, the team used a clever anchor. Rather than relying solely on 16S ribosomal ribonucleic acid genes, they hunted for the ribosomal protein S3 gene — rpS3 — and then looked for scaffolds that carried a conserved block of 16 ribosomal protein genes around it. Eighty of their 86 Chloroflexi sequences had more than half of that 16-gene block, which allowed the team to build a concatenated protein phylogeny with resolution comparable to a 16S ribosomal ribonucleic acid tree — but directly from metagenomic fragments, without needing a complete genome first. The result: 86 distinct Chloroflexi genotypes from 15 separate lineages, including members of eight classes previously known only from 16S tags. Chloroflexi turned out to be the second most represented bacterial phylum in the dataset, comprising about 14 percent of the bacterial community across depth samples. The detection limit for this approach was impressive — organisms present at as little as 0.02 percent abundance showed up. That diversity result alone reframes the phylum. But the real story is in three near-complete genomes the team reconstructed — RBG-2, RBG-9, and RBG-1351 — each estimated at over 90 percent complete based on single-copy marker analysis. Three genomes, three radically different lifestyles, all pulled from the same column of mud. RBG-9 is the largest, at roughly 3.8 million base pairs with 3,596 protein-coding genes, and it falls within the Anaerolinea class. Its genome encodes a full aerobic respiratory chain: a 14-subunit NADH dehydrogenase, succinate dehydrogenase, two cytochrome complexes, a caa3-type cytochrome c oxidase, and an adenosine triphosphate synthase. It also carries a remarkable suite of c-type cytochromes, including one with 24 heme groups. The prediction is that RBG-9 can respire sugars and amino acids using oxygen — a surprise in sediment that is predominantly anoxic. Its highest abundance is at 5 meters depth, which fits with a zone where oxygen or electron acceptors might occasionally penetrate. RBG-2 and RBG-1351 tell a different story — and a more unexpected one. Both are anaerobes, both encode the complete Wood-Ljungdahl pathway, and neither carries a standard electron transport chain. The Wood-Ljungdahl pathway is worth explaining: it is a two-branch biochemical route that stitches carbon dioxide into a compound called acetyl-CoA, the central metabolic currency that cells use to build biomass or generate energy. Organisms running this pathway can grow on carbon dioxide and hydrogen alone. Hug and colleagues note explicitly that this is the first documentation of the Wood-Ljungdahl pathway in Chloroflexi — a major metabolic capability no one had attributed to this phylum before. RBG-2, at around 2.3 million base pairs, is predicted to be a homoacetogen — an organism that ferments sugars and plant-derived compounds, channeling the products through to acetate, and uses the Wood-Ljungdahl pathway to fix carbon dioxide in the process. Its genome encodes glycolysis, beta-oxidation for fatty acids, multiple ferredoxin-dependent oxidoreductases, and an adenosine diphosphate-dependent acetyl-CoA synthetase for producing acetate while capturing adenosine triphosphate by substrate-level phosphorylation. It also carries a group four membrane-bound nickel-iron hydrogenase, which the authors propose could generate a proton gradient to drive an archaeal-type adenosine triphosphate synthase — and that adenosine triphosphate synthase origin is one of the study's stranger findings. RBG-1351, at 1.5 million base pairs, is similarly predicted to be a fermenter and acetogen, also encoding a complete Wood-Ljungdahl pathway, beta-oxidation, and the acetyl-CoA synthetase. It differs from RBG-2 in its hydrogenase — it carries a group one membrane-bound nickel-iron hydrogenase typically associated with respiratory hydrogen oxidation, though the canonical electron partners are absent, leaving its in vivo function unresolved. Phylogenetically, RBG-1351 sits sister to the Dehalococcoidia — it is among the closest known relatives of the organohalide respirers, yet it apparently does something entirely different. Both RBG-2 and RBG-1351 are most abundant at 4 meters depth, consistent with the anoxic, carbon-rich zone where fermentation would dominate. Now for the genuinely strange part. Both RBG-1351 and RBG-2 carry genes apparently borrowed from Archaea. In RBG-1351, there is an operon encoding the majority of an archaeal flagellar apparatus — motor proteins FlaH, FlaI, and FlaJ, assembly proteins FlaF and FlaG, and three flagellins. This is archaeal-type motility machinery, encoded as fla genes, distinct from the flg-type flagellar genes found in bacteria. Paired-end read mapping confirmed this operon sits on a scaffold that is otherwise Chloroflexi in character. Whether RBG-1351 can actually make a functional flagellum is uncertain — the preflagellin peptidase FlaK is absent — but the genes are unambiguously there and unambiguously archaeal. In RBG-2, the adenosine triphosphate synthase the team identified has crenarchaeotal origin, meaning it clusters with the synthases of a major archaeal lineage rather than with bacterial ones. Horizontal gene transfer across the bacterial-archaeal boundary is rare, and finding it for core machinery like motility and energy generation suggests these organisms have had deep, intimate associations with archaeal communities in the subsurface. The study also dismantles a simpler narrative about Chloroflexi. Of 82 predicted reductive dehalogenase genes found across the entire sediment dataset, only five were on Chloroflexi scaffolds — and those five cluster near a protein from the sulfate reducer Desulfobacula toluolica, not within the Chloroflexi lineages. RBG-2 does carry a haloalkane dehalogenase, but it is an aerobic enzyme, not a respiratory one. The conclusion Hug and colleagues draw is direct: organohalide respiration does not appear to be a major metabolic lifestyle for these sediment Chloroflexi. The Dehalococcoidia are the exception, not the template for the phylum. What emerges from all of this is a revised picture of carbon cycling in the shallow subsurface. Chloroflexi in these aquifer sediments appear to be primary processors of complex organic matter — breaking down plant-derived compounds, sugars, and fatty acids — and funneling the products, particularly acetate, to downstream consumers like methanogens and metal-respiring bacteria. The Wood-Ljungdahl pathway in RBG-2 and RBG-1351 adds another dimension: these organisms are potentially net carbon fixers in an environment most people picture as purely decomposing. They take the carbon dioxide that fermentation produces and pull some of it back into biomass. Eighty-six genomes from fifteen lineages, recovered without growing a single organism in a flask. That number captures what metagenomics makes possible — and what it revealed here was a phylum hiding in plain sight, flagged for years by 16S surveys, finally legible. This lecture was created by ennepō. Go to https://ennepo.ai to Discover, Create and Follow the latest research in your field. Read when you can. Listen when you want to.