A complete-genome view of phylum Nanobdellota and recurrent Form III RuBisCO transfer between archaea and Patescibacteriota

The archaeal phylum Nanobdellota (formerly Nanoarchaeota) was previously represented by four complete genomes. We present 208 complete Nanobdellota genomes from Oxford Nanopore metagenomes of the Baltic Sea water column and Fennoscandian groundwater (69-201 m below sea level), rotated to the ORC1/Cdc6 replication origin -- a 52-fold expansion of complete-genome representation. Across the ar53 supermatrix and a Nanobdellota-tuned 71-marker supermatrix on 1,239 taxa, the named GTDB orders within Nanobdellota are recovered as monophyletic clades, including the three orders that dominate our environmental sampling: Woesearchaeales, Pacearchaeales, and the GTDB placeholder order SCGC-AAA011-G17. This is consistent with the existing GTDB R232 order-level circumscription. We retire the SCGC-AAA011-G17 placeholder name, replacing it with a complete-genome-anchored SeqCode nomenclatural chain (Maxwellarchaeales ord. nov., Maxwellarchaeaceae fam. nov., Maxwellarchaeum gen. nov., and Maxwellarchaeum balticum sp. nov.) without altering the order-level circumscription. Pacearchaeales and Maxwellarchaeales retain no central or energy metabolism beyond Form III RuBisCO, PEP synthase, and ferredoxin; Woesearchaeales retains partial glycolysis and a V/A-type ATPase. A 4,262-tip phylogeny of rbcL (the RuBisCO large-subunit gene) identifies nine candidate archaea-to-Patescibacteriota Form III RuBisCO transfer events -- including one to a Baltic Minisyncoccia -- versus two reciprocal candidates, consistent with archaea-to-CPR being the more frequently identified direction in our data. All 256 Nanobdellota genomes (208 complete + 48 high-quality non-circular), the ar71 marker set with its 1,239-taxon ML tree, 154 Nanobdellota-trained HMMs for KEGG-ortholog detection in DPANN proteomes (94 ROBUST), and the 4,262-tip rbcL reference tree are released as a community resource, alongside the full analysis archive -- alignments, intermediate trees, structural predictions, and per-step scripts -- at Zenodo (DOI 10.5281/zenodo.20174424; see Using the resource).