Rhodolith metagenome diversity shifts across the Great Amazon System

Rhodolith-forming coralline algae in the Great Amazon Reef System (GARS) occur under strong light and redox gradients imposed by the Amazon River plume. We tested whether a conserved microbial and metabolic core persists across sectors while functions reorganize with local conditions. We conducted shotgun metagenomics on rhodolith holobionts collected in the South, Central, and North sectors and profiled taxonomic composition and pathway markers (KEGG/SEED; METABOLIC). Bacteria dominated the holobiont, with Proteobacteria, Chloroflexi, and Bacteroidetes prevailing, and Thaumarchaeota as the main archaeal lineage. Functional profiles showed structured not random variation among sectors. In the South, high water transparency supported oxygenic phototrophy (psa/psb, rbcL/S; phycobiliproteins) and stronger coupling between carbon fixation and respiration. The Central sector displayed a transitional configuration combining oxygenic and anoxygenic phototrophy (pufL/M; bch genes) with co-occurring nitrification-denitrification (amoA, nxrAB, nirK, nosZ), indicating tight NS cycling. The North was enriched in sulfur redox pathways linked to suboxic microzones, with sulfate-reducing and sulfur-oxidizing lineages and contributions from methanogenic archaea. Across sectors, high diversity and functional redundancy likely underpin holobiont persistence in mesophotic settings. Our results indicate a resilient, sector-specific reorganization of rhodolith-associated microbiomes along plume-driven gradients, with implications for biogenic calcification and biogeochemical stability under climate change and ocean acidification scenarios.