Conserved rhodolith microbiomes across environmental gradients of the Great Amazon Reef

The Great Amazon Reef System (GARS) covers an estimated area of 56,000 km² off the mouth of the Amazon River. Living rhodolith holobionts are major benthic components of the GARS. However, it is unclear whether environmental conditions modulate the rhodolith microbiomes. Previous studies suggest that environmental parameters such as light, temperature, depth, and nutrients are drivers of rhodolith health. However, it is unclear whether rhodoliths from different sectors (northern, central, and southern) from the GARS have different microbiomes. We analysed metagenomes of rhodoliths (n = 10) and seawater (n = 6), obtained from the three sectors, by illumina shotgun sequencing (total read counts: 25.73 million). Suspended particulate material and isotopic composition of dissolved organic carbon (δ¹³C) indicated a strong influence of the Amazon river plume over the entire study area. However, photosynthetically active radiation at the bottom (PARb) was higher in the southern sector reefs, ranging from 10.1 to 14.3 E.m^-2 day^-1. The coralline calcareous red algae (CCA) Corallina caespitosa, Corallina officinalis, Lithophyllum cabiochiae, and Hapalidiales were present in the three sectors and in most rhodolith samples. Rhodolith microbiomes were very homogeneous across the studied area and differed significantly from seawater microbiomes. However, some subtle differences were found when comparing the rhodolith microbiomes from the northern and central sectors to the ones from the southern. Consistent with the higher light availability, two phyla were more abundant in rhodolith microbiomes from southern sites (Bacteroidetes, and Cyanobacteria). In addition, two functional categories were enhanced in southern rhodolith microbiomes (iron acquisition and metabolism, and photosynthesis). Phycobiliprotein-coding genes were also more abundant in southern locations, while the functional categories of respiration and sulfur metabolism were enhanced in northern and central rhodolith microbiomes, consistent with higher nutrient loads. The results confirm the conserved nature of rhodolith microbiomes even under pronounced environmental gradients. Subtle taxonomic and functional differences observed in rhodolith microbiomes may enable rhodoliths to thrive in changing environmental conditions.