Insights into the water microbiome dynamics of a large tropical estuary transition

Tropical estuarine systems play several ecological roles, such as acting as a nursery for biodiversity and cycling nutrients and greenhouse gases. However, the phylogenetic and metabolic diversity of estuarine microbiomes is not completely known. Furthermore, it is unclear how seasonal patterns may affect microbial diversity in these systems. The aim of the present study was to determine the metagenomic diversity and its major drivers in a large tropical estuarine system dominated by a mangrove forest in the South Atlantic around the Paraiba do Sul River. In total, 12.16 million shotgun sequences were generated (dry season: n = 8; wet season: n = 5), and water-quality parameters were evaluated for all locations. Metagenomic sequences were distributed between two patterns: (1) the dry season, in which the families Rhodobactereaceae and Flavobactereaceae increased, and (2) the wet season, in which Moraxellaceae, Pseudomonadaceae, Pseudoalteromonadaceae were more abundant. The dry season was characterized by higher salinity, nitrogen fixation, nitrification, and photosynthetic potential. In contrast, the wet season had higher carbon dioxide (CO₂) and methane (CH₄) production and a higher abundance of methanogenic, methylotrophic, and chemoorganotrophic bacteria in samples with low salinity. These findings suggest a possible relationship with the production of greenhouse gases during the wet period.