Geochemical controls on hydrocarbon distribution and associated bacterial assemblages in petroleum-influenced mangrove sediments, northeastern Brazil

Mangrove sediments function as efficient sinks for hydrophobic contaminants due to their fine-grained texture, elevated organic matter content, and low hydrodynamic energy. In petroleum-influenced coastal systems, identifying the geochemical controls that regulate hydrocarbon retention is essential for understanding contaminant persistence and environmental behavior. This study investigated sediment geochemistry and associated bacterial assemblages in a chronically oil-impacted mangrove system in São Francisco do Conde, northeastern Brazil. Total petroleum hydrocarbons (TPH), total organic carbon (TOC), grain-size distribution, and hydrocarbon fractions were quantified and evaluated using correlation analysis and principal component analysis (PCA). TPH concentrations varied markedly among sampling sites, with the highest values observed in the São Francisco sector. A strong and statistically significant positive correlation between TOC and TPH (r = 0.864, p = 0.012) indicates that sedimentary organic carbon exerts a primary control on hydrocarbon distribution. Although silt–clay fractions predominated across samples, textural parameters alone did not explain the observed variability. Multivariate ordination revealed a structured geochemical gradient separating sectors primarily along hydrocarbon-associated axes. High-throughput 16S rRNA gene sequencing demonstrated spatial variability in bacterial community composition consistent with the geochemical gradient. The concordance between organic carbon content, hydrocarbon accumulation, and community structuring highlights the importance of sedimentary carbon dynamics in regulating hydrocarbon fate in tropical mangrove environments.