Bacterial diversity and microbial functional responses to organic matter composition and persistent organic pollutants in deltaic lagoon sediments

Lagoons in river deltas are highly productive systems that receive high loads of organic matter and nutrients. Among the major environmental stress factors and human health-related issues, the anthropogenic contamination is of particular concern, since coastal lagoons are intensively exploited for aquaculture activities. Although microbial communities constitute the most abundant fraction of the benthic biomass, also providing valuable ecosystem services, the links between sediment quality and microbial processes were largely disregarded. In this study, we aimed to elucidate whether different levels of riverine influence could provide favourable or adverse environmental conditions to sustain microbial diversity and processes. Sediments collected from four lagoons of the Po River delta were analysed to assess biochemical composition (biopolymeric carbon, Bio-PC), target organic pollutants (polycyclic aromatic hydrocarbons, PAHs; nonylphenols; bisphenol A), and microbial community properties (bacterial community composition, prokaryotic biomass, prokaryotic carbon production rate, PCP, community respiration rate, CR, extracellular enzyme activities, EEAs). The major physical and chemical sediment properties allowed identifying two statistically distinct groups of sediments with relatively low (LI) and high (HI) riverine influence. HI sediments were characterised by higher Bio-PC and PAHs concentrations, along with relatively high PAHs potential toxicity estimated by the Toxic Equivalent concentration approach. Contrasting results were obtained by linking the occurrence of dominant bacterial taxa (i.e.,Desulfobulbaceae and Desulfobacteraceae families) to sediment contamination patterns. Notably, the increasing pollution levels were likely to positively affect the occurrence of the Desulfuromonadales Sva1033 family, whereas detrimental effects were found against the family Ectothiorhodospiraceae. In addition, the concurrent increase of PCP/CR ratio along with key EEAs, highlighted that the benthic microbial community could consistently contribute to accelerate the degradation of persistent organic pollutants, with potential implication on the sediment self-purification processes.