Microbial responses to PAH in Mediterranean and Antarctic coastal waters
Large quantities of polycyclic aromatic hydrocarbons (PAH) and other semivolatile aromatic-like compounds (SALCs) are atmospherically deposited onto the global ocean. Microbial degradation is a key sink that modulates the occurrence and fate of these airborne pollutants in the upper ocean. However, little is known on the geographical variability of this process under realistic field conditions. Microbial communities from two contrasting habitats at coastal sites in the Mediterranean and the Maritime Antarctica, were incubated with environmentally relevant concentrations of PAHs for 48h in order to assess both PAH degradation and the impacts of PAHs on microbial communities at the compositional and functional levels. A minimal response was observed in Antarctic waters, either in terms of PAH degradation or changes in microbial composition. In contrast, Mediterranean bacteria readily degraded the less hydrophobic PAHs, with rates averaging 4.72 ± 0.5 ng L h-1. Metatranscriptomic analyses of Mediterranean communities after three hours of incubation showed significant enrichments of genes associated to horizontal gene transfer, stress response and PAH degradation, mainly harboured by Alphaproteobacteria. In contrast, 16S amplicon sequencing analyses revealed an increase of Bacteroidota and Flavobacteriales after exposure to PAHs.  Considering the large differences in pollution levels, nutrients availability and temperatures between the two sites, these results provide experimental evidence of the regional-dependent divergences in biodegradation modulating PAH concentrations and the effects of PAH on microbial communities in the global upper ocean.