Two-year systematic investigation reveals alterations induced on chemical and bacteriome profile of PM2.5 by African dust incursions to the Mediterranean atmosphere.

PM2.5 atmospheric samples were regularly collected between January 2013 and March 2015 at a central location of Eastern Mediterranean (Island of Crete) during African dust events (DES) and periods of absence of such episodes as controls (CS). The elemental composition and microbiome DES and CS were thoroughly investigated. Fifty-six major and trace elements were determined by inductively coupled plasma-mass spectrometry. Relative mass abundances (RMA) of major crustal elements and lanthanoids were higher in DES than in CS. Conversely in CS, RMAs were higher for most anthropogenic transition metals. Lanthanum-to-other lanthanoids concentration ratios for DES approached the corresponding reference values for continental crust and several African dust source regions, while in CS they exceeded these values. USEPA's UNMIX receptor model, applied in all PM2.5 samples, established that African dust is the dominant contributing source (by 80%) followed by road dust/fuel oil emissions (17%) in the receptor area. Potential source contribution function (PSCF) identified dust hotspots in Tunisia, Libya and Egypt. The application of 16S rRNA gene amplicon sequencing revealed high variation of bacterial composition and diversity between DES and CS samples. Proteobacteria, Actinobacteria and Bacteroides were the most dominant in both DES and CS samples, representing ~88% of the total bacterial diversity. Cutibacterium, Tumebacillus and Sphingomonas dominated the CS samples, while Rhizobium and Brevundimonas were the most prevalent genera in DES. Mutual exclusion/co-occurrence network analysis indicated that Sphingomonas and Chryseobacterium exhibited the highest degrees of mutual exclusion in CS, while in DES the corresponding species were Brevundimonas, Delftia, Rubellimicrobium, Flavobacterium, Blastococcus, and Pseudarthrobacter. Some of these microorganisms are emerging global opportunistic pathogens and an increase in human exposure to them as a result of environmental changes, is inevitable.