Airborne microorganisms in muddy rain: microbe-mineral interactions and their ecosystem impact

The Sahara Desert and the Sahel region in North Africa contribute approximately 50–70% of global atmospheric dust emissions. Microorganisms can attach to dust particles and be dispersed into exogenous environments, being subsequently deposited by gravitational sedimentation (dry deposition) or through aqueous precipitation (wet deposition) also known as muddy rain. In the present work, five muddy rain samples were collected in Granada (Spain) during different episodes in 2021–2022. The SEM-EDX study demonstrated a high content of fine clay particles which may facilitate the atmospheric transport of microorganisms. The colonization of strategic microsites and the formation of mineral aggregates might be possible mineral-bacteria interactions. According to metagenomic analysis, Pseudomonadota (64%), Bacteroidota (13%), and Bacillota (6%) were the main phyla. At the genus level, extremophiles, plant-beneficial bacteria, and others involved in soil biogeochemical cycles have been described. Fourteen cultivable microorganisms were isolated and identified by means of 16S rRNA sequencing. Members of the phyla Pseudomonadota, Bacillota, Actinomycetota and Bacteroidota have been found. Among the isolates, Stenotrophomonas rhizophila and Brevundimonas bullata potentially exert beneficial effects at the ecosystem level. In general, muddy rain facilitates the transport and dispersal of microorganisms from different environments, with a potential positive influence on soils and vegetation in terrestrial ecosystems.