From the Atlantic to the Mediterranean: what factors shape diatom diversity in the plastisphere.

Plastic substrates are ubiquitously colonized by complex microbiomes, referred to as the plastisphere. Diatoms, despite being considered the main autotrophic taxa and key early colonizers involved in biofilms, currently remain poorly studied compared to other microbes plastic pieces were immersed in coastal systems at six locations across five countries in the Atlantic Ocean and the Mediterranean Sea. Spatial and seasonal dynamics of plastic-attached diatom communities were investigated by incubating polyethylene (HDPE) at all sites for 84 days. Temporal dynamics at each season were also investigated using HDPE, polypropylene, and polystyrene in Toulon only (North-Western Mediterranean Sea). Using amplicon sequencing of the rbcL gene, geographic location appears to be the primary driver of diatom diversity, followed by season and incubation time, with intra-site variations always smaller than those between sites. Intra-site variations were mostly due to relative abundance changes, while inter-site variations corresponded to shifts between species. Biofilms from the Atlantic shared neither species nor ASVs, whereas biofilms from the Mediterranean shared 5 species and 5% of reads. Pioneer taxa changed depending on the season, while mature taxa tended to converge irrespective of the season. Finally, the nature of conventional plastics plays a minor role in shaping the diatom community, as shown previously for heterotrophs in the plastisphere. Overall, the shaping of diatom communities in biofilms could be observed at different taxonomic levels, down to the genetic variant, highlighting the extent to which genetic divergence occurs. Finally, potential ecological consequences of diatom presence and diversity in the plastisphere are discussed.