Biodiversity of protists in coastal areas of the Mediterranean Sea affected by nutrient pollution

Diversity of protists was researched in different coastal areas of the Mediterranean Sea by means of high throughput sequencing technologies based on the amplification of a specific region of the 18S rRNA gene. A total of 25 samples were obtained in the Mar Menor coastal lagoon (MMS) in 2016-2017 after the ecosystem suffered a so-called 'ecosystem disruptive algal bloom' which was attributed to nitrate pollution. Another set of samples (9) was harvested in 2018 at a coastal area of the northern Alboran Sea, which received urban wastewater (CS). The community structure in these two areas was compared with samples collected at stations of the Alboran Sea following a trophic gradient from coast to open sea (UP; 15 samples; OAS) located far from pollution sources. On average, Alveolata, Stramenopila and Rhizaria counted for more than 85% of the total sequence reads in the three areas although the contribution of the three groups was different. The relative abundance of Alveolata reads averaged 62% in OAS but increased notably in CS (80%) and MMS (70%). In contrast, both diversity and abundance of Rhizaria were reduced in CS (1.4% and 4.7% of reads and OTUs) and MMS (3% and 7%) compared to OAS (5.5% and 9.7%). The highest contribution of Stramenopila to total reads was obtained in OAS (20% compared to 8% and 13% in CS and MMS, respectively). a-diversity of autotrophs, dinoflagellates and heterotrophs decreased in the areas impacted by nutrient pollution. However, these was not a consistent variability pattern in b-diversity.