Characterization of protistan plankton diversity in ancient salt evaporation ponds located in a volcanic crater on the island Sal, Cape Verde.

Salinity is an important factor when exploring the limits known for life. Therefore, hypersaline systems have attracted much attention in recent years. In this study, we investigated the protistan diversity and community composition in two natural salt evaporation ponds (27-30% salinity) located in an ancient volcanic crater on the Cape Verde island Sal using high-throughput DNA sequencing. Our study revealed a broad range of protistan taxa and a high taxonomic diversity within the Ciliophora, Dinophyceae, and Chlorophyta. We detected a total of 23 Dinophyceae families, although Dinophyceae were generally considered to be only this diverse in aquatic environments of less than 10% salinity. Moreover, we uncovered a high degree of genetic novelty in this habitat. The mean similarity of all detected OTUs to previously described sequences was only 93.6%. These findings strongly dispute the traditional view that extreme hypersaline environments generally maintain low protistan diversity. A meta-analysis covering our and previously published data from other inland and coastal salt ponds clearly showed that our samples clustered according to salinity and not biogeography. This result further supports the claim that salinity is a major transition boundary for protistan communities, regardless of their biogeographic origin.