Protist diversity along a salinity gradient in a coastal lagoon

The importance of microbial eukaryotes to aquatic systems has been widely acknowledged in the last decade, and the application of high-throughput sequencing techniques has revealed an astonishing diversity and high proportions of novel taxa. Most studies have focused either on marine or freshwater ecosystems; thus, information on estuarine communities is either incomplete or missing. We assessed the composition of microbial eukaryotes along a South Australian coastal lagoon affected by a broad (7 to 65 PSU) salinity gradient, the Coorong Lagoon. This lagoon extends for over 170 km from the mouth of the River Murray (Murray Mouth) southwards, where the salinity increases up to hypersaline values. We sampled 5 stations during the austral summer and winter and sequenced the amplified V4 region of the 18S rRNA gene using Ion Torrent. Genetic libraries were mostly represented by reads from 5 phyla, with Chlorophyta prevailing in summer, diatoms in winter and Haptophyta in the southernmost sampling sites. In spite of the broad spatial and temporal salinity changes observed, the communities of small eukaryotes clustered in 2 groups reflecting the sample location. Moreover, dissimilarities between samples were unaffected by differences in salinity, but increased with increasing geographic distances. Microbial exchanges from the Coorong Lagoon towards both freshwater and seawater occur via the Murray Mouth and are likely to prevent the formation of communities adapted to local salinity conditions. However, such exchanges likely decrease with increasing distances from the Murray Mouth, resulting in distance-driven eukaryotic communities.