Plankton biodiversity experimental responses to temperature and salinity variations

We used mesocosm experiments and Next-Generation Sequencing (NGS; Illumina MiSeq Sequencing) to study the response of the natural Baltic and Mediterranean unicellular eukaryotic plankton communities to a heat shock and subsequent salinity changes. Firstly, we applied the heat shock and then we used the resultant control (kept in ambient temperature of each study site) and heated communities to study how changes in salinity affect them. Our results demonstrate strong effects on OTU richness, community taxonomic and functional composition and rRNA:rDNA ratio of the plankton unicellular eukaryotes. We observed loss of OTUs together with significant compositional changes after the heat shock. At the same time increased rRNA:rDNA ratios for heterotrophic related OTUs were observed. Very few shared OTUs were found across the salinity treatments and reduced OTU richness under high and low salinity in the Baltic experiment and under low salinity in the Mediterranean experiment. We also found high variability on the response of rRNA:rDNA ratios after salinity changes among the taxonomic and trophic groups. The difference in the estimated resistance against salinity changes for the unheated and the heated communities between the two experiments highlighted the importance of the diverse species co-tolerance.