• horizontal transmission
• Paramecium
• salinity stress

Changes in salinity concentrations present an environmental stress especially important for the ecology of brackish microorganism. This study is focused on Paramecium, a unicellular organism (Philum Ciliophora) that lives in freshwater and brackish environments. Paramecia harbor frequently endosymbionts, which can be beneficial or harmful depending also on environmental conditions. Here I address the question if the acquirement of horizontally transmitted bacterial symbionts is more frequent under salinity stress conditions by using different strains of Paramecium either infected with endosymbionts (donor) or not infected (receiver).
First the salinity tolerance for each Paramecium strain used in this study was tested. Obtaining the growth curves of different strains at different salinity concentrations allowed me to determine weak and strong salinity stress according to the relative reduction of maximally reached Paramecium cell numbers. A reduction between 0 and 20 % was defined as no osmotic stress, between 20 and 50 % as weak and from 50 to 80 % as strong osmotic stress condition, whereas a relative reduction between 80 to 100 % represents lethal conditions. The results of the salinity tolerance test showed differences between the used species regarding to the salinity tolerance. Freshwater species, like P. biaurelia and P. caudatum, can survive and multiply at salinity concentrations in the range from 0 to 6 ‰, higher salinity concentrations seem to be lethal. On the contrary, the brackish water species P. duboscqui is able to grow at salinity concentrations in the range from 0 to 9 ‰.
Subsequently I co-cultivated donor and receiver strains in different combinations and with appropriate controls at no, weak and strong salinity stress conditions. I determined the population and infection dynamics (with FISH, Fluorescence in situ Hybridization). Paramecium populations were influenced by salinity and competition. The salinity stress conditions influence the endosymbiont’s life. Likewise the endosymbiont maintenance and transfer from donor to receiver is affected.
At freshwater conditions (0 ‰, no salinity stress) I observed no transmission and the endosymbiont’s maintenance in the donor strains. Horizontal transmission was determined for certain combinations at weak salinity stress (1.8‰). This indicates that transmission is influenced by environmental stress but depends also on the endosymbiont and receiver species. The highest prevalence of infected receiver was observed after 42 days where 50% of the receiver cells were infected. At high stress conditions (4.5‰) the donor strains lost the endosymbionts and consequently no horizontal transmission occurred.
In conclusion, salinity stress influences the endosymbionts transmission frequency and likewise its maintenance.
This is the first study testing direct exposure of Paramecium with basically no previous adaptation to various salinity stresses. Furthermore it documents its influence on the horizontal transmission and maintenance of bacterial endosymbionts.