Tolerance of testate amoeba species to rising sea levels under laboratory conditions and in the South Pacific

摘要

Testate amoebae are ubiquitous unicellular eukaryotic micro-organisms. They occur in high abundances in terrestrial habitats and contribute to important ecological and biogeochemical processes like silicon cycling. Little is known about the growth of testate amoeba populations under hyperosmotic stress, as caused by salinity pulses due to coastal flooding. Given the increasing sea-levels in response to climate change we address this knowledge gap by jointly considering results from a laboratory experiment and a field study in Tuvalu (South Pacific). In the laboratory experiment, Trinema galeata cultures tolerated up to 1 g NaCl L-1 but showed a pronounced decrease in growth rate probably due to interactions with other micro-organisms. Tracheleuglypha dentata cultures had the highest growth rate at 0.5 g NaCl L-1. At higher salt concentrations (5-10 g NaCl L-1), a strongly reduced growth and rapid closing of the shell aperture with a plug occurred, which can be considered as rapid pre-encystment. Euglypha rotunda cultures had the highest growth rates at 1 g NaCl L-1. At higher salt concentrations (5-10 g NaCl L-1), a strongly reduced growth and spherical shrinking of cell plasma occurred, which is again considered as rapid pre-encystment. After seven weeks of exposure, a replacement of the 5 and 10 g NaCl L-1 culture medium by NaCl-free medium resulted in a quick recovery of T. dentata and E. rotunda cultures, but not of T. galeata cultures. Generally, cultures of T. dentata showed the highest growth rates and shortest generation times (up to 0.37/1.91 day(-1), respectively). In the field study, different flooding regimes resulted in fundamental changes of the composition of soil amoeba communities. In conclusion, terrestrial testate amoebae can tolerate at least short periods of exposure to moderate salinity caused by flooding stress due to their ability to form cysts and fast growth rates.