| Summary: | Choanoflagellates are the protozoan antecedents of multicellular animals, but little is known about their response to environmental change. Recently, the arctic choanoflagellate Diaphanoeca grandis was found in a supersulfidic, anoxic fjord in Norway, suggesting that at least some choanoflagellates are highly stress tolerant. We investigated this by testing the 24 h survival of D. grandis exposed to variations in pH (7.3–8.7), temperature (4, 10, 15, 20, 25, 30°C), Po 2 (normoxia vs. anoxia), salinity (13, 19, 26, 32, 48, 64‰), H 2 O 2 , (0.01, 0.1 and 1 mM) and H 2 S (0.03, 0.3, 1%). Cell death was assessed by morphology and propidium iodide labeling. Compared to control conditions (normoxia, 10°C, 32 ppt salinity, pH 8.0), cell death was significantly increased only at the highest pH (8.7, p = 0.023), temperatures of 20°C and above (p < 0.0001), the lowest and highest salinity (13 and 64 ppt; p < 0.0001), H 2 O 2 of 0.1 mM and above (p<0.0001), and H 2 S of 0.1% and above (p=0.0002). Furthermore, 65% of D. grandis were alive after 24 h of anoxia (p= 0.0023). These results indicate that D. grandis is surprisingly tolerant of typical environmental stressors. Whether this is characteristic of all choanoflagellates is still unknown.
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