Effects of acidity on the growth of two Euglena species

Our study separates the effects of elevated protons (at pH < 3) and elevate d metals (Al, Cd, Cu, Fe, Ni, Zn) on the growth of E. mutabilis Schmitz, a pioneering phototroph in acid mine drainage (AMD) and E. gracilis Klebs, a closely-related species rarely found in severely AMD-impacted sites. Both s pecies were acid tolerant, growing optimally at pH 2.5-7. At pH values typi cal of AMD (pH 2.5-4) in the absence of elevated metals, E. gracilis outcom peted E. mutabilis (growth rates of 1.0 and 0.8 div d(-1), respectively). R elative metal toxicities were evaluated based on the Effective Exposure cau sing 50% growth reduction (= EE50). With total metal additions similar to A MD levels, E. mutabilis demonstrated significantly greater tolerance to all metals, except Cu. E. gracilis showed two-fold higher tolerance to Cu2+ th an E. mutabilis (EE50 of 91.6 vs. 45.7 pmol cell(-1)). The EE50 for Zn2+ wa s similar for both species (368 pmol cell(-1) for E. gracilis and 423 pmol cell(-1) for E. mutabilis). With Cd and Ni, E. mutabilis tolerated an order of magnitude higher exposure than E. gracilis (EE50 of 1.6 vs. 0.2 pmol Cd 2+ cell(-1); EE50 of 942 vs. 87 pmol Ni2+ cell(-1)). Al and Fe were tolerat ed at high total metal concentrations (up to 100 mM) by E. mutabilis, but t oxicity was evident with E. gracilis at much lower levels. E. mutabilis gre w at double the Al3+ exposure tolerated by E. gracilis (EE50 of 398 vs. 188 pmol Al3+ cell(-1)). There was an 18-fold difference in Fe tolerance level s between E. mutabilis and E. gracilis with EE50s of 8773 and 502 pmol Fe2 cell(-1), respectively. We conclude that differential metal tolerance, par ticularly to Fe2+, accounts for the mutually exclusive distribution of E. g racilis and E. mutabilis in AMD-impacted habitats.