Effects of acidity on the growth of two Euglena species

Abstract

Our study separates the effects of elevated protons (at pH <3) and elevated 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 species were acid tolerant, growing optimally at pH 2.5–7. At pH values typical of AMD (pH 2.5–4) in the absence of elevated metals, E. gracilis outcompeted E. mutabilis (growth rates of 1.0 and 0.8 div d−1, respectively). Relative metal toxicities were evaluated based on the Effective Exposure causing 50% growth reduction (= EE50). With total metal additions similar to AMD levels, E. mutabilis demonstrated significantly greater tolerance to all metals, except Cu. E. gracilis showed two-fold higher tolerance to Cu2+ than E. mutabilis (EE50 of 91.6 vs. 45.7 pmol cell−1). The EE50 for Zn2+ was 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 Cd2+ cell−1; EE50 of 942 vs. 87 pmol Ni2+ cell−1). Al and Fe were tolerated at high total metal concentrations (up to 100 mM) by E. mutabilis, but toxicity was evident with E. gracilisat much lower levels. E. mutabilis grew 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 levels between E. mutabilis and E. gracilis with EE50s of 8773 and 502 pmol Fe2+ cell−1, respectively. We conclude that differential metal tolerance, particularly to Fe2+, accounts for the mutually exclusive distribution of E. gracilis and E. mutabilis in AMD-impacted habitats.