Effect of pH on Photosynthesis of Euglena mutabilis Schmitz, an Acidophilic Benthic Flagellate

The dependence of the photosynthetic rate of Euglena mutabilis Schmitz on pH across a range of 2.0–10.0 was investigated. Populations of E. mutabilis isolated from sediments of acidic mine drainage in Sensui (pH = 3.95) and a volcanic cold spring in Bougatsuru (pH = 5.32) were cultured in a pH-adjusted growth media for 96 h, and photosynthetic rate and dark respiration rate were measured. The maximum gross photosynthetic rate of E. mutabilis cells from the Sensui drainage population did not vary significantly over the pH range of 2.0–7.0, and their dark respiration rate showed high values at pH = 7.0. The maximum gross photosynthetic rate of E. mutabilis cells from the Bougatsuru spring population did not vary significantly within the pH range of 2.0–6.0, and their dark respiration rate tended to show high values at pH = 7.0. E. mutabilis can colonize under circumneutral conditions up to and including pH = 6.0.

Ubiquity of Euglena mutabilis Population in Three Ecologically Distinct Acidic Habitats in Southwestern Japan

Three strains of Euglena mutabilis were isolated from sediments in acidic inland water systems (pH = 3.4–4.7), in Southwestern Japan—acid mine drainage in Sensui (Fukuoka), cold sulfidic spring in Bougatsuru (Oita), and a temporal pool in the Ebinokogen volcanic area (Miyazaki). All strains grew well in acidic media at pH 3.07. Phylogenetic analysis among these three strains showed high similarities to plastid SSU and nuclear SSU rRNA gene sequences (99.86% and 99.76%, respectively). They were closely related to the cultured isolates from other highly acidic habitats (pH = 2.0–5.9). Concentration of sulfate, aluminum, calcium, and iron had 7–70 fold of differences among the three studied habitats. Our results imply that the rRNA genes of E. mutabilis have compensated for their low genetic diversity by adapting to a wide pH range, as well as various water chemistry of habitats.

Mechanisms of inorganic carbon acquisition in two Euglena species

The mechanism of inorganic carbon uptake was examined in Euglena gracilis Klebs. and the acidophilic species Euglena mutabilis Schmitz. Both species, whether grown in acidic (pH 3.5) or alkaline (pH 7.5) media lack external carbonic anhydrase. Acid-grown E. gracilis was shown to have no capacity for bicarbonate transport, but transport was induced on transfer to alkaline medium (pH 7.5) in the light over a period of 8 h. In contrast, acid-grown E. mutabilis appears to have no capacity for bicarbonate transport even at neutral pH. The overall internal pH of the cells was determined by equilibration with 14C-labelled benzoic acid over the pH range 3.5–5.0 and with 14C-labelled 5,5-dimethyloxazolidine-2,4-dione over the range pH 5.5–7.5. The acidophilic species maintains an internal pH range of 6.6–6.8 in an external pH range of 3.5–5.5, whereas the acid-tolerant species E. gracilis maintains a neutral internal pH in an external pH range of 3.5–7.5. Measurement, by mass spectrometry, of the fluxes of CO2 and O2 in photosynthesizing cells at pH 3.5 demonstrated a rapid uptake of CO2 by both species that was completely blocked by iodoacetamide, an inhibitor of CO2 fixation. Uptake of CO2 by E. gracilis, grown at pH 7.5, was not completely inhibited by iodoacetamide and O2 evolution was sustained when the cells reached the CO2 compensation concentration, indicating a direct uptake of bicarbonate. These data indicate that the acidophilic species, E. mutabilis, takes up CO2 by diffusion, whereas the acid-tolerant species, E. gracilis, takes up CO2 by diffusion at acid pH levels but has some capacity for active bicarbonate uptake when grown at alkaline pH levels.Key words: acidophilic alga, acidotolerant alga, bicarbonate uptake, CO2 uptake, Euglena gracilis, Euglena mutabilis, internal pH.