THE SIGNIFICANCE OF HYDROGEN ION CONCENTRATION IN THE BIOLOGY OF EUGLENA GRACILIS KLEBS

Abstract

1. The alleged tolerance of high concentrations of citric acid by Euglena gracilis Klebs is associated with H⁺-ion concentration. This species of Euglena tolerates H⁺-ion concentrations as high (or higher) when produced by hydrochloric, sulphuric or oxalic acids. 2. Even less acid solutions of valeric and salicylic acids cause death, however. This effect may be connected with the greater rates of penetration of these acids into living cells. 3. Increased growth of Euglena gracilis at high acidities is due to the elimination of bacterial or other competition at such pH values. It does not occur in a series of sterile cultures. 4. A sterile, pure-line, well-aerated culture of Euglena gracilis, in Bacto-peptone medium, showed little difference in division-rate between pH 7.7 and 4.5 in the light, and between pH 7.7 and 3.0 in darkness. An only slightly greater rate of division at about pH 6.7 suggests that a true optimum may exist at that point. The absolute limits of life proved to be approximately pH 2.3 and 11.0+. 5. There is a marked optimum pH for photosynthesis in this Euglena. Sealed cultures in the light invariably developed maximum growth at pH 6.5 to 6.8. Comparison with aërated cultures in darkness shows that the difference between growth of sealed cultures in light and in darkness is actually not due to the products of photosynthesis in the former, but to the use of the oxygen produced during photosynthesis in the oxidation of organic foods in the medium. 6. Quantitatively, enough oxygen is produced at pH 6.7, in photosynthesis by this Euglena to maintain its own reproduction at maximum or nearly maximum rate. Euglena may, therefore, be a significant element of the carbon cycle where it occurs in nature. Under conditions of nearly complete anaërobosis in darkness it does not grow, but encysts, and subsequently dies. 7. In cultures of about pH 3.5 or lower, many individuals fail to complete normal division, remaining attached together by the surface at the posterior tip (the last part to divide in euglenoid fission). Two, three, four and even as many as six individuals have been found attached together, the higher numbers of individuals forming rosettes. This condition depends on pH, as such, occurring equally in light and darkness, in sealed and aërated cultures. Furthermore, it is apparently only a surface effect, as, although occasionally other types of abnormalities appear, in the type here described there is no connection between the cytoplasm of one individual and that of another. 8. More or less complete etiolation occurs, even under continuous illumination, below pH 5 ±. This is not permanent, the chlorophyl reappearing after the organisms are returned to a medium of lower H⁺-ion concentration. 9. In cultures of low pH (below about pH 3.5) a photodynamic effect is present. This is far more marked with green than with etiolated individuals, which suggests that the contained chlorophyl, or a derivative of it, is the photodynamic compound involved. 10. Encystment does not occur as an effect of pH anywhere in the range studied, pH 8.5 to 2.4. 11. The so-called "euglenoid movement" is not induced by any particular pH, but may be brought about by transferring the organisms from the culture in which they have been living to one in which the pH is markedly different. This type of movement should not be considered a form of locomotion, but, rather, a modified avoiding reaction.