Fig. 1. Instantaneous growth rate of Chlamydomonas sp. (filled squares, solid line) and Chlamydomonas reinhardtii (open squares, broken line) at different pH values, measured as change in chlorophyll fluorescence per time elapsed; see Materials and methods. The acidophilic Chlamydomonas sp. does not grow at pH=1.0 or 7.5. Growth at pH 2 is not significantly different from growth at pH 3, 4, 5.5 or 6.0 (t-tests, P>0.2 for all). C. reinhardtii does not grow at pH 2 and has lower growth at pH 3 and 4 but is growing faster than Chlamydomonas sp. at pH 6.0. Values are means ± S.E.M. (N=3).