Effects of pH on growth, photosynthesis, respiration, and copper tolerance of three Scenedesmus strains

Abstract

Our objective was to investigate the sensitivity of growth to acid pH, and to compare acid-sensitivity of growth, photosynthesis, and respiration of copper-tolerant (B-4, X-Cu) and copper-intolerant (X-72) strains of Scenedesmus acutus f. alternans. Further, we wanted to assess the effects of pH on copper toxicity. We hypothesized that B-4 and X-Cu will be more acid tolerant than X-72, and that growth will be more acid-sensitive than photosynthesis and respiration. Optimal growth of the three strains occurred at circumneutral pHs where growth rates were similar. Growth rates responded differently to decreasing pHs. In B-4, the most acid-tolerant strain, growth was reduced by 50% at pH 4.42. X-Cu was less tolerant since it did not grow at pH 4.5, and showed 50% growth reduction at pH 4.81. X-72 was not acid-tolerant. Growth was reduced by 50% at pH 5.05, and no growth occurred at pH 4.8 or less. Photosynthesis was less inhibited by acid pHs than growth but the strain ranking was the same: B-4 was most acid-tolerant, followed by X-Cu and X-72. A reduction in respiration with declining pH was observed in X-72 and B-4 but not in X-Cu, where respiration was unaffected by pH. CO 2 compensation points for photosynthesis in all three strains showed a general decrease with pH of growth, and were lower in B-4 and X-Cu than in X-72. The inhibitory effect of copper on photosynthesis in X-72 and B-4 was reduced at lower pHs; copper toxicity in X-Cu was unaffected by pH. These findings suggest that algae such as X-Cu with a metal-exclusion mechanism can also exclude protons. In such algae, high H + concentration in the medium does not ameliorate copper toxicity. In contrast, in non-tolerant species (e.g. X-72) and in species that accumulate copper (e.g. B-4) high H + concentrations in the range suitable for growth ameliorate copper toxicity, possibly by competing with the metal for binding sites on the cell surface thereby decreasing copper uptake.