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Table 1 Analysis of hemolymph buffer curves and determination of PCO2 from pH.

From: Physiological responses of Daphnia pulex to acid stress

Group SID
(meq L-1)
CA (mM) pK'A rmse
pH 7.8 acclimation 21.1 ± 0.2 2.3 ± 0.3 8.18 ± 0.11 0.003
  24.9 ± 0.3 5.2 ± 0.3   
  23.9 ± 0.2 3.2 ± 0.3   
pH 6.0 acclimation 20.9 ± 0.8 10.4 ± 0.9 7.92 ± 0.11 0.011
  25.0 ± 1.2 14.4 ± 1.1   
  17.1 ± 0.5 4.4 ± 0.7   
pH 5.5 acclimation 13.6 ± 0.4 7.4 ± 0.4 7.46 ± 0.06 0.006
  9.2 ± 0.2 4.4 ± 0.2   
  14.6 ± 0.4 7.4 ± 0.4   
  1. Buffer curves were obtained by measuring the pH of hemolymph samples in dependence on CO2 partial pressure (PCO2). The three buffer curves of each acclimation group were simultaneously fitted by the binary buffer model (equation 1), using pK'A as a shared parameter. This means that the value of this parameter was forced to be the same for all three buffer curves. Given are the best-fit parameter values (mean ± standard error) for the strong ion difference (SID), the concentration (CA) and pK'A value of the non-bicarbonate buffer, and the the standard error of the fit (rmse, root mean squared error). The number for the degrees of freedom (i.e. the number of data points minus the number of fitted parameters) was 8. The reverse determination of PCO2 from pH is exemplified as follows. Given the in vivo pH of 8.334, the three calibration buffer curves of the pH 7.8 acclimation group yielded PCO2 values of 0.519 ± 0.005 kPa, 0.573 ± 0.006 kPa, and 0.575 ± 0.005 kPa. These individual means were finally averaged to yield an overall mean of 0.556 kPa with a standard error of 0.018 kPa. This standard error therefore reflects the variability among the buffer curves.