Use of cavity ring‐down spectrometry to quantify 13C‐primary productivity in oligotrophic waters

Cavity ring-down spectroscopy (CRDS) is a highly sensitive laser technique that allows the analysis of isotopic signals and absolute concentration of individual molecular species in small-volume samples. Here, we describe a protocol to quantify photosynthetic C-13-uptake rates of marine phytoplankton by using the CRDS technique (C-13-CRDS-PP). We validated our method by comparing the C-13-PP rates measured between CRDS and isotope ratio mass spectrometry (IRMS) in samples with different carbon content (30-160 mu gC). The comparison revealed that C-13-CRDS-PP rates were highly correlated with those obtained by IRMS (Spearman correlation coefficient, rho = 0.95, p < 0.0001, n = 15), with a mean difference between the two estimates of +/- 0.08 mgC m(-3) h(-1). Moreover, the slope of the relationship between CRDS and IRMS results was not significantly different from 1 (F = 0.03, p = 0.86), and the intercept did not differ from 0 (F = 1.4, p = 0.24), indicating that there was no bias in the CRDS relative to the IRMS-based measurements. A separate analysis also showed that despite the difference in volume and carbon content between samples (40 +/- 10 mu gC and 160 +/- 40 mu gC, respectively), the C-13-CRDS-PP technique provides similar results (Mann-Whitney test, U = 30.5, p = 0.90, n = 8). In addition, C-13-CRDS-PP rates measured along the Red Sea (similar to 176 mgC m(-2) d(-1)) agreed with C-14-based PP rates previously reported for similar locations. Thus, this study evidenced that the C-13-CRDS-PP method is sensitive enough to quantify carbon fixation rates in oligotrophic regions.