Determination of the particulate absorption of microalgae using a point-source integrating-cavity absorption meter: verification with a photometric technique


The determination of the spectral light absorption coefficient of planktonic algae in seawater is crucial for many applications, including optical remote sensing. Common techniques are adversely affected by light-scattering effects, or by low particle concentrations in situ. Measurements with a point-source integrating-cavity absorption meter (PSICAM) can overcome both difficulties. In our study, we assessed the performance of a PSICAM for measuring the absorption coefficients of microalgae by a comparison with state-of-the-art techniques. The PSICAM was evaluated by comparative measurements of diluted algal cultures with a photometric setup, where the sample cuvette is placed in the center of an integrating sphere (CIS). The accuracy of both the CIS and the PSICAM technique was calibrated against a commercial spectrophotometer by measurements of dense algal cultures. The remaining sources of error for the PSICAM technique were the bleaching procedure and chlorophyll fluorescence. We propose an improved bleaching technique and introduce a correction procedure for the fluorescence effects. Applying both, the overall differences between PSICAM and CIS measurements were less than ± 1%. We further demonstrate that the accuracy of the PSICAM is at least as good as that of a commercial spectrophotometer, but its precision should be two to three orders of magnitude higher. Due to its high sensitivity and accuracy, and to the fact that sources of error can be identified and accounted for, the PSICAM has a great potential to significantly improve the determination of absorption spectra of particulate matter in natural seawater.
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