Light Absorptance of Algal Films for Photosynthetic Rate Determinations

A Reflectance Absorptance Transmittance (RAT) meter, developed for routine measurements of Abt λ nm on leaves, was used to measure absorptance of algal films mounted on glass fibre filter disks for use in PAM (Pulse Amplitude Modulation) fluorometry. Absorptance readings (Abt λ nm ) are essential to quantitate the photosynthetic electron transport rate from PAM data and are also helpful if algal disks are used in an IRGA (InfraRed Gas Analyzer). Measurements were made on Synechococcus R-2 (Cyanophyceae), Acaryochloris marina (Chlorobacteria), Chlorella vulgaris (Chlorophyceae), Chaetoceros sp. (Bacillariophyceae) and Isochrysis sp. (Haptophyta) in blue (445 nm), green (525 nm) and red (625 nm) light. Plots of Abt λ vs. Chl a (mg Chl a m − 2 ) or Chl d for Acaryochloris (mg Chl d m − 2 ) of the algae followed exponential saturation curves (Abt% λ = Abt% λ, ∞ ( 1-e − k×Chl )) usually reaching asymptotes in the blue light used by blue-diode PAM machines. Synechococcus , Chlorella , Chaetoceros and Isochrysis Abt% 445nm saturates ≈ 50–100 mg Chl a m − 2 ( Synechococcus Abt %445 nm, max = 77.3 ± 0.94, ± 95% conf. limits, Chlorella Abt %445 nm, max = 77.0 ± 2.06, Chaetoceros Abt% 445 nm, max = 80.6 ± 1.87; Isochrysis 83.2 ± 8.06 mg Chl a m − 2 ); Acaryochloris above about 100 mg Chl d m − 2 . Asymptotic loading of cells was in general easily achieved in blue light in the case of Synechococcus , Chlorella and Acaryochloris but not in green, red or “white” light. It was difficult to achieve asymptotic loading for Chaetoceros and Isochrysis due to mucilaginous blockage of the filter disks.