Quantitative in vivo absorption spectra of phytoplankton: Detrital absorption and comparison with fluorescence excitation spectra1

Abstract

Spectra1 absorption of phytoplankton from cultures and natural samples was measured on filters with various optical setups including collimated and diffuse irradiation and measurements ofwetted filters within an integrating sphere. In suspensions within an integrating sphere, specific absorption coefficients for laboratory cultures varied by a factor of only two.Measurements on filters yielded values dependent on filter load. Specific absorption coefficients derived from measurements of sample filters were considerably higher than values obtained from suspensions in an integrating sphere due to increased diffuseness of irradiance and to pathlength amplification by filter-particle and particle-particle interactions. Measured absorption of phytoplankton in the blue can be increased greatly by absorption of detritus, evident from absorption spectra of depigmented samples on filter. After subtracting detrital absorption, absorption spectra of phytoplankton are qualitatively similar to the corresponding quantum-corrected fluorescence excitation spectra. The detritus-corrected ratio of absorption at 440 vs. 675 nm shows average values between 1 and 1.5.Comparison with published values shows that specific in vivo absorption coefficients of phytoplankton arc mostly overestimated as a result of the methodology applied. In the blue region of the spectrum, overestimation of phytoplankton absorption in field samples is possible if detrital absorption is neglected.

Specific in vivo absorption coefficient of chlorophyll a at 675 nm1

“…This absorption was not detected in cultures and can be shown to be of particulate detrital origin (Maske and Haardt 1987).…”

“…was obtained from an aquaculture project at KieVBulk. Species composition of samples from Kiel harbor are given elsewhere (Maske and Haardt 1987).…”

“…show considerable variability (Maske and Haardt 1987). Possible explanations could be methodological artifacts, pigment composition, or the absorption characteristic of the individual cells, their optical density, and diameter (packaging or "sieve" effect: Duysens 1956).…”

“…In this context the Chl a absorption band at 675 nm is of specific interest because of its interference with fluorescence emission at the 680-685-nm peak wavelength. The variability of literature data and results of comparative measurements of absorption with other methods (Maske and Haardt 1987) led us to measure absorption with an integrating sphere. To our knowledge these results are the first absorption coefficients measured at close to natural concentrations.…”

Effects of solar ultraviolet radiation on biogeochemical dynamics in aquatic environments : report of a workshop, Marine Biological Laboratory, Woods Hole, Massachusetts, October 23-26, 1989

“…In several studies, using a number of approaches, the absorption spectra of particulate material from natural waters have been partitioned into two components: 1) the absorption by phytoplankton pigments, and 2) the absorption by what has been classified as "organic detritus" (e.g., Kiefer and SooHoo, 1982;Kishino et aL., 1984;Maske and Haardt, 1987). Spectrally, these two operational classes of organic chromophoresare quite different.…”