The response of photosynthetic absorption coefficients to irradiance in culture and in tidally mixed estuarine waters

The accuracy of models for primary production and light propagation depends on correct assignment of absorption to photosynthetic pigments. The phytoplankton absorption coefficient is comprised of two components: photosynthetic and photoprotective absorption coefficients. A method based on the fluorescent excitation of chlorophyll a is used to quantify the photosynthetic absorption coefficient for phytoplankton grown in culture and sampled from Puget Sound, Washington. The difference spectrum between total phytoplankton and photosynthetic absorption should be equivalent to photoprotective absorption. For cultures, the difference spectra exhibit peaks near 460 and 490 nm and broad-band absorption between 400 and 450 nm. However, for field samples an additional pronounced peak is observed around 440 nm, similar in shape to the chlorophyll a Soret peak. If the 440-nm peak were associated with photosystem I chlorophyll a, the photosynthetic absorption coefficient will be underestimated by < 15% for these samples. Variability in both coefficients is predictable as a function of irradiance. The photosynthetic coefficient varies inversely with growth irradiance, and the photoprotective coefficient varies directly with irradiance. This direct relationship with irradiance accounts for much of the variability in the spectral shape of the total phytoplankton absorption coefficient. The ratio of the photosynthetic absorption coefficient to the total phytoplankton absorption coefficient increases as a function of decreasing irradiance for cultures and for held samples collected from stratified regions of the water column. This ratio is a photoadaptive parameter that can serve to integrate physiological response to irradiance and has the potential to provide estimates of mixed layer dynamics.