Influence of zeaxanthin on quantum yield of photosynthesis of Synechococcus clone WH7803 (DC2)

Abstract

Synechococcus clone WH7803 (DC2) was grown on a 12:12 h light-dark cycle of either blue-green fluorescent, white fluorescent or daylight-filtered tungsten light. Integrated irradiance for each culture was set at 15 .mu.Ein m-2 s-1. Subsequent measurements of absorption, pigmentation and carbon action spectra were used to examine wavelength-dependence of photosynthetic quantum yield. Comparison of directly-measured and reconstructed absorption spectra suggests that ''pigment packaging'' effects are minimal in Synechococcus. Spectral quality had a marked effect on pigmentation and quantum yield. Cellular concentrations of chlorophyll a, .beta.-carotene and phycoerythrin were all .apprx. 2-fold lower in daylight-grown Synechococcus relative to blue-green and white light-grown cells; ratios of .beta.-carotene- and phycoerythrin-to-chlorophyll a were markedly constant for all 3 illuminations. Blue-green light grown Synechococcus cells had a .apprx. 2-fold higher zeaxanthin content than those grown under white light or daylight illuminations. These results indicate that cellular zeaxanthin content is not an implict constant and its concentration is dependent on irradiance levels of blue-green light. Zeaxanthin probably serves an important function as a photoprotectant pigment in Synechococcus, and as such, can also produce significant decreases (20 to 40%) in the apparent quantum yield for photosynthesis in the blue-green region of the visible spectrum. In contrast, highest quantum yields were rontinely measured between 525 and 650 nm suggesting that light absorbed by phycobilins (phycoerythrin and phycocyanin) drive the majority of carbon fixation in DC2-like coccoid cyanobacteria. In calculating the spectral quantum yield for natural phytoplankton populations, it is suggested that (1) carbon action spectra be determined under ''enhanced'' conditions and (2) photosynthetically absorbed radiation for phytoplankton be estimated using spectral reconstruction techniques where absorption contributions by non-photosynthetic chromophores are removed from whole cell absorption signatures.