Quantum yield,relative specific absorption and fluorescence in nitrogen-limited Chaetoceros gracilis

Decreases in cell-nitrogen quota resulted in changes in the carbon-based quantum yield of photosynthesis, the chlorophyll a-specific absorption coefficient, and in vivo fluorescence in the marine diatom Chaetoceros gracilis in laboratory experiments performed in 1983 and 1984. The three parameters were independently determined for the two spectral regions dominated by either chlorophyll a or fucoxanthin absorption. As cell-nitrogen quota decreased, the quantum yield for both pigments decreased; the specific absorption coefficient for chlorophyll a and the in vivo chlorophyll a fluorescence excited by each pigment increased. The observed increase in the in vivo fluorescence per chlorophyll a could be partially attributed to the increased specific absorption coefficient for chlorophyll a; the remainder of the fluorescence increase was related to a decline in photosystem activity. Energy transfer efficiency between light-harvesting pigments appeared to be maintained as cell-nitrogen quota decreased. The decrease in a fluorescence index (F _DCMU-F _O)/F _DCMU] with nitrogen starvation suggested a decrease in Photosystem II activity. These results imply that decreases in reaction center and/or electron-transport system activity were responsible for the decline in rates of photosynthesis under conditions of notrogen deficiency.