Responses of the photosynthetic apparatus of Dunaliella tertiolecta (Chlorophyceae) to nitrogen and phosphorus limitation

Nitrogen (N) and phosphorus (P) limitation affect the photosynthetic appara tus of Dunaliella tertiolecta in markedly different ways. When grown at 0 . 25 d(-1) (18% of the resource-saturated maximum rate, mu(max) = 1 . 39 d(- 1)) in chemostat cultures, N- and P-limited cells were chlorotic relative t o nutrient-replete controls. The lutein-to-chlorophyll a ratio increased un der both N and P limitation, whereas the neoxanthin-to-chlorophyll a ratio increased only under P limitation. The ratio of accessory photoprotective p igments (alpha- and beta-carotene) to chlorophyll a increased under N-limit ed conditions. Despite differences in accessory pigment complement, chlorop hyll a-specific light absorption coefficients of N- and P-limited cultures did not differ significantly, and were greater than in nutrient-replete con ditions. In contrast, the initial slope of the photosynthesis-irradiance (P E) response curve (alpha(Chl)) declined under nutrient-limiting conditions. There were slight reductions in the maximum quantum efficiency of photosyn thesis (phi(m)) in N- and P-limited cells. Reductions in phi(m) were accomp anied by reductions in the ratio of Variable to maximum fluorescence (F-v/F -m),and the ratio of the photosystem II reaction centre protein DI to the l arge subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Differences in light-saturated gross photosynthesis rate, as measured by li ght/dark oxygen exchange, could be accounted for by changes in the abundanc e of the carboxylating enzyme Rubisco. Oxygen exchange and (CO2)-C-14 assim ilation appeared to measure different processes in P-limited and N-limited cultures. At light-saturation, C-14-bicarbonate assimilation approximated g ross photosynthesis (as measured by light/dark oxygen exchange) in P-limite d cultures. In contrast, C-14-bicarbonate assimilation approximated net pho tosynthesis in N-limited cultures. When all culture conditions were compare d, there was linear covariation of the rates of reductant supply via light absorption and photochemical charge separation with the rates of reductant demand for CO2 fixation and NO3- reduction.