Nitrogen (N) and phosphorus (P) limitation affect the photosynthetic apparatus of Dunaliella tertiolecta in markedly different ways. When grown at 0·25 d−1 (18 % of the resource-saturated maximum rate, μmax=1·39 d−1) in chemostat cultures, N- and P-limited cells were chlorotic relative to nutrient-replete controls. The lutein-to-chlorophyll a ratio increased under both N and P limitation, whereas the neoxanthin-to-chlorophyll a ratio increased only under P limitation. The ratio of accessory photoprotective pigments (α- and β-carotene) to chlorophyll a increased under N-limited conditions. Despite differences in accessory pigment complement, chlorophyll a-specific light absorption coefficients of N- and P-limited cultures did not differ significantly, and were greater than in nutrient-replete conditions. In contrast, the initial slope of the photosynthesis–irradiance (PE) response curve (αChl) declined under nutrient-limiting conditions. There were slight reductions in the maximum quantum efficiency of photosynthesis (ϕm) in N- and P-limited cells. Reductions in ϕm were accompanied by reductions in the ratio of variable to maximum fluorescence (Fv/Fm), and the ratio of the photosystem II reaction centre protein D1 to the large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Differences in light-saturated gross photosynthesis rate, as measured by light/dark oxygen exchange, could be accounted for by changes in the abundance of the carboxylating enzyme Rubisco. Oxygen exchange and 14CO2 assimilation appeared to measure different processes in P-limited and N- limited cultures. At light-saturation, 14C-bicarbonate assimilation approximated gross photosynthesis (as measured by light/dark oxygen exchange) in P-limited cultures. In contrast, 14C-bicarbonate assimilation approximated net photosynthesis in N-limited cultures. When all culture conditions were compared, 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.