Photosynthetic and growth responses of three freshwater algae to phosphorus limitation and daylength

1. Three common species of freshwater phytoplankton, the diatom Nitzschia sp., green alga Sphaerocystis schroeteri and cyanobacterium Phormidium luridum, were grown under contrasting daylengths [18 : 6 h light : dark cycles (LD) versus 6 : 18 h LD] and phosphorus (P) regimes (P-sufficient versus 1 lM P). The rates of growth and photosynthesis, as well as growth efficiencies and pigment concentrations, were compared among treatments. 2. The growth and photosynthetic parameters of the three species depended on both P status and daylength in a species-specific way. The responses to P limitation depended on daylength and, conversely, the responses to daylength depended on P status. 3. Growth rates and the maximum rates of photosynthesis (Pmax) of all species decreased under P limitation under both light regimes. However, the decrease of Pmax because of P limitation was greater under long daylength. The Pmax of the green alga S. schroeteri decreased the most (ca. sixfold) under P limitation compared with the other two species. The photosynthesis saturation parameter Ik also decreased under P limitation; the decline was significant in Nitzschia and Sphaerocystis. P-limitation significantly increased photoinhibition (b) in Nitzschia and Sphaerocystis, but not in Phormidium. The excess photochemical capacity (the ratio of the maximum photosynthesis rate to the photosynthesis rate at the growth irradiance), characterising the ability to utilise fluctuating light, was significantly lower under P limitation. 4. The growth efficiency (growth rate normalised to daylength) declined with increasing daylength in all species. Under short daylength the cyanobacterium Phormidium had the lowest growth efficiency of the three species. 5. The cellular chlorophyll a concentration in both Nitzschia and Sphaerocystis was significantly higher under short daylength, but only under P-sufficient conditions. In Nitzschia, under short daylength, P-limitation significantly decreased cellular chlorophyll concentration. In contrast, P-limitation increased cellular chlorophyll concentration in Sphaerocystis, but under long daylength only. The ratio of chlorophyll a to b in the green alga also declined under short daylength and under P-limited conditions.