Effect of irradiance on growth, photosynthesis, pigment content and nutrient consumption in dense cultures of Rhodomonas salina (Wislouch) (Cryptophyceae)

The effects of different incident irradiances on growth, photosynthesis, pigment content and nutrient uptake were studied in dense cultures of the cryptophyte Rhodomonas salina (Wislouch) Hill y Wetherbee. Cells were grown for ten days at different incident irradiances: 11, 33, 75, 260 and 320 µmol photon m-2s-1. The average pigment content (phycoerythrin and chlorophyll a) was lower when cells were grown at high irradiances (260 and 320 µmol photon m-2s-1) compared with cells grown under low irradiances (11, 33 and 75 µmol photon m-2s-1).The maximum photosynthetic rate (Pmax) was maximal at 75 µmol photon m-2s-1 , decreasing considerably at the highest irradiance in which apparent photosynthetic efficiency per cell and per chlorophyll a unit (αcell and αchla) also decreased. Results showed that dense cultures of R. salina grew optimally in a range between 75 and 260 µmol photon m-2s-1 with optimal photosynthetic performance at 75 µmol photon m-2s-1. Nitrogen and phosphorus depletion occurred as a consequence of the exponential growth, except under the lowest and the highest irradiances (11 and 320 µmol photon m-2s-1, respectively). Associated to N-limitation, phycoerythrin (PE) and chlorophyll a decreased considerably. It is suggested that R.salina responds to nitrogen-depleted conditions by mobilizing nitrogen from PE, this behaviour being modelled by a relationship between external nitrogen availability and PE mobilization.