How phosphorus deficiency affects physiology and morphometry of Baltic Picocyanobacteria

Inorganic phosphorus (P inorg ) starvation and limitation experiments were performed in two unicellular picoplanktic cyanobacteria strains, isolated from the Central Baltic Sea, using P inorg -free medium and a range of phosphate concentrations (1, 0.02, 0.002 μM K 2 HPO 4 ). The effects of P inorg -deprivation and P inorg -reduced availability on growth, pigment content, photosynthesis, alkaline phosphatase activity and cell morphometry were investigated. Weak differences in growth and pigment content were detected among the cultures grown at 1 and 0.002 μM phosphate in both strains. An increase of photosynthetic efficiency and alkaline phosphatase activity was observed in a phycoerythrin-rich strain at low concentration and in P ιnorg -free medium. Growth, chlorophyll a and phycobiliproteins content of the two strains decreased in response to P inorg -starvation. Morphometrical analysis of both strains showed a reduction of cell volume at the lowest concentration tested.