Differential impacts of organic and inorganic phosphorus on the growth and phosphorus utilization of Microcystis aeruginosa.

Phytoplankton growth in freshwater is often limited by the availability of phosphorus (P), and thorough understandings of P availability are essential to prevent algal blooms. However, the differences in bioavailability and utilization mechanisms of different P forms remain unclear, especially whether organophosphorus could be used as P sources. This study investigated the effects of 0.5, 1.0, and 2.0 mg/L P on Microcystis aeruginosa, including dissolved organic P (DOP) (1-hydroxyethane 1,1-diphosphonic acid) and dissolved inorganic P (DIP) (dipotassium phosphate). Compared with DIP, intracellular P content absorbed in DOP treatment was significantly lower. DOP was more conducive to the synthesis of soluble protein and the release of extracellular polymeric substances. Alkaline phosphatase activity was generally enhanced in response to DIP deficiency. Both DIP and DOP promoted carbon uptake to the same extent. DOP groups absorbed carbon to synthesize energy and proteins in response to stress, while DIP groups were mainly used carbon for growth. They all reduced the content of microcystin releasing into the aquatic environment and therefore reduced ecological risk caused by microcystin. Compared with DIP, the expressions of photosynthesis-related genes were significantly down-regulated in DOP group, while the expressions of nucleoside phosphate catabolism, P transporter, and amino acid biosynthesis and metabolism were significantly up-regulated in response to P deficiency environment and the stress of 1.0 mg/L DOP concentration. In summary, the bioavailability of different P forms on cyanobacteria is different, so it is not sufficient to only use total P for assessing environmental risk. P forms should also be considered for risk management of freshwater ecosystems.