RESUMEN
Conventional assays to measure phosphorus in freshwater systems are sometimes not sufficient to quantify the actual bioavailable P for aquatic biota since some inorganic or organic P species may not be detected by chemical methods, and their bioavailability can be affected by a range of environmental factors. This situation could lead regulatory agencies to be unable to detect imminent ecosystem-degrading phenomena such as cyanobacterial blooms. It could also be an obstacle in studying the ecophysiological requirements of freshwater communities. P bioavailability in five rivers located in central Spain was analysed by a polyphasic approach (combinations of different marker types) based on cyanobacteria. This approach included a parallel study with the use of a self-luminescent P-cyanobacterial bioreporter based on a phosphatase alkaline promoter, determination of in situ alkaline phosphatase activities from cyanobacteria found at sampling sites, and the characterisation of cyanobacterial morphological features related to P bioavailability (hairs, polyphosphate granules and calyptras). An inverse relationship was found between values of bioavailable P, measured by the bioreporter and phosphatase activities. Cyanobacteria from sampling sites with low bioavailable P showed high phosphatase activity and vice versa, although some differences in values of this activity were observed in different cyanobacteria found at the same place, in relation to different growth strategies. Morphological characteristics associated with P limitation or P enrichment also varied between sampling locations. Cyanobacteria collected from sampling sites with reduced P bioavailability, measured by bioreporter and phosphatase activity, had a lower abundance of polyphosphate granules; those cyanobacteria capable of developing hairs or calyptras showed a greater abundance of these structures. Conversely, polyphosphate granules in cyanobacteria increased as P bioavailability increased as measured by the bioreporter and phosphatase activity. The study shows that the results of genetic, physiological and microscopic analyses based on these methods complement each other, implying that combining their findings would provide a more complete analysis of the nutrient status of running waters.
