Luxury uptake and biomass phosphorus
A summary of the biomass polyphosphate and organic phosphorus concentration for each test scenario is shown in Table 4. The mean biomass organic phosphorus was 1.1% (g P/g dry biomass). This was consistent with a previous estimate that found that the critical growth level for microalgae was 1% phosphorus (Borchardt and Azad, 1968). One of the trials (PAR-150 μmol/m2/s, Temperature-10 °C, Total Phosphorus-15 mg P/L) resulted in an elevated biomass organic phosphorus concentration (2%). The reasoning for the elevated concentration is unclear, but it is not representative of the rest of the dataset. Biomass polyphosphate, which represents luxury uptake, ranged from 0.9 to 2.3%. Therefore, the accumulation of polyphosphate is a significant removal mechanism as it represented 53 ± 8% of microalgae biomass phosphorus. Similar polyphosphate concentrations have been found at temperate climates. Powell et al. (2011) found the phosphorus fraction of sludge from three WSPs in New Zealand consisted of 33–73% polyphosphate.