## Results
### Microbial Community Assessment  
The decrease in the efficiency of selecting a PHA-storing MMC from Period III onwards was also sustained by the quantitative FISH analysis results (Figure 3). The significant decrease in the probe-targeted PHA-storing organisms was observed throughout the study. During Period I, the probe-targeted organisms covered 47% of the bacterial population, comprising mostly members of the _Thuera_ and the _Lampropedia_ genera. _Thuera_, _Azaercus_, and _Paracoccus_ were previously described as important PHA-storing organisms in the PHA-producing systems fed with fermented feedstocks [28; 33].  
Figure 2: Typical profiles of biomass concentration (\(X\), \(\times\)), specific concentrations of organic acids (_H.Org._\(\blacktriangle\)), proteins (_Prot_, \(\Circinus\)) and ammonia–nitrogen (_N-NH3_, \(\square\)), and biomass PHA content (_PHA_, \(\blacktriangle\)) obtained in SBR’s cycles in Period I (**a**) and Period IV (**b**).  
_Lampropedia_ was found to be capable of storing PHA in an enhanced biological phosphorus removal systems [35], and was identified as an important population in the PHA-production MMC through denaturing gradient gel electrophoresis (DGGE) [36]. Their ability to produce PHA was also confirmed in this study through Nile Blue staining (Figure 4). Members of this genus were previously described in an activated sludge-treating dairy wastewater [35] and cheese factory wastes [37], and were also detected through 16S rRNA gene clone sequencing, in a PHA-producing system, with fermented molasses [28].  
When the supplemented nutrients were reduced to half of the reference (Period II), the abundance of _Thuarea_ and _Lampropedia_ were reduced to about one-third of their original value. However, the PHA production rate (q\({}_{\text{PHA}}\)) and productivity (Table 3) were not significantly affected, suggesting that other populations took over in adaptation to the new conditions. Indeed, the abundance of _Azoorcus_ was temporarily increased, suggesting that this genus was more resilient to a decrease in N and P, in the medium. Nevertheless, after Period III, the total number of probe-targeted organisms decreased, which correlated with the sharp decrease in PHA-storing performance, both in the selection and in the accumulation reactors. This may indicate an outgrowth of the PHA-storing organisms by other heterotrophs that are capable of using proteins as the nutrient source, as was hypothesised above. Indeed, the fraction of cells with a positive signal after Nile Blue staining, clearly decreased  
Figure 4: The typical tablet-shaped clusters of _Lampropedia_ observed in this study: (**a**) FISH micrograph with Cy3-labelled _Lampropedia_ (_magenta_) and Cy5-labelled Bacteria (_other Bacteria in blue_); and (**b**) Nile Blue staining. _Bar_ = 10 μm.  
Figure 3: Summary of the microbial population-changes of the enriched MMC, in the selection reactor during the five different periods (I–V).  
between Period I and V (Supplementary Materials Figure S1). Among the assessed PHA-storing organisms, _Thuera_ and _Azoarcus_ were those that prevailed at the end of the study, suggesting that, among the probe-targeted PHA-producers, these organisms had the highest capacity to survive in a non-supplemented feed.