## 3 Results
### Batch Cultivations  
For an initial evaluation of the PDW spectroscopy signals, batch cultivations with different C/N ratios were performed to trigger different PHB and CDW accumulation. The results of the three batch cultivations are shown in Figure 2. Detailed graphs of the cultivations can be found in the in Supplementary Materials in Figures S1-S3. The first batch cultivation was performed as a reference batch containing 3% (w\(\cdot\)v\({}^{-1}\)) rapeseed oil and 75 mM nitrogen (2.25 g\(\cdot\)L\({}^{-1}\) urea). Nitrogen limitation was indirectly detected by ammonia quantification, which is released from urea cleavage prior nitrogen uptake [24; 28]. Nitrogen was depleted between 9.2 and 14.2 h, which triggered PHB accumulation. Within this period, the CDW increased from 6 to 15 g\(\cdot\)L\({}^{-1}\) and the PHB content increased from 7 to 35 wt% (0.4 to 5.3 g\({}_{\textrm{PB}}\)\(\cdot\)L\({}^{-1}\)). The PDW spectroscopy signals \(\mu_{\textrm{a}}\) and \(\mu_{\textrm{s}}\)' did not show any analyzable signals until 6.5 h. Subsequently, both signals exponentially increased until 10.5 h. After this time point, \(\mu_{\textrm{a}}\) linearly increased until 16 h and did not further rise afterwards. An increase of the \(\mu_{\textrm{s}}\)' signal was detected from 10.5 to 20 h and remained constant afterwards. The maximum CDW (25.5 g\(\cdot\)L\({}^{-1}\)) was achieved after 33.2 h containing 65 wt% PHB (16.8 g\({}_{\textrm{PB}}\)\(\cdot\)L\({}^{-1}\)), which represents an overall yield of 0.56 g\({}_{\textrm{PB}}\)\(\cdot\)g\({}_{\textrm{GB}}\)\(\cdot\) Gu\({}^{-1}\). Over the entire cultivation, the urea consumption for biomass accumulation was 0.26 g\({}_{\textrm{urea}}\)\(\cdot\)g\({}_{\textrm{CDW}}\)\({}^{-1}\). The purpose of Batch 2 was to decrease the C/N ratio compared to the reference batch for an increased accumulation of active biomass (RCDW) and decreased PHB content. Nitrogen was depleted between 11.6 and 19.8 h. Within this period, the CDW and PHB content increased from 12.3 to 38.3 g\(\cdot\)L\({}^{-1}\) and 7 to 62 wt% (0.8 to 23.7 g\({}_{\textrm{PB}}\)\(\cdot\)L\({}^{-1}\)), respectively. The PDW spectroscopy signals \(\mu_{\textrm{a}}\) and \(\mu_{\textrm{s}}\)' simultaneously increased from the beginning of the cultivation until 12 h in an exponential manner. No significant changes of \(\mu_{\textrm{a}}\) were detected after that time point, whereas \(\mu_{\textrm{s}}\)' increased until 18 h and subsequently remained constant. After 23.8 h the maximum CDW of 40 g\(\cdot\)L\({}^{-1}\) containing 63 wt% (26.2 g\({}_{\textrm{PB}}\)\(\cdot\)L\({}^{-1}\)) was achieved. Overall, a PHB yield of 0.66 g\({}_{\textrm{PB}}\)\(\cdot\)g\({}_{\textrm{OII}}\)\(\cdot\) Gu\({}^{-1}\) and urea consumption of 0.29 g\({}_{\textrm{URE}}\)\(\cdot\)g\({}_{\textrm{RCDW}}\)\({}^{-1}\) was achieved. The C/N ratio in the third batch was increased compared to the reference batch by keeping the initial nitrogen concentration constant (75 mM) but increasing the rapeseed oil content to 4% (w\(\cdot\)v\({}^{-1}\)). Depletion of nitrogen occurred between 10.3 and 11.3 h. Within this period the CDW increased from 7.7 g\(\cdot\)L\({}^{-1}\) with 8 wt% PHB (0.6 g\({}_{\textrm{PHB}}\)\(\cdot\)L\({}^{-1}\)) to 9.6 g\(\cdot\)L\({}^{-1}\) with 20 wt% PHB (1.9 g\({}_{\textrm{PHB}}\)\(\cdot\)L\({}^{-1}\)). PDW signals were detectable after 2 h. A comparable increase of \(\mu_{\textrm{a}}\) and \(\mu_{\textrm{s}}\)' was detected until 10 h. An attenuated increase of \(\mu_{\textrm{a}}\) until 17.5 h was detected, whereas a diminished increase of \(\mu_{\textrm{s}}\)' was detected from 10 to 22.5 h. The CDW further increased to 33.7 g\(\cdot\)L\({}^{-1}\) containing 73 wt% PHB (24.5 g\({}_{\textrm{PHB}}\)\(\cdot\)L\({}^{-1}\)). Overall, a PHB yield of 0.61 g\({}_{\textrm{PHB}}\)\(\cdot\)g\({}_{\textrm{OII}}\)\(\cdot\) Gu\({}^{-1}\) and urea consumption of 0.25 g\({}_{\textrm{URE}}\)\(\cdot\)g\({}_{\textrm{RCDW}}\)\({}^{-1}\) was achieved.