## 2 Materials and Methods
### Phycobiliprotein Extraction and Recovery  
Figure S1 in Supplementary Materials presents a schematic outline of the preformed extractions, with the following sections describing in detail the performed experiments. 2.3.1. Phycobiliprotein Extraction Employing Maceration, Homogenization, and Freeze-Thaw  
The solvent effect was evaluated using 2.5 g of _A. platensis_ biomass loaded into 50 mL centrifuge tubes and mixed with 30 mL of either water or sodium phosphate buffer (0.1 M) at pH 6, 6.5 and 7. To compare the three different techniques, 2.5 g of _A. platensis_ biomass was loaded into 50 mL centrifuge tubes and mixed with 30 mL of sodium phosphate buffer (0.1 M, pH 7) for all extractions. Maceration was performed with constant agitation at 250 rpm with a thermostatically controlled shaker (Oriola BIOSAN Environmental shaker-Incubator ES-20, Riga, Latvia) at 25 \({}^{\circ}\)C for 1, 2, or 3 h. Homogenisation was performed with a T 25 ULTRA-TURRAX homogeniser (IKA-Werk, Staufeb, Germany) at 12 000 rpm at room temperature (25 \({}^{\circ}\)C) to evaluate the impact of the homogenisation time of 5, 15, 30, and 45 min on the yield and purity of the extract. For freeze-thaw extractions, each cycle consisted of freezing the samples at \(-18\)\({}^{\circ}\)C for 4 h, followed by thawing at room temperature (25 \({}^{\circ}\)C) for 30 min. 2.3.2. Serial Extraction to Determine the Maximum Extractable PBP Content  
The maximum extractable PBP content in the _A. platensis_ biomass was assessed following serial (repeated) extractions performed in the following manner. In a 50 mL centrifuge tube, 2.5 g of biomass was mixed with 30 mL of 0.1 M sodium phosphate buffer at pH 7. The samples were constantly agitated at 250 rpm with a thermostatically controlled shaker for 2 h at 25 \({}^{\circ}\)C and were centrifuged at 5000 rpm for 15 min. The supernatant was then collected, while the remaining biomass was resuspended in a fresh batch of solvent (30 mL of 0.1 M sodium phosphate buffer, pH 7). The extraction procedure was repeated, and, after each step, the PC and APC content were measured as described in Section 2.2. The procedure was repeated until no further increase in the PBP content was observed at a given extraction step (four cycles were performed in total). The extraction efficiency for different extraction techniques was calculated using Equation (5). \[\text{PBP recovery}\left(\%\right)=\frac{\text{Yield from a particular extraction}\left(\frac{\text{mg}}{8}\right)}{\text{Yield from serial extraction}\left(\frac{\text{mg}}{8}\right)}\times 100 \tag{5}\]  
2.3.3. Phycobiliprotein Extraction Under Optimal Conditions  
A total of 25 g of _A. platensis_ biomass was loaded into a 500 mL Duran flask and mixed with 300 mL sodium phosphate buffer (0.1 M, pH 7). Each maceration cycle was performed with constant agitation at 250 rpm for 2 h in a thermostatically controlled shaker (Oriola BIOSAN Environmental shaker-Incubator ES-20, Riga, Latvia) at 25 \({}^{\circ}\)C. The supernatants were then collected, while the remaining biomass was resuspended in a fresh batch of solvent (300 mL of 0.1 M sodium phosphate buffer, pH 7). After three cycles, the PBP content of pooled supernatants was measured as described in Section 2.2. 2.4. In Vitro Antioxidant Capacity  
2.4.1. Total Phenolic Content (TPC) from Folin-Ciocalteu's Assay  
The TPC assay for PC extracts was performed as previously described in the literature [29]. Briefly, the working solution was prepared for the assay by diluting commercial Folin-Ciocalteu's reagent in distilled water (1:9, \(v/v\)). In 2 mL Eppendorf tubes, 150 \(\upmu\)L of the sample was mixed with 750 \(\upmu\)L of Folin-Ciocalteu's reagent (2 M, 1:9, \(v/v\) in distilled water). Distilled water was used as a blank. After 3 min of reaction, 600 \(\upmu\)L of Na\({}_{2}\)CO\({}_{3}\) (75 g/L) was added, and the mixtures were left in the dark for 2 h at 25 \({}^{\circ}\)C. The absorbance was measured at 760 nm using a GENESYS 150 UV-vis spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA).