## 1 Introduction  
Polyhydroxyalkanoates (PHAs) production by mixed microbial cultures (MMCs) from industrial wastes/by-products has been gaining interest as a cost-effective approach for lowering PHA production costs [1]. Nevertheless, this process still requires improvements for achieving lower operational costs. The MMC PHA production from wastes generally comprises three stages, the acidogenic fermentation, culture selection, and the PHA production. The core of the process, though, is the selection stage, since the selected MMC is the central component for determining the efficacy of the PHA-producing process in terms of its storage capacity and productivity. The culture selection stage is grounded in two efforts, (1) to enrich a MMC in PHA-accumulating organisms capable of producing high amounts of PHA; and (2) sustain a high-growth of the enriched MMCs for the PHA production stage, in order to increase the volumetric productivity of the global MMC PHA production process [1]. Therefore, in the selection stage, both growth and PHA storage need to be supported. Consequently, all nutrients required for growth must be available during this stage. On the other hand, in the PHA production stage, the presence of various nutrients is generally related to lower storage yields and rates, biomass PHA contents, and PHA productivities [2; 3]. However, a recent study has shown that PHA productivity increases when nitrogen and phosphorus were present [4]. Many industrial wastes/by-products feedstocks (paper mill wastewater, sugarcane molasses, olive oil mill wastewater, glycerol, oxalate) used so far for PHA production, are rich in carbon compounds but poor in nitrogen and phosphorus, making nutrient supplementation necessary during the culture selection [3; 5; 6; 7; 8]. Other feedstocks, such as cheese whey, possess potential sources of nitrogen, like proteins, but its availability may be limited by the culture's capacity to metabolise these proteins. Cheese whey is a by-product of dairy industries. Due to its high protein-content, ca. 1.4-33.5 g\(\cdot\)L\({}^{-1}\)[9], CW may be valorised through the production of whey protein, whose market has been increasing, both for the concentrate, and for individual proteins [10; 11]. However, only approximately 50% of the ca. 180-190 \(\times\) 10\({}^{6}\) tons cheese whey produced worldwide, yearly, is processed into various food and feed products [12]. The remaining surplus has to be disposed of, representing a severe environmental problem due to its high pollutant characteristics [9], and high volumetric production (9 kg cheese whey is created per 1 kg of cheese produced). In Europe alone, the annual surplus of cheese whey which is not further valorised is 13 \(\times\) 10\({}^{6}\) tons, containing about 6 \(\times\) 10\({}^{5}\) tons of lactose [13]. The recovery of whey proteins from CW generates a stream containing 81% of the lactose originally present in the milk, which has been demonstrated to be a suitable carbon source for PHA production by pure cultures [14]. Recently, [PERSON] et al. [15] demonstrated that raw CW is a promising feedstock for MMC PHA production, showing that anaerobic fermentation bacteria were able to use a fraction of the protein content present in CW (ca. 30% of the initial protein content in the feed). Due to the presence of an easy fermentable carbon source, sugar (lactose), it is supposed that proteins were consumed as nutrients source, excluding the need for external nutrients supplementation, in the first stage of the process. However, despite the obtained fermented CW still containing a substantial protein content, in that study the aerobic selection stage feed was supplemented with excess nutrients, not assessing for the possibility of the aerobic culture being able to use proteins as nutrients source. Indeed, the ability of anaerobes to hydrolyse proteins is well recognised [16; 17]. Hitherto, no study on the impact of proteins in the MMC PHA production has ever been reported.