Natural carotenoids are considered as high-value added pigments, comprising an ever increasing global market (Global Market Insights 2023). Besides their coloration, carotenoids demonstrate several biological functions and actions (Papapostolou et al. 2023). For instance, β-carotene is well known for its vitamin A activity, which is higher compared with other carotenoids (Grune et al., 2010, Aburai et al., 2018). Moreover, carotenoids are able to reinforce human’s immune system acting as strong antioxidants and as antimicrobial agents (Mohan et al., 2018, Varghese et al., 2023). A plethora of plants in nature are able to synthesize carotenoids; however, their exploitation in industrial scale has several limitations, mainly due the dependence on geographical variability and the seasonal conditions (Igreja et al. 2021). In addition, the process is time consuming and large areas of cultivation are necessary in order to reach high production levels (Paul et al. 2023). The major factor hampering their employment in a larger scale, is the difficulty of manipulating the whole cultivation parameters such as the temperature and the soil content (Zhang, 2018, Bogacz-Radomsk et al., 2019).

Chemically synthesized carotenoids have been widely produced in order to meet market demands for colorants. Even though the technology for their chemical synthesis is inexpensive and efficient, synthetic carotenoids have been accused of causing various health implications (Mussagy et al. 2019). Moreover, artificial pigments provoke substantial and hazardous waste when disposed of; causing environmental pollution (Santos Ribeiro et al. 2019). As an alternative, natural carotenoids could also be synthesized biotechnologically from a variety of microorganisms such as bacteria, fungi and yeasts. Actually, the rate of their biotechnological production is nowadays facing an increase, as new strains demonstrating a high capacity for producing carotenoids have been discovered, and the production technology is constantly evolving (Alexandri et al. 2022). Microbial carotenoids offer a viable strategy for stable, efficient and safe production with regard to human health and the environment. Another advantage of microbial synthesis includes the ability of high product yields together with vast color diversity (Torregrosa-Crespo et al., 2018, Wang et al., 2021). Among the carotenogenic strains, yeasts have arisen as the most promising candidates from both a commercial and economic point of view. High growth rates, the ability of assimilating a variety of substrates, and the fast production rate of secondary metabolites constitute some of the main advantages of yeasts (Fakankun and Levin 2023). Many studies are focusing on Rhodotorula and Rhodosporidium species, also named as red yeasts, owing to their ability to accumulate high amounts of carotenoids like β-carotene, torulene and torularhodin (Mannazzu et al. 2015; Kot et al. 2018; Sereti et al. 2023). These yeasts are also considered as oleaginous due to the fact that they are able to synthesize lipids up to 70% of their dry cell weight (Kot et al. 2019a). Likewise, lipid accumulation could also occur through red yeast fermentations, utilizing a wide range of substrates (Patel et al. 2015). The produced oil is comprised of a fatty acid profile which is similar with plant-derived oil and is mainly characterized by the presence of palmitic acid (C16:0), stearic acid (C18:0), oleic acid (C18:1), linoleic acid (C18:2) and linolenic acid (C18:3) (Wu et al. 2023). Recent studies have focused on optimization of yeast fermentation conditions in order to enhance the lipid accumulation by directing their metabolic pathway to fatty acid synthesis instead of carotenogenesis (Zhang et al., 2019, Gong et al., 2022). However, microbial oil produced by red yeasts could also be characterized as carotenoid-rich, due to the lipid soluble nature of carotenoids. Therefore, the simultaneous production of carotenoids and fatty acids might enhance their perspective utility. In particular, carotenoids-rich microbial oil has been utilized in the development of novel food formulations such as wax esters and oleogels (Papadaki et al., 2019a, Papadaki et al., 2019b), while their inclusion in novel formulations (e.g. edible films) could enhance their bioactivity, increasing their potential applications (Papadaki et al., 2023, Papadaki et al., 2024).

The purpose of this study was the investigation of two novel R. palidigenum strains, NCYC 2663 and NCYC 2664, in order to evaluate their ability to produce carotenoids assimilating different carbon sources. Four carbon sources, namely glucose, sucrose, fructose and a mixture of glucose: galactose, were selected in order to assess growth patterns and metabolite synthesis (lipids and carotenoids) of these unexplored yeasts. Aiming to unravel potential applications of the produced carotenoids in food industry their antioxidant and antimicrobial activities against common bacterial and fungal pathogens was also investigated. Notably, this research stands as the first to comprehensively analyze the kinetic behavior and the composition of both carotenoids and lipids produced by two novel R. paludigenum strains alongside specific bioactive properties. This study offers a thorough analysis of specific microbial carotenoids, providing new insights in their synthesis by marine yeasts, paving the way towards sustainable production of these value-added compounds.