## 1 Introduction  
It is well understood that human activity on the planet is causing rising global environmental risks and for the first time constitutes the largest driver of planetary change [1]. Increasingly tightening legislation governing waste disposal makes conventional treatment of waste streams more expensive and energy demanding, creating the need for alternative solutions [2]. Circular economy transition is ongoing and increasingly supported and urged by policy makers, as a response to the global environmental problems earth is facing [3, 4]. Circular bioeconomy is the term currently used to describe the bioconversion of waste streams into added-value products [5]. The European Union has published detailed reports quantifying the agro-industrial residual biomass production in the country members to promote their utilization as feedstock in the future bioconomy [6]. Microbial biotechnology is indispensable for the creation of processes that will pave the transition to closed-loop circular production by exploiting residual biomass and re-introducing them into the food chain as novel products [7]. Various pathways to valorize biowaste into value-added products such as biofuel, biochar, and biomaterials that have emerged from research labs are currently being studied for their upscaling feasibility [8]. Bioconversion of lignocellulosic streams with the use of macrofungi can be a sustainable alternative to the energy-consuming production of biofuel and bioenergy. Especially, the concept of biorefineries that integrate various conversion and separation steps to utilize all process intermediates and by-products has been proposed as a feasible strategy for the valorization of lignocellulosic residues [9; 10; 11; 12]. Although many side-streams are being utilized as animal feed, it is considered that they can potentially be used to produce higher-value products [7]. The high degree of competition in national and international markets provides another motive for the agri-food industry to seek economically sustainable solutions that will produce added-value products [13]. It is widely considered that fungi play an important role in tackling food security challenges [14]. Fungi are well-known for their ability to degrade lignocellulosic material in nature [15] and can be protagonists in this circle of transforming low-value substrates into a variety of valuable bio-compounds [14; 15; 16; 17; 18; 19; 20]. The metabolism of many macrofungi has been studied in synthetic media with various carbon and nitrogen sources to determine growth patterns and their productivity [21; 22]. Current research indicates that satisfying quantities of macrofungi mycelial biomass can be produced in liquid cultures using agro-industrial side-streams as the sole carbon source [23; 24; 25; 26; 27]. Submerged cultivation of macrofungi is an efficient, reproducible, and potentially scalable method to produce fungal biomass and metabolites (Figure 1). Macrofungi can biosynthesize highly nutritional biomass containing a great variety of valuable bioactive compounds. Using fungal metabolites or biomass in industrial applications, known as fungal white biotechnology, is a field that has been gaining increased attention lately due to its eco-friendly nature. Applied fungal biotechnology is used for food, feed, biocontrol, bioenergy, chemicals, pharmaceuticals, detergents, enzymes, proteins, paper and pulp, textiles, materials for construction, and the automotive and transportation industries [14; 28]. This article aims to provide a critical review of what has been achieved in the field of liquid state fermentation of macrofungi up to now, using agri-food residues as the substrate to produce several bioactive metabolites from a wide range of higher edible or/and medicinal fungi. In particular, Section 2 provides an overview of the volumes of agri-food side streams produced, as well as the key physicochemical characteristics that characterize them.