## 1 Introduction  
Marine species are reported to provide many constituents relevant to the human diet [1, 2]. However, seafood deteriorates rapidly postmortem via several biochemical and microbial breakdown mechanisms [3, 4, 5]. Reducing the temperature via ice or mechanical refrigeration is the most prevalent method to delay microbial and biochemical spoilage in fish. Despite its widespread use, neither method can fully ensure the preservation of fish quality, particularly during extended storage periods or if the cold chain is not consistently maintained. Therefore, advancements in refrigeration techniques have been developed to address these challenges [6, 7, 8]. One such strategy is the use of active packaging films, which can release preservative compounds (such as antimicrobials and antioxidants) into seafood substrates [9, 10, 11]. In general, packaging maintains the freshness of seafood and preserves it against adverse agents, such as water vapor, microorganisms, gases, odors, dust, and mechanical shock and vibrations, during distribution and storage [12, 13]. Notably, the release of preservative compounds extends the shelf life of packaged seafood [14, 15, 16]. The most commonly used packaging materials include synthetic polymers such as low- and high-density polyethylene and polyethylene [17]. However, adverse health problems (i.e., cancer development and toxicity) resulting from the persistent consumption of synthetic antioxidants (butyl-hydroxytoluene, butylhydroxyanisole, etc.) have led to the recommended use of natural antioxidants as alternatives to synthetic antioxidants [18; 19]. Therefore, biodegradable and edible materials derived from plants and animals, including peptides, polysaccharides, and lipids, have proven to represent remarkable alternatives [20; 21]. The film-forming ability of several polysaccharides, such as cellulose, chitosan, starch, pectin, and alginate, has been revised [22]. In particular, gelatin from diverse animal sources can be effectively used to develop active packaging strategies [23]. Marine macrolgae contain a wide variety of natural constituents with potential antimicrobial and antioxidant activities and are suitable for application during seafood processing and storage [24; 25; 26]. Among macrolgae, red varieties (i.e., _Gracilaria_ sp. and _Gelidium_ sp.) are chiefly recognized for their industrial application in extracting phycocolloids (such as agar, algin, furcellaran, and carrageenan) [27; 28], which are utilized to improve the physical properties of edible films in innovative food packaging solutions [29; 30]. Agar extraction typically involves initial alkaline pretreatment, followed by water extraction (90 and 120 \({}^{\circ}\)C) under pressure [31; 32]. This process generates a considerable amount of solid byproducts characterized by a moderate protein content and high polysaccharide content [33]. In the _Gelidium_ genus, different species have been reported to contain antimicrobial and antioxidant components. Thus, a high quantity of flavonoid compounds was detected in both aqueous and ethanolic extracts of _G. pusillum_ (Stackhouse), which was attributed to their antioxidant and antibacterial properties [34]. Similarly, a _G. corneum_ powder-whey protein isolate was shown to have a remarkable antimicrobial effect on fish paste [35]. Sulfated polysaccharides from _G. pacificum_ Okamura also had beneficial effects on mice with antibiotic-associated diarrhea [36]. Moreover, aqueous and ethanol extracts from _G. coliense_ were reported to exert notable inhibitory effects on different bacterial species [37]. Recently, an aqueous extract of flour obtained from _Gelidium_ sp. demonstrated antimicrobial activity in fish during chilled storage [38] and antioxidant properties in heated fish [39]. The present study focused on the use of the red alga _Gelidium_ sp. flour, a waste substrate resulting from the industrial extraction of phycocolloids. The potential preservative effect of an aqueous extract of this alga flour (AF) was investigated.