Novel insights into hákarl: A deep dive into the microbiological and physico-chemical features of Iceland's traditional fermented shark

Hakarl, the traditional Icelandic product obtained from the fermentation of Greenland shark (Somniosus microcephalus), represents a unique food item from both cultural and microbiological perspectives. This study investigated commercial samples of h´akarl using an integrated approach, combining physico-chemical analyses, lipid and volatile profiling, metataxonomic sequencing, and microbial isolation. Results confirmed the alkaline nature of the product (pH ~8) and a water activity (aw) of 0.96, sufficient to sustain an active and diverse microbial community. Lipid profiling revealed the predominance of monounsaturated fatty acids, with differences in PUFA and DHA levels between the analyzed producers, consistent with nutritionally favorable characteristics. Microbiological analysis highlighted bacterial communities dominated by Firmicutes, particularly Bacilli and Clostridia, with significant abundances of Tissierella creatinini and Atopostipes suicloacalis. Culturedependent methods led to the isolation of Carnobacterium antarcticum cultures, which were subsequently characterized for their enzymatic activities. These findings suggest potential biotechnological applications of the isolates, especially in fermentation and aroma development. Volatile compound analysis identified thirteen VOCs including alcohols, aldehydes, ketones, phenols, sulfur- and nitrogen-containing compounds. Trimethylamine was the predominant metabolite responsible for the strong ammonia-like odor, followed by phenol and sulfurcontaining compounds, which also contributed to the sensory profile. Overall, the results provide novel insights into the microbial ecology, physico-chemical traits, and volatile characteristics of h´akarl, confirming its variability linked to artisanal production methods. The study emphasizes h´akarl's role as a reservoir of protechnological microorganisms and advances current understanding of the factors influencing its safety, quality, and identity as traditional fermented food.