## 1 Introduction  
The primary motivation of the present study was to accelerate wound healing in aquaculture fish by using cold atmospheric pressure plasma (CAP). CAP has been demonstrated to be beneficial for wound healing processes by reducing the microbial load, suppressing inflammatory events, reducing oxidative stress, and enhancing angiogenesis [1, 2, 3, 4] in human and other vertebrate models [5, 6]. These CAP-induced benefits support the natural regenerative capacity of the integument, but have not yet been tested on aquatic organisms.  
The integument of vertebrates consists of the three layers epidermis, dermis and hypodermis [7, 8], which represent a solid barrier against a potentially detrimental environment. To encounter the different spectra of terrestrial and aquatic environmental influences, the skin of land-living tetrapods and amphibians or fish, respectively, has specific adaptations [9]. Since lesions of the skin may result in a loss of homeostasis, a complex regeneration program is initiated immediately after traumatic [10]. This wound-healing program involves several well-orchestrated events, including blood clotting, inflammation, re-epithelialisation and the generation and remodelling of tissue [11], which display class- and species-specific characteristics [12, 13]. Skin injuries in farmed fish frequently occur as a consequence of intensive fish farming, e.g., due to fungal, bacterial or parasitic infections [14, 15], aggressive conspecific interactions, overstocking-related abrasions or inadequate handling [16, 17, 18]. The immediate treatment of skin wounds rapidly reduces the wound size and stimulates the healing process to prevent bacterial secondary/co-infections, maintain fish welfare and eventually prevent considerable economic losses [19]. However, there are hardly any treatment approaches for this problem so far. While wound healing processes have been investigated in a few in-vitro models from bony fish [20; 21; 22], to the best of our knowledge therapeutic approaches involving CAP have never been analysed in fish and fish cell lines to date.  
Cold atmospheric pressure plasma is an ionized gas formed by the input of high energy and consists of ions, electrons, radicals, ultraviolet photons and uncharged atoms or molecules. CAP can be generated using various plasma devices, such as plasma jets or a dielectric barrier discharge [23]. CAP generates short- and longer-lived molecules, such as reactive oxygen and nitrogen species (RONS), one of which is hydrogen peroxide. Longer-lived species are generated especially by the interaction of CAP with liquids [23; 24]. In mammalian cell models, the treatment with CAP is conducted either directly, or indirectly via liquids that were previously treated with plasma. Our former studies proved that CAP mainly affects the cells via the surrounding medium. We showed that the efficacy of plasma-activated medium (PAM) is maintained for up to 21 days when stored at standardized conditions of 37 \({}^{\circ}\)C [25; 26]. This medium-mediated effect of CAP was also observed by other authors for different fluids (cell culture medium, phosphate buffered saline, water, Ringer's solution) [27; 28; 29; 30; 31]. In general, similar experimental setups are recommended [32; 33] to allow for discrimination of cell type-specific reactions from medium-mediated effects, particularly when comparing tumour cells with normal cells.  
We wanted to find out whether plasma exerts an effect on fish cells and how this differs from mammalian cells. In our study, we compared different culture conditions and their influence on cell viability. To this end, we conducted parallel studies on cells from different origins in PAM prepared from the same culture medium or in PAM prepared from medium with different antioxidant potential to test whether these parameters influence the cellular sensitivity towards cold atmospheric pressure plasma.