###### Abstract
Coupled aquaponics systems' operation in terms of biological filtration monitoring of the water quality is similar to recirculating aquaculture systems. Water is transported from fish tanks to the biological filter by gravity and via a pump to the plants. Lastly, the water returns from the plants in fish tanks. Alternative water sources, such as brackish or saltwater can be used in aquaponic systems (maraponics), in which euryhaline or saltwater fish, seaweeds, haplolytic plants, and salinity-tolerant glycopures can be utilized [6]. To date, euryhaline species, such as gilthead sea bream _Sparus aurata_[7], sea bass _Dichentrachus labrax_[8; 11; 17], flatheded grey mullet _Mugil chephalus_, leaping grey mullet _[PERSON] saliens_[18], or Nile tilapia _Oreochromis niloticus_[19], and red tilapia hybrid _Oreochromis mosambicus_[20; 21] were used in saline aquaponic systems. Several euryhaline teleosts, such as European sea bass (_Dicentrachus labrax_) can adapt to a wide range of environmental salinity [22]. These marine species possess the capability of sensing environmental osmolality and further transducing the sensory stimulus to signaling pathways that generate many physiological changes for adjusting their osmoregulatory strategy [23]. Mitogen-activated protein kinases (MAPK) constitute evolutionary conserved intracellular signal transduction protein cascades that are activated by numerous extracellular stimuli, including salinity levels [24]. Several MAPK subfamilies are involved in the signaling of osmotic stress, thus indicating the important role of cascade in osmoregulatory mechanisms [25]. Furthermore, several members of another intracellular protein family, namely heat shock proteins [HSPs], are highly expressed under stress. HSPs act as molecular chaperones and participate in the prevention of stress-induced protein unfolding and degradation of damaged proteins, thus maintaining cellular protein homeostasis [26; 27]. Miscellaneous abiotic and biotic factors may contribute to the induction of HSPs, such as heat, drugs, pathogen infections, and osmotic stress [26; 28]. From this angle, it is important to further investigate the expression levels of these groups of proteins (MAPKs and HSPs), revealing their chronic expression under different salinity levels in an aquaponic system. On the other hand, the growth of plants is closely related to their environment and, in particular, the cultivation conditions, including salinity levels and nitrogen availability as major issues. Nitrogen is an essential element affecting plant growth in a critical way as it is the building block of proteins, enzymes, chlorophyll, coenzymes, and nucleic acids. Plant nitrogen uptake is affected by many factors, such as nutrient concentrations, light intensity, plant growth stages, and genetic factors [29; 30]. Regarding salinity, halophytic plants developed adaptive mechanisms to cope with the saline environment. Effectiveosmoregulation is a key process employed by halophytes to tackle osmotic stress, thus avoiding dehydration. Moreover, the accumulation of osmotically active compounds protects sensitive cellular substances from Na+ toxicity and plays a major role in reactive oxygen species detoxification [31]. Although many halophytes, such as _Criithmum martimum_ L. (Apiaceae), can grow even at seawater levels of salinity, they show reduced growth at high salinities compared to lower salinities [7; 32; 33]. In addition, salinity affects the plant's photosynthetic activity, either directly by targeting photosynthetic apparatus and causing changes in chlorophyll, carotenoids, and proteins levels or indirectly though the deterioration of plant's water relations [34; 35]. The halophyte rock samphire (_Criithmum martimum_), a salt-tolerant Mediterranean plant of coastal areas, was used in the present study as it can survive and grow at 12 ppt or higher salinities [7; 36]. Rock samphire, therefore, fulfills the requirements as one of the candidate plants in saline aquaponics (maraponics and brackaponics).