Improving water quality, growth performance, and modulating some stress physiological biomarkers in Cyprinus carpio using raw date nuclei as a zinc adsorbent agent

Adsorption of heavy metals by affordable adsorbents has recently become one of the most often applied method for removing these metals from contaminated water. The purpose of this study was to evaluate the efficacy of using raw date nuclei (RDN) as a natural and inexpensive adsorbent to remove Zn ions from contaminated water and reduce its impact on water quality, Zn bioaccumulation levels in fish organs, growth performance, and some physiological aspects of common carp (Cyprinus carpio) under sublethal concentrations. Five experimental groups were examined for eight weeks; CNT, control group rearing in clean water; RDN₀, fish group rearing in Zn-contaminated water; RDN₃, fish group rearing in Zn-contaminated water and treated with 3 g RDN/L; RDN₄.₅, fish group rearing in Zn-contaminated water and treated with 4.5 g RDN/L; RDN₆, fish group rearing in Zn-contaminated water and treated with 6 g RDN/L. Water quality measures were monitored in a continuously manner without removing fish excreta or renewing water. The results showed that sublethal concentration of Zn alone significantly elevated the level of antioxidant biomarkers (SOD, superoxide dismutase; GST, glutathione transferases; GPX, glutathione peroxidase; CAT, catalase and MDA, malondialdehyde), significantly declined fish performance, increased Zn-accumulation in fish organs (gills and flesh) and increased the ammonia secretions in fish ponds compared to the control group. Zn concentrations in RDN-containing aquariums were significantly lower than in RDN-free aquariums, and Zn removal from polluted water was dependent on RDN dosage. Consequently, fish performance and serum bio-indicators enhanced in the presence of RDN in a fish group reared under sublethal Zn concentrations. In conclusion, although Zn caused toxicity in C. carpio at sublethal levels, treating Zn-contaminated water with RDN at a dosage of 6 g per liter effectively prevented oxidative stress and the harmful impacts of Zn exposure.