Cadmium (Cd) is known as the non-element since it lacks essential nutritional properties such as Cu, Zn, Co, and Mn (McGeer et al. 2012). Cd is naturally found in water bodies in trace concentrations of around 0.003 mg/L; however, background levels are increasing, particularly in areas polluted by major industrial, agricultural, and mining activities, and have reached 2 mg/L. (Cao et al. 2012; Adesiyan et al. 2018). Cd rapidly bioaccumulates and bioconcentrates in all tissues and organs of aquatic species, with relatively high levels in the liver, kidney, and gills and substantially lower levels in muscle tissue (Heydarnejad et al. 2013; Soegianto et al. 2022a).
Fish exposed to waterborne Cd may experience a variety of deleterious effects, including changes in respiration (Shaffi et al. 2001), decreased growth (Okorie et al. 2014), alteration in plasma ion regulation (McGeer et al. 2000), hematological abnormalities (Adhim et al. 2017; Handayani et al. 2020), and enzyme activity (Nursanti et al. 2017; Ma’rifah et al. 2019). There has been limited research on the effect of Cd on acid-base balance in fish. Previous studies reported that some heavy metals such as Zn, Cu and Pb can affect blood acid-base balance of rainbow trout (Salmo gairdneri) (Spry and Wood, 1985), cod (Gadus morhua) (Larsen et al. 1997), and groovy mullet (Liza dumerili) (Mzimela et al. 2002). Other forms of inorganic pollution, such as nitrite, increased blood nitrite, methaemoglobin, and oxygen partial pressure (pO2) in European eels (Anguilla anguilla), although blood pH, pCO2, and HCO3− were negatively correlated to nitrite concentrations in media (Huang and Chen 2002). Following nitrite exposure, giant river prawn Macrobrachium rosenbergii showed an increase in haemolymph pO2 and ammonia excretion as well as a reduction in haemolymph pH (Chen and Lee 1997). Alteration in acid-base balances and other hematological parameters induced by heavy metal and other pollution exposure occurred from a quick series of events in which hypoxemia, most likely caused by gill destruction, led in tissue hypoxia and a combined acidosis, both were lethal (Brauner and Rummer 2011).
All animal generates approximately the same quantity of CO2 that O2 consumes during metabolism. The blood transports O2 from the environment to the tissues, while the tissues release CO2 and the blood transports it back to the environment. The red blood cell (RBC) contains hemoglobin (Hb), which is required for O2 and CO2 transport in the blood of all vertebrates (Brauner and Rummer 2011). Since acid–base compensation relies mostly on the direct movement of H+ and HCO3− through the gill in exchange for Na+ and Cl−, acid–base regulation is also associated to ionic regulation in fish. In consequence, ensuring ionic and osmotic balance in fish requires regulation of NaCl transport through the gill. CO2 excretion, ionic regulation, and acid–base balance is all regulated by carbonic anhydrase (CA) (Gilmour and Perry 2009). Many in vitro studies have indicated that heavy metals suppress CA activity in fish (Soyut and Beydemir 2012; Kaya et al. 2013; Caglayan et al. 2020; Kurici et al. 2021), but there has been very few in vivo research on the influence of heavy metals on fish CA.
The tilapia, Oreochromis niloticus, was selected as the experimental test species in this research because it is among the most commonly consumed freshwater fish in Indonesia and the world's second most popular fish after carps, according to the Food and Agriculture Organization. It can endure a variety of environmental conditions and is widespread in many areas of the country (Pabru et al. 2019; Handayani et al. 2020). Nonetheless, because tilapia farms utilize waters from rivers that are regularly contaminated by heavy metals from anthropogenic sources, the consequences of Cd in tilapias are a major issue. In order to investigate the physiological and hematological effects of sub-lethal Cd on O. niloticus reared in freshwater, we evaluated acid-base regulation, CA level, plasma osmolality, ions level, and blood parameters over a four-day exposure period.