Cadmium induced bioaccumulation, histopathology, gene regulation in fish and its amelioration – A review

In anthropogenic epoch, modern agriculture practices and land use patterns, industrialization, mining, and overpopulation have accelerated to the contribution of varying degree of environmental hazards including air, water and soil pollution, greenhouse gas emissions, land degradation and loss of biodiversity on the earth, which impact on biophysical thresholds for the “safe operating space” in the earth for humanity and many other biosphere [1]. Several pollutants including non-degradable toxic heavy metals from natural sources as well as anthropogenic activities have immensely increased in aquatic habitats, which are increasingly posing health challenges and threats to aquatic life as well as humans via entering into the food webs [2]. In terrestrial animals, the main route of heavy metals entry to the body is gastrointestinal tract through contaminated foods and water [3], [4], [5], whereas in fishes, toxic elements can enter into body through gastrointestinal tract, gill and skin [6], [7], [8], [9]. Moreover, phytoplanktons and zooplanktons that can accumulate heavy metals may be ingested by different fish species [10], [11]. Compulsion of aquatic habitat and source of feeds including phytoplanktons and other invertebrates in the food chains make fishes more vulnerable for forced accumulation of several heavy metals and metalloids including cadmium (Cd), lead, arsenic, copper, zinc, and cobalt. Moreover, intensification of aquaculture to cater the need of rising global fish-food consumption at the rate of 3.1% per year [12] has made it to be a risky affair in terms of environmental health. Among heavy metals, Cd is one of the most prevalent and toxic elements commonly found in soil, water, air and foods [13], [14]. For example, lethal concentration to 50% (LC50) of common carp (Cyprinus caprio) for Cd is 0.20 µM, whereas LC50 values for copper and zinc are 0.77 and 29.9 µM, respectively [15]. Moreover, long half life of heavy metals and its tendency to accumulate in vital organs are of great concerns to environmental and biological health. For Cd, such hazard may continue up to 35 years [16], [17], [18]. Through biomagnifications via food chains, Cd can cause serious health issues in humans and is even considered as a potent carcinogen affecting multiple organ functions [19]. In fishes, Cd exposure exerts several adverse effects on general growth and development, reproductive processes, osmoregulation, morphological structures, histology, stress tolerance, and endocrine system [20].

Major organs involved in bioaccumulation of Cd in fishes are kidney, liver, gill, spleen, intestine, ovary, testis and muscle. Several factors, namely, temperature, pH, organic content and salinity level of water and their seasonal variations regulate Cd bioaccumulation in tissues [21]. Cellular or tissue level alterations may be evident prior to any external or physical changes are observed along with the bioaccumulation process indicating the level of exposure of the heavy metals and in other way, fish species are also considered as a biomarker of polluted water bodies [22]. Considering the species variation in fishes, numerous studies have been conducted to investigate the histopathological alterations in various tissues due to Cd exposure. Oxidative stress induced by Cd toxicity, when the Cd threshold is exceeded, may cause immunosuppression, inflammation and genotoxicity along with changes in their gene regulation [23], [24]. Cd toxicity not only affects the fish health and production but its long term accumulation in fish tissues may be a serious threat to humans though water habitats may contain lower Cd concentrations than the maximum allowable limit in natural water bodies [25]. Fish is an important source of high quality protein, omega-3 fatty acids, and many essential micronutrients that confer several human health benefits and balanced nutrition [26]. World fish food supply has increased by an average annual growth rate of 3.2% with per capita consumption from 9.9 kg to 18.4 kg in the period 1961–2009 [27]. Fish accounted for 16.6% of animal protein intake and 6.5% of total protein intake in 2009 (FAO, 2012). Due to the importance of fish foods in human nutrition and its growth, amelioration of Cd by an acceptable measure hence is needed for fish health and ultimately for the benefit of humans. Considering the importance of the subject and ever expanding knowledge pool, this review explores and summarizes extant research findings in bioaccumulation, ultrastructural alterations in tissues, oxidative stress, synergism-antagonism, and gene regulations caused by Cd and their amelioration in fishes.

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