The dynamic changes of arsenic bioaccumulation and antioxidant responses in the marine medaka Oryzias melastigma during chronic exposure

Arsenic (As) is a highly toxic metalloid to aquatic organisms, but the effects of low-dose chronic inorganic As exposure on marine fish are still unclear. A 28-day study was conducted on chronic exposure of 100 μg/L inorganic As [As(III) and As(V)] in the marine medaka Oryzias melastigma to quantify the effects of chronic inorganic As exposure on its bioaccumulation, biotransformation, oxidative stress, and antioxidant enzymes response. During the exposure period, chronic inorganic As exposure had no significant effect on the total As bioaccumulation except at 7 d for As(V) treatment. Based on the toxicokinetic data, the low As bioaccumulation was caused by low dissolved uptake (ku), internal transfer (k12, k21) and high efflux (ke1). The organic As were the predominated species (77 - 80%) and remained constant, while the inorganic As increased 35% (from 0.26 μg/g to 0.35 μg/g) during the initial 4-d exposure and then recovered. The increased inorganic As could be covered by the high contribution of organic As especially arsenobetaine (AsB) to the total As accumulation. Coincidently, the malondialdehyde (MDA) levels followed similar trends as the inorganic As concentrations, indicating that inorganic As bioaccumulation induced oxidative stress at the initial stage. Both the contents of the nonenzymatic antioxidant (glutathione, GSH) and the activities of the enzymatic antioxidants (superoxide dismutase (SOD), catalase (CAT), peroxidase (POD) and glutathione-S-transferase (GST)) increased initially and then decreased as the inorganic As concentrations, thus lowering the hydrogen peroxide (H2O2) levels and displaying a typical antioxidant defense mechanism. In summary, this study elucidated that although the marine medaka had a limited ability to accumulate waterborne As, the increase in the inorganic As at the early stage of exposure still caused toxic effects, which could be ignored by constant total As concentrations. Therefore, the toxicity of As could be underestimated if only the total As instead of the inorganic As is monitored in marine fish.