Arsenic (As) is one of the most concerning environmental contaminants because it originates from both natural and anthropogenic sources and is therefore ubiquitous in the environment. Arsenic contamination is a major environmental concern in China, South Asia, Southeast Asia, and many other regions. Pentavalent arsenic [As(V)] is commonly the predominant inorganic As species in oxic environments and is associated with toxic, teratogenic, and carcinogenic effects. Arsenic can accumulate in aquatic organisms and be transferred through food webs, thereby compromising seafood safety, threatening ecosystem integrity, and posing potential health risks to consumers.
Globally compiled data indicate that total As concentrations in the muscle of marine fish are, on average, substantially higher than those in freshwater fish. However, the mechanisms underlying this well-documented pattern remain unclear. To address this knowledge gap, this study combined global-scale data analysis with a series of controlled laboratory experiments. We first constructed an experimental marine food web comprising Thalassiosira weissflogii, Artemia salina, Nereis sp., and Epinephelus sp. and identified contrasting trophic transfer patterns among As species. Inorganic As exhibited biodiminution along the food web (biomagnification factor, BMF < 1), whereas arsenobetaine (AsB) showed significant biomagnification (trophic magnification factor, TMF > 1). These results indicate that AsB is the principal As species contributing to elevated total As concentrations in marine fish. In contrast, freshwater fish, represented here by zebrafish (Danio rerio), efficiently absorbed inorganic As through the intestine but exhibited a limited capacity for AsB biosynthesis. Consequently, their internal As pools were dominated by inorganic species, whereas their total As concentrations remained comparatively low. Collectively, our findings suggest that the high As accumulation observed in marine fish arises from the combined effects of salinity-associated regulation, genetic control, intestinal biotransformation, and sequestration in muscle tissue. These processes facilitate the conversion of As into AsB, a species prone to biomagnification along marine food webs. Freshwater fish, by contrast, display a distinct As bioaccumulation pattern because of their limited capacity for this biotransformation pathway. These findings elucidate a fundamental mechanism underlying the difference in As accumulation between marine and freshwater fish and provide a scientific basis for more accurate assessments of As related ecological and human health risks.