Arsenic bioaccumulation and biotransformation in macroalgae: Arsenosugar complexity and multifactor variability

Arsenic (As) is a toxic metalloid, and its accumulation in macroalgae has raised concerns for aquatic ecosystem health and human food safety given the increasing consumption of macroalgae. While previous studies have predominantly focused on quantifying total and inorganic As levels, the mechanisms underlying As bioaccumulation and biotransformation in macroalgae have remained fragmented. This review systematically synthesizes current knowledge on As accumulation levels, speciation, and biotransformation pathways in macroalgae. Macroalgae exhibit a high capacity to accumulate inorganic As from ambient water but mitigate its toxicity through extensive biotransformation into diverse organic As species, particularly arsenosugars. While key genes and enzymes involved in inorganic As conversion and methylation have been identified in microorganisms and microalgae, the enzymatic and genetic determinants of arsenosugar formation and degradation in macroalgae remain largely unexplored. These processes are influenced by a complex interplay of biological factors (e.g., algal taxonomy, growth stage, metabolic activity) and environmental conditions (e.g., temperature, pH, light intensity, salinity, nutrient availability). By integrating current knowledge on As speciation and biotransformation mechanisms, this review aims to advance mechanistic understanding of As environmental behavior and biological fate in macroalgae, and such knowledge is essential for informed risk assessment and the sustainable development of macroalgae aquaculture.