Elevated N/P Ratios Enhance Arsenic Bioaccumulation and Biotransformation in Marine Microalgae

Marine microalgae, as the primary producers in ocean ecosystems, play a critical role in global carbon cycling and are efficient accumulators of arsenic (As) from seawater, raising concerns about its trophic transfer through marine food webs. Rising terrestrial nutrient inputs, particularly excess nitrogen, have elevated nitrogen-to-phosphorus (N/P) ratios, representing a widespread ecological concern that alters marine biogeochemical cycles and threatens the survival of marine microalgae. A deeper understanding of how arsenic uptake and transformation biogeochemistry in marine microalgae respond to intensifying N/P ratios is thus crucial to predict the response of marine ecosystems and their contribution to global climate change. Despite these concurrent trends, the impact of altered nutrient dynamics on the arsenic biogeochemistry in marine microalgae remains largely unexplored. Here, we show that appropriately increased N/P ratios markedly enhance arsenic accumulation and promote the intracellular conversion of inorganic As to organic species. Transcriptome data further demonstrate that marine microalgae could significantly upregulate gene expressions associated with arsenic transport-related genes to cope with increased N/P ratios. Metabolomic flux analysis revealed that elevated N/P ratios promoted photosynthetic carbon fixation, ATP synthesis, and carbohydrate metabolism, while the amino acid metabolism was suppressed in marine microalgae, and the redirected energy flow may facilitate arsenic metabolism. Collectively, the reallocation of metabolic energy under high N/P conditions contributed to increased arsenic uptake and biotransformation in marine microalgae. An analysis of the Tara Oceans database confirmed these patterns and showed that arsenic metabolism-related genes are widely distributed across global surface oceans. Continuous nitrogen inputs and rising N/P ratios may alter arsenic speciation and mobility, benefiting cellular detoxification in the short term but potentially increasing organic arsenic accumulation, trophic transfer, and associated ecological and human health risks over longer time scales.