Arsenate biotransformation in six marine macroalgae species under dual salinity regimes: Comparative insights across phyla

Aquatic inorganic arsenic (As), particularly arsenate (As^V), poses a significant threat to marine ecosystems due to its potential for bioaccumulation and biomagnification. Marine macroalgae can uptake, bioaccumulate, and biotransform As from the marine environment. While climate-induced stressors, such as fluctuating salinity, can influence As^V uptake and biotransformation in the marine macroalgae. However, the effects of reduced salinity on As^V methylation remain poorly understood. This study investigated the impact of salinity fluctuations (5 and 34 psu) on As^V (1.0 μmol L^-1) uptake, biotransformation, and the release of organic As compounds by six marine macroalgal species (Ulva pertusa, Ulva fasciata, Sargassum ringgoldianum, Corallina crassisima, Grateloupia livida, and Gelidiella acerosa); representing three phyla: Chlorophyta, Heterokontophyta, and Rhodophyta. Among these species, the impact of salinity variation was most pronounced for S. ringgoldianum, which exhibited a complete inhibition of As^V uptake at a salinity of 5 psu. In contrast, species U. pertusa and U. fasciata exhibited good resilience to both 5 and 34 psu salinities, with 100 % of their As^V uptake capacity. Species-specific characteristics further influence As dynamics, revealing a hierarchy of responses: Chlorophyta exhibited the highest biotransformation capacity and DMAA^V release, with U. fasciata achieving nearly complete conversion (0. 98 μmol L^-1 at 34 psu). The species from Heterokontophyta, S. ringgoldianum, demonstrated the highest bioaccumulation of As. Rhodophyta exhibited intermediate responses, with variable uptake and biotransformation depending on salinity, and C. crassisima had minimal capacity for DMAA^V release (0.025 μmol L^-1). Notably, U. pertusa and U. fasciata maintained consistent As^V uptake and DMAA^V release under both 5 and 34 psu salinities, highlighting their resilience and potential for phytoremediation. Overall, these findings emphasize that biotransformation or bioremediation by macroalgae must consider both abiotic factors, particularly salinity, and biotic factors, including macroalgae species, to improve the efficiency of As^V uptake and biotransformation in coastal environments.