Evolution of bacterial diversity during Sargassum biomass decomposition

The landfill and decomposition of Sargassum biomass washed ashore induce major concerns in terms of health and environmental risks because the release of arsenic (As). The fate of As during natural decomposition of Sargassum was studied from a mesocosm experiment during 120 days, simulating the natural conditions encountered on Sargassum stockpiled sites: temperature, sunlight and raining. In fresh Sargassum, total As concentrations were 85 mg/kg (dw). AsIII was the more abundant species followed by TMAsO, AsV, AsB and DMAsV. In leachates, As concentration was 10 mg.L-1 during the first days and then decreased to 1.5 mg.L-1 after 90 days of experiment. According to the French legislation, the discharge limits in terms of As concentration in the leachate of the decaying Sargassum biomass from hazardous waste treatment facilities would be most often exceeded. AsV was more abundant than AsIII. Variable AsB and DMAsV proportions were found. High content of arsine was measured in chemotraps, mainly trimethylarsine ((CH3)3As) released during the first weeks. After 120 days, 50% of the total As initially present in Sargassum biomass had been released as dissolved forms in the leachate, and 51 µg of As volatilised in the atmosphere (0.06‰). The final As content remaining in the Sargassum was still high (about 55 mg/kg), due to the significant mass loss of Sargassum during its degradation. A temporal increase in bacterial diversity is observed in the leachate. In the decomposed Sargassum biomass, the bacterial diversity increasing with the deepness. A major role of bacteria in the fate and speciation of As is evidenced.