Release of toxic sulfur gases from decaying seaweed (Ulva intestinalis and Sargassum muticum) presents a health risk that can be mitigated by freshwater additions

Seaweed strandings can cover several km ² with many 1000s of tonnes of biomass. They release malodorous corrosive and toxic volatiles causing respiratory illness and death. The toxic gases hydrogen sulfide and ammonia have been suggested as the main hazardous gases but limited information on the diversity and quantity of volatiles released from decomposing seaweeds is available. Here we use PTR-MS and GC-FPD analyses to quantify sulfur gases released from decaying Ulva intestinalis (Chlorophyta) and Sargassum muticum (Ochrophyta) in seawater and freshwater under hypoxic conditions for up to 18 d. The sums of sulfur gases in seawater incubations were 13,168 and 6.4 µmol g ^-1 in U. intestinalis and S. muticum , respectively. Production rates in freshwater were often significantly lower than in seawater, suggesting that freshwater addition to seaweed strandings may mitigate against the release of harmful volatiles. Analysis of the bacterial composition of decaying U. intestinalis revealed that seaweed strandings can promote the growth of facultative anaerobic pathogens and these may pose additional health hazards to the public, particularly when transmitted via bioaerosolisation. It is timely to include data on the microbial composition and flux of toxic gases when assessing the substantial health risks associated with seaweed strandings.