Health risks associated with seaweed strandings: sulfur gases and antimicrobial resistant (AMR) bacteria from decaying Ulva intestinalis and Sargassum muticum.

Seaweed strandings can cover several km2 with many 1000s of tonnes of biomass and are of international concern. They have a substantial environmental impact locally, including through the release of 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 proton-transfer reaction mass spectrometry (PTR-MS) and gas chromatography with flame-photometric detection (GC-FPD) 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 amounts of sulfur gases in seawater incubations were 131,653 and 64 nmol g-1 seaweed freshweight in U. intestinalis and S. muticum, respectively. Production rates in freshwater were often significantly lower (range 68-99% decrease) 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 including antimicrobial resistant (AMR) bacteria and these may pose additional health hazards to the public. It is timely to monitor the microbial composition and flux of toxic gases when assessing and mitigating the substantial health risks associated with seaweed strandings.