Once again, there has been a massive influx of Sargassum weed in the Caribbean. It is coming ashore on Atlantic facing beaches and forming piles up to 3m deep. As it dries and decays, hydrogen sulphide is being emitted. Why?


It would seem from a quick perusal of the literature that this is caused by sulphate-reducing micro-organisms. To quote from the eponymous Wikipedia article:

Sulfate occurs widely in seawater, sediment, or water rich in decaying organic material. Sulfate-reducing microorganisms are common in anaerobic environments where they aid in the degradation of organic materials.

which is consistent with the situation described.

In biochemical terms, these anaerobic bacteria are using sulphate rather than oxygen as the final electron acceptor (oxidizing agent) in the elctron transport chain which generates ATP through the chemiosmotic mechanism. The reaction for the reduction of the sulphate to hydrogen sulphide (dissimilatory sulphate reduction), according to the Wikipedia article already quoted, is:

dissimilatory sulphate reduction

A simple way of thinking about this is that in both aerobic and anaerobic respiration the oxidizing agent itself becomes reduced. Whereas oxygen is reduced to H2O, sulphate is reduced to H2S.

  • $\begingroup$ Thanks. I think you’re right. That suggests that agitation might stop formation of the gas. $\endgroup$ – Dave May 23 '18 at 22:35
  • $\begingroup$ Decomposing plants in fresh water also generate H2S. Anaerobic bacteria in deep underground strata generate H2S. , Also, sometimes in home water wells pumping water from shallow strata. The darn bacteria are everywhere. $\endgroup$ – blacksmith37 May 24 '18 at 1:19
  • $\begingroup$ What’s the best way to stop the process? Add oxygen (air)? $\endgroup$ – Dave May 24 '18 at 10:54
  • $\begingroup$ @Dave No, remove the rotting stuff. $\endgroup$ – RHA May 24 '18 at 19:04
  • $\begingroup$ @Dave — I personally wouldn’t know, but if you could arrange to fly me out to the Caribbean I would do my best to help. ;-) $\endgroup$ – David May 24 '18 at 19:09

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