Reef Chemistry Question of the Day #25 Hydrogen Sulfide 1

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #25

As many reefers know, hydrogen sulfide is a toxic molecule that can form when water becomes stagnant and anaerobic. It can happen in a sand bed if organic matter gets covered over and degrades anaerobically, or even in bulk water that has no oxygenation (such as in a pipe when the flow stops for long enough).

There will be another question tomorrow relating to hydrogen sulfide, but today's question is...

Where does the sulfur atom come from that is the centerpiece of hydrogen sulfide (H₂S).

Good luck.







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[redfishbluefish]

Catabolism of proteins....specifically the two thiol containing amino acids....cycteine and methionine.

 

[Randy Holmes-Farley]

Just a hint...there are more good answers available.

 

[leptang]

Several groups of bacteria can use hydrogen sulfide as fuel, oxidizing it to elemental sulfur or to sulfate by using dissolved oxygen, metal oxides (e.g., Fe oxyhydroxides and Mn oxides) or nitrate as oxidant.

 

[leptang]

Bacteria that converts it from water and organic material is my answer.

 

[DRThompson]

Organic sources: bacteria, algae, fishfood

How about all the inorganic forms: Magnesium sulfate, Potassium sulfate, cobalt sulfate, copper sulfate, manganese sulfate. I'd say there's plenty of sulfur to go around

 

[Randy Holmes-Farley]

Any more thoughts?

 

[cee]

The burning of coal

 

[SantaMonica]

Would seem to be a source that takes the lowest amount of energy for the bacteria to get the sulfur from.

 

[Randy Holmes-Farley]

Lots of very good answers!

There are actually two main ultimate sources of sulfur for hydrogen sulfide.

The first, and the main one that applies also in freshwater systems, is the anaerobic degradation of organics that contain sulfur, such as the amino acids cysteine and methionine as Redfishbluefish pointed out.

The second, and possibly the one that produces more hydrogen sulfide in marine systems, is the conversion of sulfate to hydrogen sulfide from the anaerobic degradation of any organic material. Just as reefers know that nitrate can be used in this way as a source of oxygen (i.e., as an electron acceptor) to produce N2 gas, sulfate can also be used, albeit with more difficulty. So bacteria will usually use the nitrate first, and if organics are still available and the nitrate runs out, sulfate can be used (as DRThompson pointed out).

Here's a section from the article linked below detailing that process:

Hydrogen Sulfide Production

All organisms oxidize organic compounds in various ways to gain energy or to produce new organic biomolecules that they require. Most of the large organisms kept in reef aquaria (fish, corals, algae, etc.) perform this oxidation using oxygen (O2) as the ultimate electron acceptor in the process. For example, the oxidation of methane, CH₄, to CO₂, can be done with oxygen:

CH₄ + 2O₂ --> CO₂ + 2H₂O

The reason that oxygen is called an electron acceptor is that in the reaction above, electrons are transferred from the carbon and hydrogen atoms to the oxygen atoms.

Organisms typically carry out more complicated oxidations, but the process is the same. The equation below shows the overall chemical reaction involved in the oxidation of organic molecules with oxygen, shown specifically for an organic molecule representing typical plankton.8

(CH₂O)₁₀₆(NH₃)16(H₃PO₄) + 138 O₂ --> 106 CO₂ + 122 H₂O + 19 H⁺ + PO₄^--- + 16 NO₃^-

Under conditions where O₂ is limited in supply, organisms need to turn to other electron acceptors. These include nitrate (the way that nitrate is reduced by a deep sand bed), various metals such as iron and manganese, and most important to the context of this article, sulfate. For organisms that use sulfate instead of oxygen, the oxidation can be described as:

CH₄ + SO₄^-- --> CO₂ + S^-- + 2 H₂O (which is the same as HCO₃^- + HS^- + H₂O)

The more generalized reaction for a typical "organic" can be described as:

(CH₂O)₁₀₆(NH₃)₁₆(H₃PO₄) + 53 SO₄^-- --> 56 CO₂ + 50 HCO₃^- + 53 HS^- + 16 NH₃ + 53 H₂O + PO4^---

Note that the ammonia produced is not oxidized to nitrate under these conditions as it is under aerobic conditions. Each of these processes produces different amounts of energy for the organisms involved. Oxidation with O2 produces the most energy, followed by nitrate, then manganese, iron, sulfate and finally carbon dioxide with methane as the product. Organisms (or more correctly, ecosystems that evolve containing many different organisms) often get as much energy as they can from a food supply, so until the O₂ runs out, few organisms carry out anything other than oxidation processes. The other processes are each carried out in turn, either chronologically in certain situations, or with depth into a substrate. Sulfate is one of the last usable electron acceptors available in seawater. However, it is also available in far higher concentration (2700 ppm) than any of the other acceptors (which are often sub ppm), so it can sometimes be used in parallel with all the other electron acceptors besides O₂.

These processes are typically carried out by bacteria and archaea under low oxygen conditions. They take up sulfate via transporters in their cell membranes, and then use it internally in a series of separate chemical processes ending with it being transformed into hydrogen sulfide.

In addition to these biological processes, purely chemical reactions also produce hydrogen sulfide in the ocean. The heat of hydrothermal vents, for example, can drive the reaction between organic materials and sulfate to produce hydrogen sulfide. While the process can theoretically proceed at as little as 25°C, it is so slow at that temperature as to be unimportant. At 200°C, typical reactions along these lines can take months to years, and it has not been demonstrated at less than 125°C. Such reactions should consequently have minimal importance in most reef aquaria.

This has more:

Hydrogen Sulfide and the Reef Aquarium by Randy Holmes-Farley - Reefkeeping.com

Happy Reefing.