Low Oxygen Substrate turns gray / black. Why?

[taricha]

Right is grungy substrate from my tank, sealed and carbon dosed. Left are two samples that are left open but very small water-air interface - light one is the control, and the dark one is carbon dosed (vodka) in excess. Consumption of Carbon rapidly depletes oxygen. The color difference is apparently due to the super-low oxygen levels.

But Why?
Color changes can tell us a lot about chemistry, but I don't know what specifically this very obvious change is telling me.
guesses...

1. Dead aerobic organisms turn black? If so, why?

2. aerobic organisms are replaced by anaerobes? ok, but why would anaerobes have much darker pigments?

3. change in metal species from oxidized to reduced? Which metals are likely here? Fe? Aren't these trace metals super low concentration? that's a lot of color change!

4. Change in form from oxidized to reduced of some nonmetal causing this color change? like what? sulfur?

 

[Val Shebeko]

Good question

 

[brandon429]

Have no idea great job

 

[brandon429]

See that’s a good test for aquabiomics.

 

[Val Shebeko]

See that’s a good test for aquabiomics.

Like Lucy, can you do some ‘splainin?

 

[taricha]

See that’s a good test for aquabiomics.

Unless it's not bacteria responsible!

 

[Greg P]

Something I've never though about before. I just accepted it as a 'thing that happens', but always assumed it was organic in nature.

Wiki states;
Hydrogen sulfide reacts with metal ions to form metal sulfides, which are insoluble, often dark colored solids.

 

[Randy Holmes-Farley Verified Community Expert]

The black is heavy metal sulfide precipitation.

 

[Randy Holmes-Farley Verified Community Expert]

http://reefkeeping.com/issues/2005-12/rhf/index.php

Stability of Hydrogen Sulfide in Water: Metal Sulfide Precipitation

Sulfide in seawater is also unstable toward precipitation with certain metals. The black deposits often seen in anoxic sediments are typically metal sulfides, especially ferrous sulfide (FeS) and pyrite (FeS2), with much smaller amounts of copper, manganese, zinc, nickel and cobalt sulfides. The exact processes whereby these metal sulfides form in marine sediments and elsewhere is complicated and still under study.6 In some areas, like the Orca basin in the Gulf of Mexico, deposited iron sulfides make up as much as 0.7% of the sediments' mass.7 So, iron sulfides are not necessarily a trace component.

 

[Randy Holmes-Farley Verified Community Expert]

Unless it's not bacteria responsible!

Anaerobic respiration by bacteria converts sulfate into sulfide, which then turns black when it combines with heavy metals.

 

[brandon429]

toxicity test too / where’s sacrificial cerith

a snail wont die crawling around in aerated muck, but I’m interested in knowing if differing forms of detritus / destratified sandbed states can kill when upwelling occurs. Clearly a snail put into the vial will die in rot water but is that substrate toxic when exposed to typical aerated and safe tank water scaled down, transferred into an otherwise legit snail chamber

 

[Randy Holmes-Farley Verified Community Expert]

toxicity test too / where’s sacrificial cerith

a snail wont die crawling around in aerated muck, but I’m interested in knowing if differing forms of detritus / destratified sandbed states can kill when upwelling occurs. Clearly a snail put into the vial will die in rot water but is that substrate toxic when exposed to typical aerated and safe tank water scaled down, transferred into an otherwise legit snail chamber

Some substrate organisms have defenses against hydrogen sulfide. I show some tox data in the article linked above.

 

[Randy Holmes-Farley Verified Community Expert]

http://reefkeeping.com/issues/2005-12/rhf/index.php#11

Organisms with Special Tolerance to Hydrogen Sulfide

Many marine species live in close proximity to sediments that often contain hydrogen sulfide. Some even live in them. From the data in Tables 2 and 3, it is clear that the range of susceptibility to hydrogen sulfide poisoning is huge, and those species more prone to natural exposure often have higher tolerance. How do they accomplish that? Likely by more ways than are presently known, but even so, some of these organisms' mechanisms are known.

In terrestrial mammals (dogs, cats, rabbits), hydrogen sulfide is primarily detoxified by reaction with oxyhemoglobin to form colloidal sulfur, hemoglobin and water.17 However, if hydrogen sulfide concentrations are high, or if oxygen concentrations are low, that mechanism is not adequate. In fact, conditions of anoxia increase the negative effects of hydrogen sulfide on many aquatic organisms. 15,16,18

In the marine worm Urechis caupo, for example, it has been shown that excess hydrogen sulfide is oxidized to thiosulfate (-SSO3-).19 The thiosulfate is then allowed to passively diffuse out of the organism's hindgut. Interestingly, this worm also appears to make hydrogen sulfide out of sulfur-containing amino acids and use it as a gasotransmitter, controlling its body wall's muscle tone.20 Whether this mechanism also helps it limit exposure to ambient hydrogen sulfide is unclear.

The marine worm Halicryptus spinulosus has an even more elaborate system. During exposure to hydrogen sulfide under oxic conditions, it oxidizes the hydrogen sulfide to thiosulfate, just as the Urechis caupo does. Under anoxic conditions, it has a multi-pronged strategy. Its first defense is to supply iron for precipitation of iron sulfide on its surface and in its blood. Under these conditions, the animal's surface and its blood blacken considerably, but this process is reversible once oxic conditions return. Finally, these worms apparently bind sulfide to an unidentified chemical in the hemolymph, providing additional protection for its mitochondria.

Another way that organisms detoxify hydrogen sulfide is to incorporate bacterial symbionts that themselves detoxify the sulfide. Some bivalves and annelids, for example, have special hemoglobin structures that bind sulfide, thereby detoxifying it, and that also serve to transport it to such bacteria that can remove the sulfide from the hemoglobin.21

 

[taricha]

The black deposits often seen in anoxic sediments are typically metal sulfides, especially ferrous sulfide (FeS) and pyrite (FeS2), with much smaller amounts of copper, manganese, zinc, nickel and cobalt sulfides

That's fascinating. Excellent.
given that Sulfur (as sulfate) is so abundant (SOEST hawaii)...

Sulphate 2.712 g/Kg

Then I'm guessing the limiting factor in how much dark material is produced (assuming enough oxygen depletion) is likely the amount of these metals?

I'll be curious to see if adding trace metal supplement products visibly increases that dark material production.

 

[Randy Holmes-Farley Verified Community Expert]

That's fascinating. Excellent.
given that Sulfur (as sulfate) is so abundant (SOEST hawaii)...


Then I'm guessing the limiting factor in how much dark material is produced (assuming enough oxygen depletion) is likely the amount of these metals?

I'll be curious to see if adding trace metal supplement products visibly increases that dark material production.

I don't know what typically limits the amount of precipitate, but would guess it is the metals when substrate is anoxic.

 

[taricha]

I'm guessing the limiting factor in how much dark material is produced (assuming enough oxygen depletion) is likely the amount of these metals?

Or maybe there's plenty of metal to produce tons of dark color.
it looks like the amount of dark color in the material under these conditions may be more of a (weak) indicator of the amount of Organic material.

The two on the right got 1 drop of vodka per 10mL water - a huge organic carbon dose.
The two on the left got nothing. Unsealed, stayed the original color, the far left sealed has gotten slightly darker.

So here's my attempt to make sense of it. From Randy's article...
"In general, as seawater becomes depleted of oxygen, a series of chemical transformations takes place, largely due to biological activity continuing to consume oxygen and other electron acceptors....

As mentioned above, this order of electron acceptors used to oxidize organic material is oxygen (O2), then nitrate (NO3-), then manganese (Mn++++), then iron (Fe++), then sulfate (SO4--)....

Exactly what processes take place at what depths in sediments depends on many factors, such as the nature of the sediments themselves (size distribution and chemical makeup), the amount of organic material being deposited and the temperature."

and quoting out of order
"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."

So the far left has some oxidizable material, being sealed it went through the O2 and has started on the others and has gotten slightly darker as some metal sulfides are created as the sulfate got used.
Second from left had in principle the same amount of material to be oxidized but slow diffusion of O2 from surface was apparently enough to prevent much of the other electron acceptors from participating and so no sulfate used no metal sulfides produced and darkening did not occur. The two on the right, sealed or unsealed made little difference because so much Organic Carbon was being processed that all the oxygen was long gone and lots of sulfate was used and metal sulfides created.

hmm... @Dan_P a simple, (but probably very weak) indicator of organic material in a substrate sample.

 

[Dan_P]

hmm... @Dan_P a simple, (but probably very weak) indicator of organic material in a substrate sample.

Hey great potential for a working thread!

Let’s appeal in a new post to our fellow citizen scientist reefers to have them test their substrate this way, of course, after the test is sharpened up a bit and we include a couple questions about their system. The results would be an interesting survey of substrates.

I imagine if there is a control vial with fresh substrate , we can analyze the submitted photographs of test results for color and color intensity.

 

[ss30]

The black is heavy metal sulfide precipitation.

That will be why it smells so bad when you stir it up then.

 

[taricha]

That will be why it smells so bad when you stir it up then.

Absolutely. The containers that turned darker have quite the noticeable sulfur odor.

 

[Randy Holmes-Farley Verified Community Expert]

Or maybe there's plenty of metal to produce tons of dark color.
it looks like the amount of dark color in the material under these conditions may be more of a (weak) indicator of the amount of Organic material.

The two on the right got 1 drop of vodka per 10mL water - a huge organic carbon dose.
The two on the left got nothing. Unsealed, stayed the original color, the far left sealed has gotten slightly darker.

So here's my attempt to make sense of it. From Randy's article...
"In general, as seawater becomes depleted of oxygen, a series of chemical transformations takes place, largely due to biological activity continuing to consume oxygen and other electron acceptors....

As mentioned above, this order of electron acceptors used to oxidize organic material is oxygen (O2), then nitrate (NO3-), then manganese (Mn++++), then iron (Fe++), then sulfate (SO4--)....

Exactly what processes take place at what depths in sediments depends on many factors, such as the nature of the sediments themselves (size distribution and chemical makeup), the amount of organic material being deposited and the temperature."

and quoting out of order
"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."

So the far left has some oxidizable material, being sealed it went through the O2 and has started on the others and has gotten slightly darker as some metal sulfides are created as the sulfate got used.
Second from left had in principle the same amount of material to be oxidized but slow diffusion of O2 from surface was apparently enough to prevent much of the other electron acceptors from participating and so no sulfate used no metal sulfides produced and darkening did not occur. The two on the right, sealed or unsealed made little difference because so much Organic Carbon was being processed that all the oxygen was long gone and lots of sulfate was used and metal sulfides created.

hmm... @Dan_P a simple, (but probably very weak) indicator of organic material in a substrate sample.

Very interesting experiment!