Why is Anaerobic soil bad for my sealed ecosystem? It could be for two reasons. It will produce Methane and Hydrogen Sulfide. So That's what I looked into today. Here I will share what I learned about anaerobic respiration in soil. Here's my report.
Methane is the byproduct of bacteria using it's last choice (CO2) of electron acceptor for anaerobic respiration. Likewise Hydrogen Sulfide (H2S) is the product of anaerobic bacteria that is at it's second to least favorable electron acceptor (Suflate or SO4). Both gases produced by this anaerobic respiration would be harmful to organisms in the sealed container, as it would build up in the air space and then the water creating a toxic environment.
As the bacteria runs out of good options it has to choose electron acceptors aren't as efficient at supplying energy so their metabolism is much slower. By the time the bacteria are using Sulfate, they have exhausted (or don't have access to?) Oxygen, Nitrate, Manganese or Iron, in that order. The bacteria are consuming Sulfate and generating toxic Hydrogen Sulfide.
Energy yield difference shown in Y and also it's the order the anaerobic bacteria will utilize that compound and to the right, some metabolism equations for each electron acceptor.
When anaerobic bacteria are using CO2 as their electron acceptor they have exhausted their sources of N03, Manganese, Iron, and additionally Sulfate.
Bacteria using CO2 for respiration rely on anaerobic activity that utilized those better options for electron acceptors at some point (O2 -> NO3 -> Manganese -> Iron -> Sulfate). In order for the anaerobic bacteria to utilize CO2 it rely's on anaerobic bacteria that at one point did use the previous sources electron acceptors. Or for one reason or another, through lack of access or being unable to compete for better electron acceptors are using their byproducts now. Either way, The anaerobic bacteria utilizing CO2 need compounds created by previous anearobic bacteria that once fed on NO3, Manganese, Iron or Sulphate.
Better illustrated here:
The three reactions I know of so far to nuetralize the harmfulness of Methane:
Anaerobic: Using Nitrate and Nitrite:
CH4 + 4NO3- --> CO2 + 4NO2- + 2H2O
3CH4 + 8NO2- + 8H+ --> 3CO2 + 4N2 + 10H2O
Anaerobic Oxidation of Methane (AOM) isn't ideal because if methane is being produced the soil is already severely lacking in Nitrate. Using Nitrate for respiration is the second thing bacteria utilizes once it runs out of Oxygen. The first equation takes 4 Nitrates and returns 3 Nitrites, a loss. With the second reaction, the Sealed ecosystem loses 8 Nitrites. Or so it seems... But those 4N2's will go feed the Duckweed that will turn every one H2 into 2 NH3's, returning the Nitrogen to the system.
Anaerobic: Using Sulphate:
CH4 + (SO4^2-) --> (HCO^3-) + HS– + H2
This uses Sulphate (SO4) which is good because utilizing sulfate is slightly more favorable than utilizing CO2 and will produce Hydrogen Sulfide if not broken down at this point (or used by plants).
CH4 + 2O2 --> CO2 + 2H20
This is better. It requires 2 Oxygen for every Methane molecule and yields one molecule of CO2. We "lost" one Oxygen. Think about it like this, normally in the cycle of Aerobic/plant respiration every one molecule of O2 an organism breathes produces one molecule of CO2. For every one molecule of CO2 a plant breathes it produces one O2. So, it technically is bad for aerobic organisms first, because 2 oxygens are not available and only slightly better for the plant which has 1 co2 instead of later having 2 after the aerobic organism respired.
These three reactions produce Carbon Dioxide, Nitrite, Inert N2 and Water. But the aerobic reaction is better as it wouldn't lead to further Nitrogen starvation of the soil, but results in a loss of one Oxygen for every Methane molecule reduced. The aerobic reaction is a net loss in sustainability between the O2 <---> CO2 respiration cycle.
While the anaerobic reaction would deplete my soil that's already close to depleted of Nitrite and Nitrate, leading to eventual plant decline and death due to nitrogen starvation. This result is the worse of them as plants are the a cornerstone to my sealed ecosystem.
Onto Hydrogen Sulfide...
Hydrogen Sulfide is produced by anaerobic organisms using Sulfate as their electron acceptor. I haven't found a reaction that breaks H2S down anaerobically but I did find this one that can do it aerobically if it can escape through the soil and into an aerobic layer or water. As you can see it produces 1 CO2 and 2 HSO. Very good indeed, though we do "lose" an oxygen again. Oh well. I'll take what I can get.
CH4 + 2 O2 = CO2 + 2 H2O.
"Losing" nutrients, O2 or CO2 etc is unavoidable in a closed system. For every change of energy from one organism to another or one chemical reaction to another energy is going to be lost and compounds are going to break down further and further until something runs out that "breaks the camels back" and largely collapses the system. This is just the way things are.
But the more I look into each part in depth the more I can tweak the system to make it last a little bit longer.
Name of the game is the same: Limit anaerobic activity, emulate the ecosystem as closely as possible, and include lots of diversity within the system.
Bump: Truth is, despite all I learned today. There's really no way to tell how all that will come together in my sealed 1 gallon glass ecosystem. And sadly, I after all that reading, I don't feel like it got me any closer to creating a more balanced system. I'm just going to keep doing what I have been doing. Emulate nature as best I can, all I can do. I'm no scientist, I can't say what effect all these minute things I just talked about would have.
I guess I do know more about what's going on within my system with the anaerobic soil. I'm just not any closer to being able to control it like I was hoping reading about it would allow me to. Bacteria gonna bacteria, sigh. Oh well, I'll stick with my method figuring out a creative way to keep the soil oxygenated. If you're interested I am keeping a Journal in that section.