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Re: Aerobic / Anaerobic Conditions



As most microbiology text books discuss, there is a gradient from fully aerobic to fully anaerobic, and the shift from one to the other depends on the process you are discussing. 

There really aren't any assumptions here.  Lack of clarity when people don't define terms, but no assumptions. 

In general the process which defines aerobic metabolism is use of oxygen as the final electron acceptor in cytochrome respiration.  Anaerobic metabolism may still use electron acceptors, but some other molecule is the final electron acceptor, not oxygen.  Steve Diver did a good job of covering kinds of anaerobic metabolism a few days ago, so I won't repeat. 

In general, on average, the switch from aerobic metabolism to anaerobic metabolism in most organisms begins in the range around 6 mg oxygen per liter.  By the time you get to 4 or 5 mg oxygen per liter, pretty much only the facultative anaerobes are still capable of functioning, and they will be performing anaerobic metabolism, for the most part.  By the time you are lower than 2 mg/L, it's strictly anaerobic metabolism. 

As you go from the outside of a biofilm towards the middle or bottom of a biofilm, oxygen drops from aerobic levels dependent on elevation and temperature, usually around 8 to 9 mg oxygen per liter, down easily to 1 or 2 mg oxygen per liter.

Some of the earliest work done on this IN SOIL was by Sexstone and Tiedje, back in 1984 or so.   They showed oxygen dropped from fully aerobic to less than 0.2, as I recall, within just a few micrometers distance, going from outside an aggregate, to the inside of an aggregate. 

So, yes, there can be a huge mix of conditions in many environmental samples, from fully aerobic to fully anaerobic, within mere fractions of inches. 

The conditions that occur in compost can result in aggregates that would allow E. coli to thrive in anaerobic environments, while outside those aggregates, aerobic conditions were the norm, with no E. coli present.  The E. coli in the aggregates are not likely to survive for long, if the aggregate breaks open and the aerobes charge in.   

A compost tea on the other hand, is well-stirred, and well-aerated.  But still, the organisms in any liquid, compost tea included, can grow so fast  that you can outstrip the aeration capacity of almost anything if you feed bacteria enough food.

That's why the conclusion that molasses and sugar cause E. coli to grow is just silly.  We have done many tea brews where molasses was added and no E. coli were detected, while the same recipe, with a different food added, resulted in the growth of E.. coli.  It's the anaerobic conditions that are important.  Not the KIND of food. 

Yes, E. coli likes simple sugars, but as fulvic acid is used by fungi, simple sugars are released.  So, does fulvic acid cause E. coli to grow?  It can, if the amount of released food gets the bacteria growing so fast that they consume the oxygen faster than it can diffuse back into the medium. 

What has to be understood is the balance of oxygen diffusion (dependent on air pressure and temperature) and oxygen consumption by the organisms. 

So, non-aerated compost tea can cycle between aerobic and anaerobic, depending on the rate of growth of the organisms and the kind of organisms in the tea.  Can E. coli grow in anaerobic teas?  Sure, during the facultative anaerobic phases.  Will E. coli then be out-competed during the fully anaerobic periods, and the fully aerobic phases.  Yep.   So, what's the balance?  Enough to completely knock out the E. coli?   Maybe.  Maybe not.  Depends on lots of things. 

Putting an oxygen probe into soil, or compost, can be quite hazardous to the continued functioning of the sensitive and fragile membrane on the oxygen probe.  So in solids, most people protect that probe with a pipe that pulls the atmosphere  from the pile and passes it by the oxygen probe.  Carbon dioxide probes are most robust, so CO2 tends to be the preferred measurement when dealing with solids. 

Hope this helps -

Elaine Ingham
President, Soil Foodweb Inc.
www.soilfoodweb.com