Subject: Re: Aerobic / Anaerobic Conditions

From: Elaine Ingham <[email-masked]>
Date: Mon, 25 Nov 2002 23:56:28 EST

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