Joseph E Boxhorn                       
Apr-04-93 07:13PM

Organization: Computing Services Division, University
  of Wisconsin - Milwaukee
From: jboxhorn@csd4.csd.uwm.edu (Joseph E Boxhorn)
Message-ID: <1po85cINNhnf@uwm.edu>

A couple of e-mail messages which can be best summarized as "HUH?" have 
convinced me that my discussion of the ecological importance of lithotrophy
was far too technical for many readers.  I apologize for this.  Here at
CGLS a great deal of what we do is biogeochemistry.  While I am not a 
biogeochemist, I tend to get overly enthusiastic when discussing the work
that surrounds me every day.  In addition, a desire to be brief led me to
forget that, while most of us use a technical vocabulary, for each of us it
tends to be a different technical vocabulary.  Again my apologies.

What follows in an explanation of the ecological importance of lithotrophy
and chemosynthesis.  I'm posting this as a reply to Chris Colby's question
(cited in my last post).  I have broken this down into three parts: organism
modes of nutrition, the ecological importance of lithotrophy, and its 
relevance to the question of origins.

              ORGANISM MODES OF NUTRITION

A convenient way ecologists use to classify the ecological roles of organisms
is to define their modes of nutrition--that is how they obtain carbon (for 
biomass) and how they obtain energy.  A broad division that is made is between
organisms that are autotrophs and organisms that are heterotrophs.  This
division is based upon carbon source.  Autotrophs obtain carbon from the 
environment as small, inorganic molecules,  usually CO2 or HCO3.  They use
energy from their energy source to convert this inorganic carbon into larger
organic molecules.  This is referred to as "fixing" carbon.  Carbon is most
often fixed as a small sugar (plants, for example, fix it into 3-PGA, a 
three carbon sugar).  Heterotrophs, on the other hand, obtain their carbon as
organic molecules.  The details of how they obtain it vary.  Animals ingest
their carbon in the form of other organisms.  Many other groups take up
(usually through transport systems in cell membranes) organic molecules that
are dissolved in the medium the organism lives in.

A second broad division is made by how the organism obtains its energy.  The 
best known energy source for autotrophs is light.  Photoautotrophs use energy
from light to fix carbon.  Other organisms, especially animals and fungi, get
their energy by oxidizing organic molecules.  Finally, a third group extracts
energy form simple inorganic molecules.  These are the lithotrophs.  An 
example of these are sulfate reducing bacteria in the genus Desulvibrio. 
These critters derive energy from the following reaction:

      SO4(-2) + 4 H2 ---> S(-2) + 4 H2O

Many of these lithotrophic organisms are found in habitats where oxygen, as
O2, in not present (anoxic or anaerobic environments).  This is for several
reasons.  First, a number of the inorganic compounds utilized by lithotrophs
are soluble in water only at the pH's that are common under anoxic conditions.
Additionally, many of those molecules readily combine with O2 to form 
compounds that cannot be taken up by the lithotrophic bacteria.  Third, O2
inactivates some of the enzymes involved in the energy extracting biochemical
pathways.  Finally, the amount of energy an organism can gain from these com-
pounds depends on the redox potential.  This depends on the chemical environ-
ment.

In ecology it is a common practice to measure and quantify energy uptake and 
flow in ecosystems in terms of carbon.  It is difficult in ecological systems
to measure energy in terms of BTU's, joule's or even calories.  As a practical
matter it is much easier to measure energy indirectly by measuring the uptake
and metabolism of carbon compounds.  

      THE ECOLOGICAL IMPORTANCE OF LITHOTROPHY

First, I'm going to paraphrase Chris' question.  As I see it, he is asking,
"How much of the energy that is captured and used by the biosphere comes from 
sources other thatn light?".  The main point of my earlier post was that 
lithotrophy accounted for only a minor portion of the energy available to 
living things.  I put forth three arguments for this:

1) By looking at the amount of carbon fixed in a lake during the light and
comparing it to the amount fixed in the dark, can get and indication of how
much of the energy entering a lake ecosystem is due to non-photosynthetic
processes.  Robert Wetzel found at Lawrence Lake, Michigan that carbon fix-
ation in the dark was only about 5% of that in the light.  This is consistent
with other findings in the literature.  This places an upper limit on how the
amount of energy that can come from lithotrophy.  Next, much of this dark
carbon fixation is accounted for by heterotrophic uptake.  This means that 
much of this carbon does not represent new carbon fixed, that is new energy
entering the ecosystem, but a recycling of material previously fixed by
photosynthesis.  IMHO, lithotrophy probably represents less that 1% of the 
total energy input into lake water column systems.

2) The best place to look for lithotrophs is places without free molecular
oxygen.  More lithotrophy occurs in the hypolimnion (bottom) of a lake than in
the water column above, when the hypolimnion is anoxic.  This is also true
of anoxic sediments.  In these environments we would expect organisms to be 
least likely to be dependent on light in their energy metabolism.  Most 
lithotrophs, though, couple the reduction of their energy source with the
oxidation of organic material.  After all, the electrons to reduce the energy
source have come from somewhere.  The organic material that is oxidized comes
from the water above (manna from heaven, at least if you're a bacterium in
the sediment :-)).  This organic material is produced by the photosynthetic
algae in the water column.  Thus these lithotrophs are ultimately dependent 
upon photosynthesis and light for their energy.

3) Deep sea hydrothermal vent communities seem to be an ecosystem that is 
almost entirely dependent on lithotrophy.  These are the tube worm communities
that are found near hot spots on the deep ocean floor.  I argue that these
communities are probably unimportant to the global energy budget because:
a) they are rare, and b) individual communities are rather short lived.  I 
would also note that because of the pressures involved, these are very 
difficult ecosystems to study. 



My conclusion is thus that lithotrophy is unimportant _ecologically_
as an energy source.

      WHAT DOES ANY OF THIS HAVE TO DO WITH THE QUESTION OF ORIGINS?

I posted this originally as an answer to a question by Chris Colby.  The 
thread that his query was in was discussing the second law of thermodynamics
and abiogenesis.  Some of the discussion had to do with whether light was
necessary or sufficient to begin the process of life.  Chris asked about 
the importance of other energy sources.

-- 
Joseph Boxhorn (jboxhorn@csd4.csd.uwm.edu)
Department of Biological Sciences 
and Center for Great Lake Studies
University of Wisconsin--Milwaukee