Joseph E Boxhorn Apr0493 07:13PM of Wisconsin Milwaukee A couple of email messages which c

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Joseph E Boxhorn Apr-04-93 07:13PM Organization: Computing Services Division, University of Wisconsin - Milwaukee From: (Joseph E Boxhorn) Message-ID: <> 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 ( Department of Biological Sciences and Center for Great Lake Studies University of Wisconsin--Milwaukee


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