Re: Can chemoautotrophic organisms produce sugars?

Greetings Sven,

Here is some information that might help you in understanding the point the instructor was making in referring to Chemoautotrophic organisms and the Calvin Benson cycle. Some of the information below was was taken from the vast amount of work Cavanaugh did in the 1980's concerning the Symbiosis of Chemoautotrophic Bacteria and Marine Invertebrates from hydrothermal vents and reducing sediments.

Cavanaugh showed evidence for Chemoautotrophy using the electron microscope to visualize the bacterial symbionts and certain enzyme activity experiments to ascertain chemoautotrophy. When photoautotrophy is ruled out, the detection of carbon fixation enzymes,such as ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco), is very convincing evidence for chemoautotrophy. Rubisco is an enzyme of the Calvin-Benson cycle. Through the cycle of reactions in the Calvin-Benson energy is harnessed from photosynthesis or chemosynthesis to convert carbon dioxide into carbohydrate. The catalyst which forms the nexus between energy gathering and energy storage is Rubisco, which catalyzes Carbon fixation by carboxylation.

So, you are therefore correct in saying that they derive carbon from carbon dioxide and ATP and reducing power from the respiration of an inorganic substrate. The inorganic materials that support chemoautorophic growth include hydrogen sulfide and other reduced forms of sulfur, ammonia, nitrite, molecular hydrogen, carbon monoxide, and ferrous iron. This brings us to the substrate specificities of the chemoautorophs which permit the recognition of five major subgroups, Nitrifying bacteria which use inorganic nitrogen compounds as energy sources, Sulfur-oxidixing bacteria which use Hydrogen Sulfide, elemental sulfur, or its partially reduced oxides as engergy sources, Iron bactera which can oxidize reduced iron and manganese and the hydrogen bacteria that use molecular hydrogen as an energy source and finally carboxydobacteria which use carbon monoxide as an energy source.

He also referred to the "colorless bacteria" , I believe he is referring to the sulfur-oxidixing bacterium. These bacteria are quite interesting, most of them are small, polarly flagellated rods and are placed in the genus Thiobacillus; they occur in marine mud, and other environments. The chemoautotrophic growth of these organisms is rapid, some having generation times as short as 2 hours when growing at the expense of thiosulfate. A common striking feature of the group is their extreme acid tolerance; some species can grow at a pH as low as 1 to 2 and fail to grow at a pH above 6. These organisms are often found in special environments in which the pH is maintained at a low level by their metabolic activities, since the oxidation of reduced sulfur compounds to sulfate results in considerable acid formation. Now it gets interesting because obligate chemoautotrophy is not the rule among sulfur oxidizers as it is among nitrifying bacteria. Several thiobacilli can grow with organic carbon and energy sources; however, the utilizable substrates appear to be confined to GLUCOSE and a few amino acids.

So, there you have it some strains are capable of autotrophic growth with hydrogen sulfide using the Calvin-Benson cycle to fix carbon dioxide and others appear to lack the ability to fix carbon dioxide and are therefore oblibgate heterotrophic.

As far as the purple sulfur bacteria are concerned some (but not all) can use OTHER reduced inorganic sulfur compounds in place of hydrogen sulfide as exogenous reductants. The biochemistry of the oxidation of these reduced sulfur compounds by purple bacteria is complex and not well established. It probably is similar to the respiratory oxidation of these compounds by chemoautotrophic bacteria.

As far as Chemosynthesis is concerned I am not sure what you are looking for as far as information.. There is a tremendous amount of information concerning chemosynthesis on the web. It is a very exciting field right now with more information being found out everyday. One of the hot spots right now concerning these organisms is the Movile Cave, located in Romania, near the Black Sea coast. It is a unique environment, and until now the only cave ecosystem completely driven by chemosynthesis, which as you already know is the production of organic molecules using chemical energy rather than light. it is an extremely small and fragile environment which deserves our protection. More than 30 new species for science were described from this cave and all are endemic. Try going to the web page:

http://www.nsf.gov/od/lpa/news/publicat/frontier/5-96/5cave.htm

This site is the electronic newsletter of the National Science Foundation and contains and interesting article concering the remote cave in Romaina which was discovered in 1986.It is by far one of Earth's most unusual ecosystems.

I want to thank you for taking the time to send in a question to the Mad Scientists and I certainly hope that I was able to sufficiently answer your question.