On Wednesday’s class (September 16, 2009), there was a brief discussion regarding the respective evolutionary order of chemotrophs and phototrophs. As a refresher, phototrophs are organisms that use photosynthesis to acquire energy whereas chemotrophs derive energy from chemicals. Under this latter heading fall the chemoorganotrophs, which derive energy from organic chemicals and the chemolithoautotrophs, which derive their energy from inorganic energy sources (e.g., reacting with chemical compounds such as carbon dioxide or hydrogen sulphide).
Chemolithoautotrophs are almost exclusively of the Bacteria and Archaea domain, living in hostel environments such as deep sea vents and other hydrothermal systems. Have you seen photographs of the 7+ ft. giant tube worms found at the ocean’s depth? Here is a photo to whet your appetite:(Photo: http://ucsdnews.ucsd.edu/newsrel/science/GaasterlandGenomeCenter.asp)
Thermophiles survive in temperatures above 45° C (113° F) and serve as primary producers in these thermal ecosystems. In the truest sense, these organisms are Fierce Invalids from Hot Climates (with a nod to author Tom Robbins).
Evolutionary biologists have long believed that chemotrophs evolved in the oxygen-poor environment before phototrophs, creating the chemical and climatic conditions for the evolution of aerobic organisms through the release of oxygen as a chemical by-product.
More recently, some scientists are suggesting that this causal linkage is not as tight as it may seem. In the June 2001 issue of the journal, Trends in Microbiology (Vol.9 No. 6), author Beverly Pierson argues that, “interpreting the metabolism of microorganisms in ancient thermal environments is…very difficult” and points to (1) photosynthetic organisms recently found in deep sea vents and (2) the evidence that both chemotrophs and phototrophs exist concurrently in both ancient and modern hydrothermal ecosystems. In short, she suggests that we don’t have substantial evidence to assert with absolute certainty that chemotrophs evolved before phototrophs.
To read the whole journal article, you can follow this rather long link from the U of M Library website: http://www.sciencedirect.com.proxy.lib.umich.edu/science?_ob=MImg&_imagekey=B6TD0-4379RMW-R-1&_cdi=5184&_user=99318&_orig=search&_coverDate=06%2F01%2F2001&_sk=999909993&view=c&wchp=dGLbVtb-zSkWA&md5=93eb460fbc6a7879da4934941d4c95b3&ie=/sdarticle.pdf