The close spatial coupling and metabolic interdependence of microorganisms in extreme environments begs the question of whether or not there exist pairs or groups of organisms that are specifically dependent on one another because of the traits each has evolved (i.e. have consortial members co-evolved?). Alternatively, do consortia represent functional relationships of opportunity in which participants are not particular from whom they derive their needs? There are many examples of obligate symbiotic relationships between prokaryotic and eukaryotic organisms, e.g. rhizobia and leguminous plants, the Anabaena-Azolla sp. (cyanobacterial-fern) association, methanogens and sulphur bacteria with anaerobic ciliates, and sulfur bacteria and Rivularia worms near deep-sea hydrothermal vents. In each of these examples, a strong case may be made for co-evolution, whereby strong selective pressures for highly specific traits allow the pair to exploit a particular niche and survive. We do not know whether similar relationships, as opposed to opportunistic encounters, characterize microbial consortia. There are cases in which specific associations between prokaryotes have been documented. Most widely known is the relationship between heterotrophic bacteria (Pseudomonas sp. and Zoogloea sp.) with the heterocysts of an Anabaena sp. Additionally, there are instances of 'metabolic coupling between specific bacterial strains and the cyanobacteria, Oscillatoria redekii. Anecdotal evidence includes the fact that many cultured cyanobacterial isolates are lost and no longer viable once their bacterial associations are completely removed. The evolutionary basis for the development of microbial consortia is a an under explored area of research!