Microbial Consortia

Structurally, prokaryotic organisms are far simpler than eukaryotic life forms. Yet, prokaryotes are functionally or metabolically diverse with regards to the reactions they mediate or stresses they can endure. Prokaryotes do not possess membrane-bound organelles such as a nucleus, endoplasmic reticulum, mitochondria, or chloroplasts. Therefore, prokaryotes can not separate metabolically incompatible biochemical processes into discrete compartments. The inability to compartmentalize cellular processes poses a bottleneck for the biochemical and structural evolution of prokaryotes. As a result, prokaryotes have diversified with regards to biochemical abilities while remaining structurally simple.

The prokaryotic solution to the packaging problem is the formation of associations with other organisms the promote protection from potentially inhibitory environmental factors. These include exposure to adverse oxygen concentrations, ltraviolet radiation, desiccation, adverse pH, and Eh. The steep biogeochemical gradients that exist in mats allow and select for functional diversification such that fully organisms possessing aerobic, microaerophilic, and anaerobic requirements may co-exist and contemporaneously function along a gradient. Such dramatic environmental changes occurring in a small spatial scale set up association that facilitate mutualistic nutrient, gas, and metabolite exchange. Associations reflect synergistic or syntrophic lifestyles where growth and biogeochemical cycling are conducted more effectively and efficiently than on an individual population basis. Such associations may be termed microbial consortia. Functionally, a consortium exceeds the sum of its parts. Members of the consortium maintain metabolic and ecological compatibility, as long as biogeochemcal and environmental gradients allow for individual niches to exist in close proximity. Microbial mats typify these conditions and, accordingly, are the focus of research on consortial growth strategies in extreme environments.

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!