MICROBIAL ROLES: ADAPTATION OF MICROBES TO AN EVOLVING PLANETARY SETTING

Microbes were the earliest form of life on Earth. Microscopic indications of their presence include fossilized biofilms, filaments, bacilli or cocci morphotypes. Macroscopic indications are oncolites, thrombolites or stromatolites. Abundant stromatolites appear approximately 2.7 Ga. From that time forward they play a major role forming extensive reef-like platform carbonate units. It may be significant that the abrupt appearance of platform carbonates occurs shortly after the development of the first large continents and the first intracratonic basins. Prior to this time evidence for microbial activity in the form of stromatolites is minimal and equivocal.

Earliest stromatolites represent the remains of bacterial mats. It is only later in the geologic record that algae become an important component. The disappearance of stromatolites in most habitats appears to be associated with the rise of invertebrate grazers and substrate disturbers. Concomitant with the demise of these structures was the increasing diversity of invertebrates and vertebrates. With this change, the question arises as to whether or not there were potentially new life modes available for opportunistic microbes. The answer is yes, and in particular it appears that calcium carbonate exoskeletons of bryozoans, if not other invertebrates, provided substrate opportunities. Other invertebrates with similar exoskeletons could serve as alternative substrates. The advantages are increasing surface areas for incrusting and providing a variety of substrates above the sediment-water interface. Our preliminary studies show that the bryozoan exoskeleton supports a “garden” of microflora that includes filaments, diatoms, coccoliths, bacilli and cocci morphotypes, and extensive deposits of biofilm. The microbes fossilize in a similar manner to those found on the surfaces of rocks and other sediments. The fossilized forms indicate the presence of the following ions: Ca, Mg, Fe, Al, with lesser levels of K and P.

Microbial roles have changed, and as the Earth evolves, opportunistic microbes will adapt.