Fossil microorganisms and land plants: Associations and interactions

Microorganisms are critical in the bio- and geosphere today, and certainly performed similar functions in ancient ecosystems. Bacteria, cyanobacteria, microalgae, and various fungi and fungi-like organisms constitute a substantial component of these ancient communities, and have been responsible for the evolution and sustainability of the ecosystems in functions ranging from decomposition of metabolites to catalyzation of nutrient cycles. This review provides examples of associations and interactions between microorganisms and land plants, principally from the Devonian and Carboniferous. During this time span of approximately 150 myr, most of the vascular plant lineages evolved and radiated into new terrestrial niches. Several exceptionally well-preserved fossil communities are used to demonstrate a wide range of biological interactions. Although none of the land plant partners exist today, many of the microorganisms involved appear morphologically little changed. Moreover, some interactions suggest that the genetic code and biochemical pathways necessary for the associations and interactions to be successful evolved early in the lineages of microorganisms involved, and have seemingly remained unchanged to the present. The examination of microorganism/land plant associations (and interactions) provides another level of biological resolution that can be used to track coevolutionary processes and help formulate hypotheses designed to more fully understand the evolutionary history of ecosystems.