PHANEROZOIC MICROBIAL COMMUNITIES: “COLLABORATORS” AND “PROMOTERS’ OF METAZOAN RECOVERY FROM MASS EXTINCTION

Microbial carbonates in the Phanerozoic record reflect the activities of metabolically complex consortia of microorganisms representing all the domains of life. Biofilms, microbial mats, and stromatolites are more than relicts of a pre-metazoan biosphere. In the case of reefs, various metazoan taxa have taken over as the conspicuous macroarchitects, but carbonate precipitation often has been controlled or mediated primarily by the microbial constituency. Macroscopic life arose in a microbial milieu, evolved in a microbial milieu, established complex symbiotic associations with microbes, and became increasingly chimeric as it borrowed and incorporated microbial cellular machinery. It is difficult to imagine the recovery of metazoan communities in a microbial milieu without intense evolutionary pressure to achieve new stable and mutually beneficial microbe-macrobe associations.

The idea that complex microbial communities were increasingly pushed aside and relegated to refugial extreme environments bears reexamination both in terms of microbial roles in modern ecosystems and in terms of the fossil record of detailed relationships between microbial carbonates and skeletal carbonates following both local and global disasters.

There is increasing recognition that virtually all metazoan life depends upon beneficial symbiotic associations with microbes. Pertinent to the issue of recovery is the fact that marine metazoan larvae settle and metamorphose in response to molecular cues from biofilms. Variations on the theme of sponge-calcimicrobe collaborations in Phanerozoic reef building suggest repeated co-evolutionary ventures in which carbonate production was predominantly microbial. Examples of microbial carbonate environments immediately preceding the appearance of novel metazoan taxa and communities in post-extinction intervals (particularly the Early Carboniferous, Early Triassic, and Early Jurassic) further support a collaborative and preparatory role rather than the view that microbial consortia were forced to retreat as metazoans recovered. During recovery, microbial carbonate production may shift from laminar to more complex architecture, but it is by no means clear that microbes have ever lost control of their role in calcification.