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Biogeochemistry of Earth before exoenzymes
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Journal Article
Biogeochemistry of Earth before exoenzymes
2023
Overview
Microorganisms that transform and oxidize organic material (that is, heterotrophs) play a fundamental role in the geochemical cycling of key elements in the ocean. Through their growth and activity, heterotrophic microorganisms degrade much of the organic matter produced by phytoplankton in the surface ocean, leading to the regeneration and redistribution of nutrients and carbon back into the water column. However, most organic matter is physically too large to be taken up directly by heterotrophic microorganisms. Consequently, many heterotrophs secrete exoenzymes that break down large molecules outside the cell into smaller substrates that can then be directly taken up by the cell. The complex nature of the biochemical systems that microorganisms use to secrete these enzymes suggests that they were unlikely to have been present in the earliest heterotrophs. In a pre-exoenzyme ocean, heterotrophic microorganisms would only be able to access a small fraction of organic matter such that most dead phytoplankton biomass would have passed directly through the water column and settled onto the seafloor. Here we synthesize existing geobiological evidence to examine the fate of organic matter in the absence of exoenzymes in early oceans. We propose that on an Earth before exoenzymes, organic matter preservation, metal availability and phosphorus recycling would have operated differently than they do on the contemporary Earth.Exoenzymes produced by heterotrophic microorganisms early in Earth history helped unlock previously unavailable organic matter and transformed ocean geochemistry.
Publisher
Nature Publishing Group
Subject
