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Authors: Andrew H. Knoll
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small black crusts dotting a tidal flat of lime mud laced with cyanobacterial mats. The crusts formed in the upper part of the intertidal zone, built by small spheroidal cyanobacteria that secreted extracellular sheaths elongated in the downward direction ( plate 2d ). That’s right. Here, in a place predicted by Proterozoic rocks, we found the modern counterpart we sought—living but hitherto undescribed cyanobacteria whose morphology, life cycle, and environmental distribution match the ancient Polybessurus bipartitus .
The Spitsbergen example isn’t an isolated instance of good luck. Steve’s former graduate student Assad Al-Thukair (now at King Faisal University in Saudi Arabia) has discovered a half dozen new species of cyanobacteria that bore into and live within ooid grains; nearly all have exact fossil counterparts in silicified Proterozoic ooids of Spitsbergen and eastern Greenland. Because these cyanobacteria display stereotyped patterns of boring, even behavior can be included in the list of features shared by living and fossil populations. Steve and Montreal University’s Hans Hofmann have drawn equally fine-scale ancient-modern comparisons between mat-building cyanobacteria found today on arid tidal flats and fossils that lived in comparable environments 2 billion years ago.
Collectively, these discoveries put some teeth into the old saw that many Proterozoic fossils look like cyanobacteria. Because habitat range is a direct function of physiology, the close environmental similarity between ancient and living cyanobacteria suggests that the microorganisms distributed across Spitsbergen (and other Proterozoic) tidal flatswere essentially modern in morphology, life cycle, and physiology. Many of the cyanobacteria we see today are indeed survivors from the ancient Earth.
Cyanobacteria are common today in coastal habitats where very salty water or other environmental challenges restrict invasion by animals. By coincidence, the chert nodules in Proterozoic carbonates also center on coastal environments where silica was precipitated much like salt from evaporating seawater. Thus, chert’s paleontological lantern shines most brightly on just those environments where cyanobacteria have always thrived. Cyanobacteria do not live alone on modern tidal flats, however; mat communities contain a host of other organisms, especially bacteria. Why don’t we see this greater microbial diversity in chert nodules?
Tidal flats are harsh environments. At low tide, their inhabitants must tolerate the desiccating glare of a blazing sun; salty water provides an osmotic trial when the weather is dry; fresh water does the same during storms. Cyanobacteria respond to these challenges by secreting an extracellular envelope that protects the cells inside. That envelope is of particular importance to paleontologists because, unlike the cells within, it resists bacterial decay after death. Cyanobacteria, then, have the microbial equivalent of a clamshell, and in tidal-flat cyanobacteria this feature is especially well developed. While other bacteria live on tidal flats, most lack preservable walls or envelopes. And to make matters worse, they are tiny and have simple shapes that frustrate biological interpretation. The very fact that preserved fossils show evidence of postmortem decay means that heterotrophic bacteria must have lived in tidal-flat environments. As discussed below, geochemical signatures enable us to identify at least a few of these populations, but we must face the fact that what we see preserved in thin sections of chert, while extraordinary, represents only a limited sampling of the microorganisms that lived along the Proterozoic shoreline.
Fortunately, the sample preserved best is one well worth understanding. Cyanobacteria are the working-class heroes of the Precambrian Earth—the main primary producers in early oceans and the source of the oxygen that transformed terrestrial environments. We know a great deal
The Spitsbergen example isn’t an isolated instance of good luck. Steve’s former graduate student Assad Al-Thukair (now at King Faisal University in Saudi Arabia) has discovered a half dozen new species of cyanobacteria that bore into and live within ooid grains; nearly all have exact fossil counterparts in silicified Proterozoic ooids of Spitsbergen and eastern Greenland. Because these cyanobacteria display stereotyped patterns of boring, even behavior can be included in the list of features shared by living and fossil populations. Steve and Montreal University’s Hans Hofmann have drawn equally fine-scale ancient-modern comparisons between mat-building cyanobacteria found today on arid tidal flats and fossils that lived in comparable environments 2 billion years ago.
Collectively, these discoveries put some teeth into the old saw that many Proterozoic fossils look like cyanobacteria. Because habitat range is a direct function of physiology, the close environmental similarity between ancient and living cyanobacteria suggests that the microorganisms distributed across Spitsbergen (and other Proterozoic) tidal flatswere essentially modern in morphology, life cycle, and physiology. Many of the cyanobacteria we see today are indeed survivors from the ancient Earth.
Cyanobacteria are common today in coastal habitats where very salty water or other environmental challenges restrict invasion by animals. By coincidence, the chert nodules in Proterozoic carbonates also center on coastal environments where silica was precipitated much like salt from evaporating seawater. Thus, chert’s paleontological lantern shines most brightly on just those environments where cyanobacteria have always thrived. Cyanobacteria do not live alone on modern tidal flats, however; mat communities contain a host of other organisms, especially bacteria. Why don’t we see this greater microbial diversity in chert nodules?
Tidal flats are harsh environments. At low tide, their inhabitants must tolerate the desiccating glare of a blazing sun; salty water provides an osmotic trial when the weather is dry; fresh water does the same during storms. Cyanobacteria respond to these challenges by secreting an extracellular envelope that protects the cells inside. That envelope is of particular importance to paleontologists because, unlike the cells within, it resists bacterial decay after death. Cyanobacteria, then, have the microbial equivalent of a clamshell, and in tidal-flat cyanobacteria this feature is especially well developed. While other bacteria live on tidal flats, most lack preservable walls or envelopes. And to make matters worse, they are tiny and have simple shapes that frustrate biological interpretation. The very fact that preserved fossils show evidence of postmortem decay means that heterotrophic bacteria must have lived in tidal-flat environments. As discussed below, geochemical signatures enable us to identify at least a few of these populations, but we must face the fact that what we see preserved in thin sections of chert, while extraordinary, represents only a limited sampling of the microorganisms that lived along the Proterozoic shoreline.
Fortunately, the sample preserved best is one well worth understanding. Cyanobacteria are the working-class heroes of the Precambrian Earth—the main primary producers in early oceans and the source of the oxygen that transformed terrestrial environments. We know a great deal
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