Role of algal eukaryotes in subtidal columnar stromatolite formation.

Columnar stromatolites were abundant and widespread in the Proterozoic but are exceedingly rare in modern seas. Consequently, the stromatolites at Hamelin Pool in Shark Bay, Western Australia, have been widely interpreted as unique modern analogs of ancient stromatolites constructed by complex communities of cyanobacteria. However, the Shark Bay columnar stromatolites contain sediment that is unusually coarse for stromatolites both ancient and modern, and the subtidal columnar stromatolites have a significant component of algal eukaryotes dominated by motile diatoms with mucilaginous tubes. This suggests that Shark Bay columnar stromatolites are not strict analogs for most ancient cyanobacterial stromatolites, least of all for those from subtidal environments. We argue that algal eukaryotes may play a substantial role in the formation and maintenance of subtidal columnar stromatolites at Shark Bay and are capable of trapping coarse sediment. In contrast, cyanobacteria have difficulty in trapping coarse sediment and produce essentially fine-grained stromatolites. We propose that there are two major types or end members of Recent marine stromatolites: (i) eualgal-cyanobacterial stromatolites that are generally coarse-grained, and (ii) cyanobacterial stromatolites that are generally fine-grained.