Remains of silicified microbial mats composed of benthic colonial coccoid cyanobacteria similar to modern entophysalidaceans and/or pleurocapsaleans have been identified in Lower Silurian black radiolarian cherts from central and southwestern Poland. Contrary to widespread views ascribing the genesis of such deposits to permanently anoxic deep-water marine environments, the abundance of benthic mats of phototrophic cyanobacteria suggests that the water-mat interface must have been located at moderate depth, most probably close to the limit of light penetration (dysphotic zone). Depending on ambient sulfide levels, the mats could intermittently perform anoxygenic (PSI) or oxygenic (PSII) photosynthesis, thriving under anoxic, oxic, or dysoxic (microaerophilic) conditions. The open marine (offshore) character of these cherts is consistent with their paleooceanographic location and with the presence of remains of such planktonic organisms as acritarchs, radiolarians, chitinozoans, and graptolites, entrapped by the cyanobacterial mats.

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