Oxygenation of the atmosphere by early cyanobacterial oxygenic photosynthesis occurred ca 2.4 Ga, during the Great Oxygenation Event (GOE), however, geological evidence of phototrophic mats and stromatolites dates to at least 3.2 Ga [1], with the origin of Cyanobacteria estimated between 3.5 [2] & 3.4 [3] Ga. This study reports on simulation experiments utilising deep branching Cyanobacteria to gain insight into what factors may have contributed to the uncoupling of the emergence of Cyanobacteria and oxygenation of the early Earth atmosphere.
The high Fe(II) availability on early Earth may have inhibited early Cyanobacteria by encasing them in rust. We show that deep branching species that are actively photosynthesising are not encrusted by rust under Archean ocean Fe(II) concentrations [4]. The reduced growth rates observed under repeated Fe(II) exposure were offset by increased photosynthetic activity, measured by O2 release rates, when compared to modern oxygen-rich conditions.
Early oxygenic photosynthesisers may initially have endured high levels of oxidative damage, thereby restricting their expansion prior to the GOE. Diurnal expression and activity of superoxide dismutases in Pseudanabaena sp. PCC7367 was monitored under a simulated Archean atmosphere and compared to modern day conditions. We demonstrate significantly enhanced transcription and activity under an early Earth anoxic simulation [5].
In summary we demonstrate that iron encrustation and oxidative stress were not responsible for limiting the spread of Cyanobacteria on early Earth. Instead, we propose that bioavailability of essential metals, such as iron, phosphates and fixed nitrogen may have contributed to the uncoupling of the emergence of Cyanobacteria and oxygenation of the early Earth atmosphere.
[1] Homann et al. (2018) Nat Geosciences 11,665.
[2] Boden et al. (2021) Nat. Comms 12,4742.
[3] Fournier et al. (2021) Proc R Soc B 288,20210675.
[4] Herrmann et al. (2021) Nat. Comms 12,2069.
[5] Tamanna et al. (2024) Submitted to Geobiology)